CA1242037A - Large capacity, large area video imaging sensors - Google Patents

Abstract

ABSTRACT OF THE INVENTION

Deleterious effects of high capacitance in large area, raster scanner image tubes, especially when employed in video cameras, can be overcome by employing the fol-lowing features: 1. a plurality of transparent stripe signal electrodes; 2. a unique, multiple layer, solid state structure designed to provide a Displaced Electron Layer-Sensor-Target for imaging (hereinafter DELST), with and without; 3. photoconductive gain in the DELST struc-ture; and 4. with and without intensifier gain in the DELST structure, the proper combination of these four features makes possible the construction of video sensor devices of extraordinarily large capacitance, having rasters generated by "low or high" velocity scanning elec-tron beams, or a laser scanning ray. The invention pro-vides a generic approach for the selection of features and their combination with the type of scanner best suited to any one application. The choice is dependent upon system requirements such as speed, spatial resolution, dynamic range, sensor-target size and cost.

Description

3~

, LARGE CAPACITY, LARGE AREA
VIDEO_IMAGING SENSORS

Field of the Invention The present invention relates to scanning image tubes especially those employed in video cameras. It is ; directed particuLarly to those devices which benefit from or require a sensor-target to have a large area capacity ; to perform optimally in an application such as to image the chest or abdomen of an adult with X-rays.
-10 Background of the Invention In the past, video camera tubes have been desig-nated "low velocity" or "high velocity". Low velocity tubes have typically featured better detective quantum efficiency and contrast than high velocity tubes. High 15 velocity tubes have, on the other hand, typically featured better lag performance (less lag) and spatial resolution and permitted the use of larger capacity layers in the sensor-target than low velocity tubes. Such characteris-tics are discussed in 1. the Electronics Engineers' 20 Handbook, Second Edition, edited by Donald G. Fink, 1982;

2. Television Camera Tubes: A Research Review by P.K.
; Weimer in Advances in Electronics & Electron Physics, Vol.
XIII, pp. 387, Academic Press, New York and London, 1960;

3. Photoelectronic Imaqing Devices, Edited by L.M.
25 Biberman and S. Nudelman, Plenum Press, New York, 1971;
i 4. Electronic Image Storage by B. Kazan and M. Knoll with contributions by ~ittarth, Academic Press, New York and London, 1967; 5. The High Beam Velocity Vidicon by J. Dresner R.C.A. Review, 305, June 1961; and 6. Advances 30 in Image Pickup & Display, Ed. by B. Kazan, Volumes I
II, 1974, 1975 Academic Press, New York and London. The desirable features of both the low velocity tube and of the high velocity tube have not, until the present inven-tion, been incorporated into a single tube. It should be 35 noted that the low velocity tube currently is the only 'I

~L'~Z5~3 tube used in practice due to its higher efficiency and contrast.
Two classes of scanner image tubes are currently in use. The first operates "without" gain and is exempli-5 fied by the vidicon, Plumbicon, Chalnicon, Saticon,Newvicon and Silicon vidicon. The second operates "with"
gain and is used in situations where there is insufficient light to operate "without" gain. These tubes are exempli-fied by the image orthicon, SEC and SIT tubes. The unique 10 feature of this latter group is the incorporation of a front end intensifier structure designed to provide image charge multiplication. In both classes of tubes, it is observed that there is included an imaging section and an electron beam scanning section. It should be noted, however, that in the "no gain" tube, the imaging section comprises simply a disc-like layer of material. It serves the dual function of being a photon sensor and a target on which to store a layer of electronic charge. For this reason, it i9 referred to herein as the sensor-target.
In a "low velocity," no gain tube, the electron beam scans the inside surface of the sensor-target in a raster scanning fashion. It scans adjacent parallel lines of the sensor-target one after another. The electrons arrive at the sensor-target surface with low energies, and 2sin particular, too low to cause any secondary electron emission. In the process, it deposits electrons uniformly across t'ne surface and drives it to approximately the potential of the electron gun cathode which is usually at or near ground. Secondly, the electron beam generates a 30 time varying video signal as it scans through the raster.
This results from modification of the charge on the sensor target as a result of projection of the optical image onto the input sensor surface of the image tube. The process occurs on a successive pixel by pixel basis as electrons 35 lost on the target surface through image formation are replaced by the scanning electron beam.
The imaging section incorporates a medium with two functions; a sensor responsive to the incident radiation to be imaged and an insulating target having a resistivity 40 on the order of 10l2 ohm cm. The high resistivity is P1~

;

essential to maintaining electron charge storage and s immobility on the inner scanned surface of the sensor-target during a raster scanning period. On exposure of the sensor-target to incident radiation, from an image of 5 an object, there results a flow of charge throug'n the medium such that electrons are lost from the scanned surface. Accordingly, electrons are lost across the sur-face in numbers proportional to the changing intensity of irradiation comprising the incident image. The resultant 10 electronic image is "readout" by the scanning electron beam.
' The tubes "with gain" have a forefront intensifier type structure which separates the sensor from the target.
As a result, the sensor changes from a vidicon photocon-15 ductor to a photoelectron emitter.
In operation, photoelectrons generated from the sensor's absorption of incident irradiation are accele-rated in the tube vacuum by the intensifiers electron optics and are imaged onto the target's outer surface.
20 The electrons strike that surface with sufficient energy to cause charge multiplication and a loss of a propor-tionate number of electrons stored on the targets inner surface. As a result, the scanning electron beam must replace more stored electrons per absorbed photon than in 25 the "no gain" tube, and the video signal is amplified.
The scanner image tubes and the prior art have experienced various problems, especially where large tar-get area i8 required. Conventional low velocity tubes in such applications suffer when the high beam resistance is 30 coupled to the large capacity to result in undesirable excessive lag. Large capacitance can result from increasing the size of the target, increasing the mediums dielectric constant and/or decreasing its thickness.
In general, numerous factors must be considered if 35 the scanner image tube is to perform effectively and optimally. Besides the sensor-target's capacity and distributed capacity, other factors include the energy of the electron beam, its resistance and current, as well as the modulation transfer function (MTF) of the tube's 40 imaging components. The latter depends on such factors as 3~3~

target thickness, the lateral displacement of charge stored on the sensor-target inner surface during a raster period, and beam current disc'narge characteristics.
Moreover, a high detective quantum efficiency 5 (DQE) of preferably 100~ is a desired feature for an opti-mal scanner image tube. A DQE of 100~ corresponds to a sensor having a quantum efficiency of 100%; output noise - limited by the photon noise at the input where the image is projected initially--i.e. photon noise, should dominate lOother electronic sources of noise: and the MTF high enough to assure that the dynamic range matches the imaging requirement throughout the necessary spatial frequency spectrum.
For large area X-ray imaging, conventional, low 15 velocity beam scanner tubes have failed to meet the needs -I of diagnostic radiology. As an alternative approac'n, x-ray intensifiers have been developed with diameters up to 22 inches. Such intensifiers are than optically coupled to a conventional sized TV camera. These systems are used 20in fluoroscopy and diagnosis. They suffer from intrinsi-; cally poor spatial resolution compared to that of the x-ray sensor. This results from multiplication of component MTF's~ Poor spatial resolution manifests itself medically in two ways, i.e., in reduced diagnostic performance and 25increased dose to the patient. These deficiencies must be overcome.
3 The operation of a "high velocity" beam video tube differs from the "low velocity" beam in that the electrons of the scanning beam are made to strike the inner surface 30of the sensor-target with sufficient energy to cause secondary electron emission. In an ideal tube, the elec-tron bombarded surface gives off secondary electrons, which are collected by "collecting" electrodes. In this process the scanned surface becomes positively charged 35because of the lost secondary electrons, and the surface voltage rises until at equilibrium, the scanning beam is essentially equivalent to the collected beam.
When the sensor is then exposed to incident light, there is a loss of positive charge in a manner that causes 40an electronic image reproduction of the incident irradia-tion image. The scanning beam then is able to replace the positive charge lost througtl secondary electron emission until the equilibrium potential i5 reached. In the process of replaciny lost charge, it generates a corre-sponding video signal on a pixel-by-pixel basis whicn effectively creates the time varying video signal that is similar to that from the "low velocity" tube.
The Iconoscope, in the United States, and its counterpart, the Emitron, in Great Britain, were the first video tubes to incorporate the eatures of "high velocity"
scan and the principle of charge storage.
The Iconoscope, however, suffered from the low sensitivity and poor efficiency resulting in large part from poor collection and redistribution effects of 15 secondary electrons.
A "high velocity" tube with a photoconductive sensor target was demonstrated in experimental tubes in the early 1950's. The purpose was to develop a new con-cept which would overcome the problems of lag and limited sensitivity. It was discovered that under certain condi-tions of operation, the tube could be made to operate with a high capacity target and provide improved lag per-formance. Furthermore, the higher capacity (thinner) targets also offered superior spatial resolution, and in theory, were expected to provide superior quantum effici-ency. Redistribution effects were reduced in that the photoemitted electrons did not exist. The secondary elec-trons generated during scanning were less of a problem in one mode of operation and worse in the other. Shading continued to be bothersome, but again on a reduced scale due to better collector design. nevertheless, the approach was dropped as the low velocity vidicon-type tubes were irnproved to the point where they met industrial and broadcast requirements.
However, applications exist that require the use of large capacity targets. These cannot be carried out with the conventional "low velocity vidicons" where target capacity and stray capacity must be limited. The "hig'n velocity" approach can serve as t'ne basis for a new invention which offers the opportunity to provide a new i7 imager able to combine the best features of the "high and low" velocity tubes.
Diagnostic radiology, for example, requires large area imaging with attendant sensor-target capacity and - 5 distributed capacity both prohibitively large with "state of the art" video tubes.
Solid state sensor panels employed in X-ray imaging are under development but suffer from other dis-3 advantages. Some such sensor panels provide that readout 10 be performed on the same side of the panel that is exposed. Moreover, the panel undergoes a cycling of voltages to respectively effectuate charging, reading, writing, erasing, and recharging. Accordingly, the panel must be transported from one station to another, rendering 15 rapid imaging impossible. When such a system is auto-mated, a large mechanical transporter is incorporated to move the panel from the exposure platform to its readout station. Disadvantages include cost, space occupied by the system, and the time involved to complete the process.

20 Summary of the Invention J
The present invention improves over prior art - scanner image tubes by overcoming various disadvantages and shortcomings set forth hereinbefore and by incorpo-25 rating features that provide for more optimal operation.
To achieve such ends, the present invention has as an object reducing, if not eliminating, the negative effects of beam resistance and capacitance in a low velo-city image tube.
Moreover, the present invention has as another object the realization of advantages relating to both low velocity beams and high velocity beams in a scanner image tube. That is, the present invention provides for good lag performance, for good spatial resolution, and for the 35 use of large capcity layers in the sensor-target as well as providing for a high DQE and good contrast.
It is another object to provide a scanner image tube useful in radiology, especially in applications 7 ~2~

requiring a large area sensor--such as imaging a human chest. In this regard, a scanner image tube is provided that can be operated in near rçal-time or in snap shot applications. In diagnostic radiology, the large majority of X-ray pictures are simply "snap-shots" as exemplified by the simple chest radiographO Most procedures in angio-graphy require repetition rates up to 7.5 frames/sec., while studies of the adult heart and coronary arteries can require real time imaging, i.e., 30 frames/sec.
: 10 Yet another object of the present invention is to significantly reduce the effective capacitance of large area imaging tubes by employing transparent, parallel stripe signal electrodes arranged side by side, parallel to the tube's scanning raster.
Still another object of the present invention i5 to reduce the beam resistance and capacitance of large area imaging tubes.
It is still a further object to minimize the magnitude of distributed capacity associated with pre-amplifier noise. This capacity increases with the area ofthe sensor-target in conventional "low velocity" and "high velocity" video tubes. By dividing the signal electrodes into stripes, whose length may be the length of a scan line, and whose width can encompass one or more scan lines, the area and distributed capacity associated with an individual preamplifier attached to a stripe can be sharply reduced. This approach requires multiplexing or a separate preamplifier per electrode tripe for optimal performance with the total given by the number of raster lines divided by the number of raster lines per stripe.
It is yet a further object of the invention to achieve high sensitivity in a tube having a high velocity beam. Prior high velocity beam tubes have produced secondary electron emission that result in the back-scattering of some of these emitted electrons back ontothe sensor during scanning, causing image quality degrada-tion. The present invention can employ a high velocity beam without such attendant degradation.
It is still yet another object to optimize sensor quantum efficiency by permitting a thin layer of, for i i . 8 3 example, crystalline antimony trisulfide to be used as a sensitive sensor--the high capacity and high lag related thereto being compensated for in the invention, particu-; larly by providing effectively a relatively low beam resistance. Lag, it is noted, arises from the failure of 5 the beam to return t'ne surface of a target to the charged potential after a single scan. In viewing an image, undesirable smear results from lag.
'! It is still a further object to provide readout , and recharging during beam scanning and to provide many s 10 electron5 for recharging than are required.
; It is yet a further object to provide, in various embodiments of the invention, charge multiplication or gain as needed using properties of the source of the scanning beam and the target. The invention thereby 15 directs a more copious flow of electrons to effect read-out and recharge of the target than is produced by the source.
In one embodiment, these and other objects are achieved by a scanner image sensor target including a 20 first electrode; a second grounded electrode, said first - electrode being positive relative thereto; a first solid state layer; a second layer; and means for a raster scanning beam of irradiation; w'nerein said first layer is sandwiched between said first electrode and said second 25 layer; and wherein said second layer is sandwiched between said first layer and said second electroae, and wherein said raster scanning means directs irradiation beams into said second layer through said second electrode, said second layer generating electrons which are conveyed 30 toward the interface with the first layer for storage thereat; the electrons stored at the interface forming an electron layer displaced from said second electrode, the electrons of said electron layer being combinable with holes generated in said first layer in response to imaging 35 irradiation passing through said first electrode into said first layer; the combining of holes from said first layer witn electrons from the said second layer at the interface forming an electronic image thereat. Accordingly, an imaging pattern of irradiation in a given spectral ban 33~7 g strikes the first layer in the image section whic'n gene-' rates free electrons free electrons and holes. The holes, in a pattern analogous to the pattern of irradiation, drift toward the interface under the influence forming an 5 electronic image thereat. Accordingly, the free electrons drift away from the interface toward the first electrode.
The holes combine with electrons at t'ne interface to form an electronic image thereat. Scanning with the beam i results in readout and recharging at the same time.
Hence, it is an object to permit image acquisition and processing simultaneously or consecutively, as desired, at a single station. This structure avoids the negative effects of charge redistribution from both photoemission and secondary electron emission which doomed 15 the success of the Iconoscope for such uses.
It is noted that the two electrodes represent plates of a capacitor and the first layer and the second layer from the dielectric therebetween. The second layer ^; is subject to local charge multiplication where a scanning 20 beam of electrons strikes and generates more electrons (or electron-hole pairs). Hsnce, a beam striking the second layer can produce almost a s'nort circuit between the second electrode and the interface along a path determined by the beam--resulting in a low beam resistance. It is 25 thus an object of the invention to increase tube per-formance and adaptability by reducing beam resistance s alone and in conjunction with reducing capacitance by the utilization of striped electrodes.
It is to be noted that the combination of layers 30 and electrodes as described above, creates a structurewhich permits electron charge storage required for image formation to coccur at the interface of the two layers.
This location is displaced from its position in a conven-tional video tube, and is intrinsic to this invention.
35 Accordingly, the structure which provides the Displaced Electron Layer in the Sensor-Target will be referred to as DELST.
Moreover, it is observed that the beam can cause sufficient charge multiplication in the second layer along 40 a path between the second electrode and a pixel at the to interface--to provide practically a short circuit there-between. It is, therefore, an object to permit an increase in the capacity of the sensor target by effectu-ating charge multiplication up to breakdown in the second 5 layer in response to the irradiating thereof by the beam.
This mechanism is designed to permit use of the high area capacity sensor-target with minimal lag.
It is a further object of t'ne invention to provide a scanner image device that can be scanned either by a 10 laser or by a high velocity electron beam, eac'n beam passing through a beam section electrode and into a layer of selected material. The selected material is responsive to the beam irradiation and generates electrons that charge or rec'narge an electron layer displaced from the 15 beam section electrode. PreEerably, the selected material is subject to local avalanche breakdown triggered by the beam on a pixel-by-pixel basis. one breakdown provides a substantial short circuit through a pixel across the layer - of selected material. It ensures an adequate supply of 20 electrons for all applications.
It is still a further object of the invention to reduce or avoid the need for charge multiplication in those applications where the laser beam scanner provides sufficient generation of charge for storage and signal 25 readout.
It is yet a furt'ner object of the invention, when using a scanning laser beam to avoid the vacuum require-ment of the electron beam scanning tube, by system design whereby the sensor-target is made as a stand-alone compo-30 nent, as is the laser beam scanner.
It is still yet another object, in variousembodiments, to provide gain to charges approaching the interface from both the image section and the beam section of the scanner image sensor-target of the invention to 35 increase the signal-to-noise ratio and enhance other characteristics of the tube.
It is still a further object of the invention to minimize the problem of large area capacity relative to associated electronic noise by selective electrode geo-40 metrical configurations.

~2~ 3~
. 11 It is yet a further object to provide a relativelylow-cost scanner image sensor-target employable in X-ray environments and in contexts ranging from high energy applications of particle and photon radiology to low 5 energy uses in the visible and infrared spectral regions.
It is furthermore an object to incorporate suffi-cient photoconductive gain in a sensor target to increase the video signal level in a video imaging system. Such increases can be used in low light level applications and 10 in overcoming electronic noise problems.
It is a principal object to minimize inter-electrode, or shunting capacity. In a super high velocity tube (in the kilovolt range) using video display type electron-optics, the conventional field mesh and suppres-15 sor mesh of low velocity beam operation are omitted. In alaser-scanner device, all the electrodes of a typical video tube are eliminated.
It is another principal objective to reduce the requirements on the sensor thereby to avoid the limita-20 tions of space charge limited performance. This ismanaged by requiring the sensor to detect incident radia-tion and if necessary provide gain, while charge storage at the interface is required of the target and not of the sensor.

25 Brief Description of the Drawings Figure l is a side-view illustration of a first embodiment of a scanner image tube employed in the invention.
Figure 2 is a front view illustration of a sensor-30 target employed in the embodiment of Figure l.
Figure 3 is an illustration of stripe electrodeseach connected to respective circuitry in accordance with the invention.
Figure 4 is an illustration showing a plurality of 35 stripe electrodes being scanned by a fan beam according to the invention.

~2~

it Figure 5a is an illustration showing reduction in positive charge along a stripe electrode when exposed to input radiation. Figure 5b is an illustration showing secondary electrons, generated during readout, being drawn 5 away from the stripe which has undergone a reduction in positive charge.
Figure 6 is an illu.stration of an embodiment of the invention including a displaced electron layer sensor-target (DELST).
Figures 7, 8 and 9 are illustrations showing the effects of impinging input irradiation and a scanning beam on a DELST. Figure 7 shows the beam striking one surface of a sensor-target, and Figure 8 shows radiation impinging v the opposite surface of the sensor-target. Figure 9 shows 15 readout.
Figure 10 is an illustration of a modification to the DELST shown in Figure 6.
Figure 11 is an illustration of an embodiment of the invention including a DELST and an optical scanning 20 beam.
Figure 12 is an illustration of a prioximity focussed intensifier device employed in the invention.
Figure 12a illustrates a generalized arrangement that utilizes the channel multiplier of Figure 12.
-I 25 Figures 13 and 14 are illustrations showing embodiments of the invention including channel v multipliers.

Description of the Invention The essence of the invention is to provide a large 30 capacity video type imager that is able to function in a manner that overcomes the customary limitations imposed by large target and large distributed capacities.
One version mazes use of parallel stripe elec-trodes spread across the sensor-target surface to offset 35 large distributed capacity and in a further preferred embodiment, a "high velocity" beam is used in overcoming the problem of large target capacity. The stripe elec-trodes are particularly important when the cross-sectional area of the sensor is required to be large, as in the case of diagnostic radiology. The invention offers Ereedom of design even in the case of small dimension sensor applica-tions of using 'nigh dielectric constant materials for the A sensor-target to increase target capacity. The advantage derived is in the target-sensor being able to store a larger charge, and thereby offer the opportunity to increase the dynamic range of the imager.
The combination of a scanning beam with stripe electrodes provides the means for achieving maximum spatial resolution for the large dimension sensor, since readout occurs at the primary sensor-target. It avoids the intrinsic limitations of conventional systems used, for example, in diagnostic radiology. There, X-ray intensifiers are designed with a large degree of demagni-fication, and then have their output optically coupled to a video camera tube. The result is a substantial loss of resolution to the extent where toe system is limited principally to fluroscopy and to intravenous angiography.
A purpose of this invention is to provide a large area X-ray video imager able to meet the needs of diagnosis as distinct frorn fluoroscopy, for most of the procedures used in the practice of diagnostic radiology.
Another version uses super "high velocity" elec-trons (kilovolt energies) and video display type electron-optics. This type of operation eliminates much of the usual interelectrode capacitance and minimizes the total distributed or shunt capacity. It effectively reduces "gain" requirements on the sensor and also reduces, if not eliminates, the need for stripe electrodes. Furthermore, the effective RC time constant is reduceable by offsetting large target capacitance with a dramatic decrease in beam resistance.
Still, another version uses a laser scanner to generate photoelectrons in a semiconducting target for charging and recharging a storage surface. It effectively eliminates interelectrode capacity, provides reauced beam impedance, and permits operation with a large target capacity.

14 3~

The "super high velocity" scanning electron beam and the laser scanner versions use a basic two layer structure, comprising a sensor layer adjacent to a target layer. In one mode of operation, the target layer must 5 have the high resistivity associated with video charge storage as exemplified by SEC and EBIC targets in conven-tional "low velocity" scan tubes. Positive charge created during the scanning process is stored on the target sur-face adjacent to the inner surface of the sensor. The 10 sensor layer must absorb incident radiation and convert it into representative charge which is transported to the interface between the sensor and target, where it dis-charges proportionately, the beam induced stored charge.
15 Since the stored charge layer is displaced from the outer (scanned) surface where it resides in a typical video tube operation to the inner surface at the interface, the structure is referred to as a "Displaced F,lectron Layer Sensor-Target," abbreviated as DELST.
In the DELST structure, there is opportunity for sensor photoconductive gain. This is possible because the conditions of "space charge limited" performance can-be avoided in the sensor when it does not have to sustain stored charge.
1. Operation with "Low" and "High" Velocity Video Tubes Referring to Figure 1, a first embodiment scanner image tube 100 according to the invention is illustrated.
The tube 100 has essentially the same structure for opera-30 tion as a "low velocity" or a "high velocity" vidicon.
The front end comprises a supporting member 102 on which is deposited a conducting layer electrode 104 and a photo-conducting layer 106. When the incident radiation 130 is light, the front end member 102 is glass and 104 is a 35 light transparent, electrically conducting layer suitable for applying a bias voltage from source 122 to the outer surface of the photoconductor 106. The photoconductive layer 106 is selected to be responsive to the light. When the incident radiation is X-ray, the structure 102 can be metal known to be transparent to the radiation. In the simplest configuration, the metal combines the role of the supporting member 102 and the transparent electrode 104.
The p'notoconductor 106 is then selected to be responsive 5 to X-rays. The materials sensitive to the other known types of radiaton (gamma, alpha, ionic, cosmic, beta, neutrons, etc.) are well known so that the tube may be made responsive to substantially, if not all, of the various types of radiant energy. The sensor-target 106 10 comprises an efficient absorber of X-ray radiaton, CsI, which efficiently converts the incident X-ray into light emission. The light, in turn, is absorbed by an adjacent layer of light sensor photoconductor with a transparent conducting electrode at the interface between the light emitter and the photoconductor to apply the bias voltage for the photoconductor.
The electrode 104 is connected to a conventional video readout circuit 120 that includes a voltage source 122 which biases the electrode 104 relative to ground and a capacitor 126 which carries the video signal to a preamp. The resistance 124 serves as a load resistor to develop the video signal.
Also shown in Figure 1 is an electron gun cathode 110 which provides electrons to form an electron beam 114 and which is directed by electron-optics 116 to scan a raster on the inner surface of the photoconductor 106.
Depending upon the bias voltages applied through Vt to electrode 104, Vc to the cathoae 110 and that applied to the grid 118, the tube can operate either as a "low velocity" or a "high velocity" vidicon.
The biases for "low velocity" operation are selected so that the energy of the beam electrons is too low when arriving at the inner-scanned surface of the photoconductive layer 106 to cause any secondary electron emission. Typical for a vidicon would be the electrode bias 122 set at 30 volts and the gun cathode bias Vc set at ground. The grid 118 is called the "decelerating grid"
and has the function of slowiny down the electrons so that their arrival of 106 will be with energies too low to cause any secondary electron emission. Accordingly, its voltage is positive with respect to ground, and typically about lOOOV.
In operation, electrons deposited on the inner surface of 106 cause its voltage to become essentially the same as the cathode described above as ground.
The biases for "high velocity" operation are selected to ensure that tube performance is governed by secondary electron emission. An example of bias settings used by Dresner (see reference) is a bias voltage of Vt=~300 volts for electrode 104, the electron gun cathode 110 set at Vc = volts (ground) and the grid 118 (Vg) set at 320 volts. This is designed so that the beam 114 electrons have sufficient energy to cause secondary elec-trons to be emitted from the scanned surface of the photo-15 conductive layer 106. They are collected by the grid 118,and accordingly, it is called the "collector" grid.
In operation, the scanned surface of layer 106 acquires a positive equilibrium potential such that the net target current equals zero. The equilibrium voltage 20 VO is determined by the collector voltage Vg and lies a - few volts above it. For a fixed Vg, the potential across layer 106 depends upon Vt applied to electrode 104.
Accordingly, the photocurrent can be made to flow in either direction according to whether Vt is larger or 25 smaller than the equilibrium potential VO.
In dealing with low and high velocity vidicon performance, particularly where large values of target and distributed capacities apply, it is prerequisite that their operation must be properly modified for optimum 30 performance. The case of X-ray imaging for diagnostic radiology is compared with conventional light imaging to point out the difficulties in operation, and the new device conception to overcome these difficulties.
Reference to Figure 2 shows the circular sensor-35 target 206 identified in Figure 1 as 106. Traced on thistarget is a raster, whose vertical and horizontal dimen-sions, are taken equal (a square) for simplicity. The typical sensor-target capacity for a small video tube using a low velocity vidicon target using antimony tri-40 sulfide is on the order of 1000-4000 picofarads, although -,f~63~

special tubes can be found with larger values. Assume for simplicity that a l"xl" raster is used and that t'ne target capacity is 1000 picofarads. If this tube were enlarged to provide a 16"x16" raster to meet the needs of diag-5 nostic radiology, the capacity would grow in proportion tothe increased area and be as large as 256,000 picofarads.
Correspondingly, the distributed capacity can be expected to grow, at worst, in a manner proportional to the increased area. The distributed capacity of the small 10 size "low velocity" tube is in the range of 2-25 pico-farads, depending upon the manufacturer. Assuming a favorable value of 5 picofarads for the small tube, the large area tube could have a distributed capacity of about 1,280 pico~arads.
The implication of these large capacitance values can be discerned by the associated capacitive lag and the preamplifier noise. The scanning beam, instead of acting as a constant current source, acts as a resistance of the order of 107 ohms. Assuming a target capacity of 1000 picofarads for the small tube, the RC time constant 20 becomes 107 x 1000 x 10 12 = .01 secs. Since the raster period at 30 frames/sec = .03 secs., the RC time constant is clearly usable.
However, for the large diameter tube, the capacity grows to 256,0Q0 picofarads and the RC time constant to 2.56 secs. this value causes the tube to be excessively laggy for use in conventional video imaging and even in X-ray imaging using snap-shot operation.
The distributed (interelectrode-shunt) capacity is related to preamplifier noise through the expression iN (PREAMP) = 4kT. 4~ .Re.Cd2.~f3 (1) =1.38xlO 23 Joules/kO

where k = Planck-Boltzmann Constant t = Absolute Temperature = 300 Re = Equivalent Noise Resistance of 18 ~2~ 3~

First Stage Preamplifier Resistance Cd = Distributed or Stray Capacity to Ground From ~iynal Leads coupled primarily to ELECTRODES
of = Electrical Bandwidth Assume Re = 40 ohms Cd = 5 picoforads for the small tube af = 107 hertz which leads to iN PREAMP)= .264 na.

10 When Cd = 1280 picofarads for the large tube is used, the preamplifier noise current grows by a factor of 256 and the preamp noise becomes 67.6 na. A "state of the art" preamplifier has been reported for the small sized, low velocity tube with a current noise as low as 0.5 na.
15 Good preamplifiers are available at about 1 na. Relative to the 1 na value, the large area tube illustrates a value almost 70 times larger and a corresponding reduction in the signal to noise ratio.
Clearly, the excessive lag and preamp~fier noise 20 must be eliminated for video tubes to be applied, for example, to diagnostic radiology.
The "'nigh velocity" team reduces the effect of target capacity on lag. The reason can be explained in terms of tube conductance. For the "low velocity" beam, 25 where the sensor-target voltage is small, the current flowing into the target (It) is given by bVt It = a exp (2) where b = e/kT
T = effective temperature of the cathode responsible for the energy distribution of electro-ns in the beam. It is in excess of 1000K.
e = electronic charge k = the Boltzman constant -The requirement that capacity lag be reduced is equivalent to requiring that the target capacity be decreased and that the constant b increased. The latter is equivalent to requiring a large value of the beam 5 conductance near the equilibrium potential of the scanned surface. The beam conductance is given by dIt = a b expbVt = bIt (3) dVt where It is limited since Vt is small in "low velocity" operation b can only be increased by reduction of the effective temperature T.

; Recent new designs in electron guns have succeeded in reducing the effective temperature T, but not to the extent necessary for operation with the large capacities - 15 inherent in our applications.
In the case of "high velocity" operation, Dresner shows that the beam conductance is given by:

dIt = Ib N(v) dv (4) dvt ~Vt v where Ib = beam current = secondary electron emission coefficient of the sensor-target N(v) = the energy distribution of the secondary electrons produced at the sensor~target This expression can be simplified to 25 _t = Ib N
dVt ~L~4;~3~-f' .

where N represents the number of electrons effectively ; collected.
It is clear that this type of operation i8 not inhibited by a low target voltage and that the beam con-5 ductance increases linearly with beam current. The corre-- spondingly causes a decrease in lag, where compared to the "low velocity" case, to a first approximation the capaci-tive lag is independent of beam current.
; The dependence of distributed capacity on the area 10 of the sensor-target is dealt with in accordance with the present invention by using stripe electrodes as illus-trated in Figure 3. The raster lines 302 are shown simply as parallel horizontal lines. The shaded stripes 304 represent electrodes placed one next to the other, spaced 15 so that their separation distance is appreciably less than the vertical dimension of a pixel. Attached to each electrode is a preamplifier circuit exemplified by the load resistors 324, the target bias sources t and the coupling capacitors 326. The distributed capacity is now 20 associated with the area of each electrode stripe and the wiring to the preamp. The length of each stripe as shown in Figure 3 is slightly longer than the length of a raster line. The width encompasses as many raster lines as device capacity and design allows. This number may differ for "low" versus "high" velocity operation because of a "secondary electron" redistribution problem inherent to conventional "high velocity" tubes.
Limiting the distributed capacity for each stripe to that of the 1" x 1" area sensor-target referenced earlier, means restricting each stripe to having the same 1 square inch area. Thus, for a length of sixteen inches, the width must be restricted to 1/16" or 1.56 mm. A 500 line raster spread over 16 inches, would present 31.25 lines/inch, and therefore about 2 raster lines per elec-trode stripe. For 1000 and 2000 TV line rasters, thenumber of lines per stripe would grow to 4 and eight, respectively. With this approach, the number of preampli-fiers depends upon the number of stripes given by the vertical raster, height divided by a stripe width. For a 'I 21 j:
j 16" raster height, and a l/16" stripe width, the number of preamplifiers equals 256.
A vertical set of stripes may be employed in place of the horizontal stripes. Such an arrangement would , 5 require the use of digital to analog conversion and .; storage so that the picture can be reassembled at the end of each frame.
` Using stripe electrodes to minimize distributed capacity is achievable at the expense of adding a large I 10 number of preamplifiers. Although, in view of modern integrated circuit techniques such numbers are acceptable, this number can be reduced by exposing each stripe separately, reading out the rastar lines beneath each stripe, and then repeating the process for successive 15 stripes. This is a procedure compatible with diagnostic radiology where an X-ray fan beam parallel to the stripes can be made to rotate so that each stripe is exposed and readout in succession, as shown in Figure 4. This approach permits replacing preamplifiers by switches or 20 multiplexing to the extent that any increase in dis-' tributed capacity can allow. Shown in Figure 4 are the electrode stripes 404, their separation 408, the sensor-target shown here as one layer 406, the X-ray source 402 and the fan beam 410. Although for simplicity the fan 25 beam is shown in rotation, other approaches apply such as translating the X-ray source and fan beam along the length of the sensor-target, or for a fixed position fan beam and X-ray imager translating the object. Not shown in Figure

4 is the overlaying support structure shown in Figure l as 30 102. The scanner 412 is identified as the means to charge the inner-scanned surface of the sensor-target.
The invention described to this point can apply equally well to the iarge area "high" or "low" velocity tubes. The sensor-target stripe capacity and the dis-35 tributed capacity can be made essentially the same as for those found in conventional, sTnall dimension video tubes.
Thus, this invention demon.strates that the "low velocity"
electron beam scanner can be used for X-ray diagnostic radiology, when coupled to this invention comprising elec-40 trode stripes with associated preamplifiers and can be further enhanced by using a translating fan beam and associated switching or multiplexing circuits.
However, the high velocity beam offers the opportunity for increased target capacity without a

5 corresponding increase in distributed capacity. This approach permits extending the dynamic range of the video sensor, since such range depends upon the magnitude of the charge that can be stored on the target, and the ability of the electron gun to read out totally that charge. The 10 "high velocity" tube offers this improvement over the "low velocity" tube and without suffering additional lag.
Thus, for example, a porous layer of trisulfide target of 1000 picofarads could be replaced by a thin amorphous layer of lO,OO0 picofarads permitting a corresponding lS increase in the magnitude of the signal that can be managed. Furthermore, the thinner layer offers improved spatial resolution from the target with increased sensi-tivity. The principal disadvantage of the "high velocity"
tube is in the redistribution of the secondary electrons.
20 When secondary electrons emerge from a surface, they emerge in a range of velocities and angles according to the expression 2md e(Vt-vc) v COS~sin~ (6) where: R is the distance-traveled from the point of emission on the surface of the target to the point of landing, if not collected by the mesh d is the target to mesh spacing m is the electron mass e is the electron charge v is the electron velocity is the emergent angle of an electron trajectory relative to the normal to the surface Vt is the potential of the scanned surface of the sensor-target Vc is the potential of the collector It is clear that many electrons return to the surface and as a result diminish the quality of any electronic image stored on the scanned surface. If the device is operated pi in darkness and equilibrium conditions prevail, then a relatively constant secondary current reaches the collec-tor equal to the beam current and a fairly constant secondary flow of secondary electrons falls back on to the 5 surface of the sensor-target. In this case, the signal I'' current is zero excepting for non-uniformities in charge distrubution associated with shading, target surface defects and other possible spurious signals. On exposure, the charge distribution is modified dependent upon the 10 photoconductive response, pixel by pixel, in accordance with the projected incident image. Thus, the potential of each individual pixel is shifted accordingly. As each ., pixel is scanned, the number of secondary electrons s generated varies according to each pixel potential, being 15 more or less according to the degree of potential change experienced by the sensor-target. As the scanning re-charges successive elements, the capacitive output signal current is largely neutralized by the return of the secondaries to other target areas, thus limiting the out-20 put signal to 25~ of the level possible if all secondaries were collected.
Another problem is associated with collector geo-; metry which in the conventional "high velocity" tube leads ; to a serious degree of shading.
It is possible, furtl~ermore, to have secondary emission from the collector mesh made deliberately domi-nant over that from the target. This results in mesh con-- trolled range of secondary electrons being reduced to a value of R/2 and in improved contrast. Further reduction 30 in R is possible by close spacing of the collector mesh.
- The problem of redistribution effects in large part were dealt with by operation with mesh controlled secondary emission and close spacing between a fine mesh and the target surface.
With the approach described in the present invention using stripe electrodes, additional means for minimizing redistribution effects is possible. For example, the raster lines adjacent to three stripes can be scanned in a manner designed to erase charge stored as a 40 result of prior imaging or redistributed secondary elec-24 3~
, .
` trons. When the inside stripe is exposed by a fan beam, then a new imaging charge distribution occurs along the : lines within the stripe which results in the potential along the lines being depressed relative to these in the 5 two surrounding stripes. Accordingly, there is a strongelectric field set-up between the lines within the center stripe and those within the neighboring stripes. Then on scan readout of the lines within the exposed center stripe, the secondary electrons are drawn off either 10 toward the collector or toward the adjacent strip and few , electrons fall back on the lines within the exposed - stripe. As a result, the signal readout is optimized by minimizing the dramatic signal loss associated with charge redistribution in conventional operation of a high velo-15 city tube. The approach is illustrated in Figure 5 with three stripes 502, the sensor-target 504 and four scan lines per stripe 506. Each scan line is represented by a shaded region supporting a charge distribution. The cen-ter stripe shows a reduced positive charge distribution 20 because of exposure in Figure 5a by X-rays 510.
Accordingly, there is an electric field set up between the center stripe and its neighbors to draw away any secondary electrons from the center stripe generated during read-- out. This is shown in Figure 5b where the electron beam 25 512 scans the first of four lines with a stripe. The trajectories of the secondary electrons are shown going to the collector grid 508, the lines in adjacent stripes and a few falling back on a line within the exposed stripe.
Not shown is the structure supporting the sensor-target 30 corresponding to 102 in Figure 1.
The procedure to expose and read out a full raster involves repeating the process shown in Figure 5 in sequence until all stripes are individually exposed and read out.
An estimate of the total cycle time using this procedure can be provided for diagnostic radiology.
Assume 256 stripes with each stri2e exposed for 3 msec, so that the total time required to expose a 16 "long" surface scanned by a translating fan beam equals .758 secs.
40 Assume also that each stripe and, therefore each line is scanned by the electron beam three times for the cycle required to minimize the effect of secondary election redistribution. Let the dwell time of the beam on a pixel be the same as for commercial television, that is about 10 7 sec. Then, for the 500, 1000 or 2000 line rasters, respectively, readout time equals .075, .300 and 1.20 . seconds. If, to these numbers, is added the total expo-sure time of 0.768 seconds, tne total times to acquire an image for viewing and/or processing become 0.84, 1.07 and 10 1-97 secs respectively. nose values are vexy favorable for most of diagnostic radiology compared to other mechanical scanning methods being attempted today and, furthermore, they would be accomplished with a far ` superior modulation transfer function.
Another procedure for reducing the effects of redistruhution is to switch biases on the stripes. In this case, the stripe to be exposed is made to be negative compared to the others. The sequence of events is then erase, expose and read out a stripe. During readout, the 20 secondary electrons generated are collected by the mesh and the surrounding stripes. The magnitude of the signal is then determined by the exposure reduced only by the few electrons that might fall back on exposed lines.
One can readily conceive of other scan procedures 25 using more or less stripes and more or less lines per stripe. They all fall within the essence of this inven-tion designed to optimize the drawing away of secondary electrons from the exposed stripe during a readout.
It is clear that the high velocity tube operating 30 with stripe electrodes offers the advantages of 1. Minimal lag 2. Optimum spatial resolution in the sensor-target 3. Increased dynamic range 4. High Detective Quantum Efficiency (DQE) 5. Electronic Noise for the large diameter tube can be the same as for the small dimension, conventional low velocity tube.

6. High signal levels. my generating a video signal at the sensor-target using the combination of a beam scan with the stripe electrodes, the 33~

modulation transfer function approaches that of the sensor-target.
Finally, the technique of exposing a stripe with a short burst of radiation followed by immediate readout f 5 shortens the time requirement for sensor-target storage.
This offers an opportunity for wider choice of sensor-, target materials with somewhat lower resistivities com-3 patible with the shorter storage time required. This could make easier the possibility of acquiring a sensor-lo target offering photoconductive gain.
The number of stripes and associatea preamplifiers , is based on the assumption that the distributed capacity ,3 grows in proportion to the area of the sensor target.
This is not necessarily the case in comparison with target 15 capacity which clearly grows directly proportionate to area. With "high velocity" operation, considerably more target capacity can be tolerated, since the price of "increased lag" does not apply as it does for "low velocity". The price for large distributed capacity is e' 20 incrased electronic noise associated with the preampli-; fier. If, for example, the distributed capacity can be made to grow slowly with area, then the stripe widths can be increased, the number of stripes and correspondingly the number of preamplifiers reduced.
Gain before readout remains an important factor in determining ultimate design of an X-ray sensitive video camera tube. At present, the principal mechanism for obtaining such gain is with a luminiscent layer such as CsI, which generates about 1000 light photons per absorbed 30 X-ray photon. In theory, based on energy considerations of the light photon versus the X-ray photon, one mig'nt anticipate that ideal conversion would offer 15 to 20 times more light photons. The consequences would be a similar improvement in signal and the signal to noise 35 ratio (relative to distributed capacity induced noise).
Correspondingly, an X-ray sensitive photoconductor offering a similar range in gain from photons directly into charge carriers would provide the same advantages in signal and the signal to noise ratio. With such improve-40 ment, the design can trade off more raster lines per .
stripe (that is, wider stripes) and fewer preamplifiersversus some loss of image contrast due to recording an increased number of scattered events. Channel multipliers may also be employed to reduce the number of stripes 5 required.
- Another approach to relievlng the problem of dis-tributed capacity is to improve the signal by adding photoconductive gain in the sensor-target. none has been evidence of such gain in tubes such as the Chalnicon and 10 Newvicon. However, these designs did not require high gain for their application. Earlier attempts to incorpo-rate gain were unsuccessful for operating under the condi-tion of "space charge" limited currents. Recent experience has shown the existence of high gain, high 15 resistive photoconductors suited to video applications.
These permit lifting the signal level of protons as high as hundreds. T'neir performance will be described in detail in the sections below related to operation with DELST devices.
Increased signal, however, provides a problem for electron guns in low velocity tubes, which are not designed to provide many micoramperes of signal. "Hiqh velocity" gun operation could be more easily adapted to the requirements imposed by such high signal currents.
25 Nevertheless, the "overwhelming" of distributed capacity induced noise by such large signal currents permits increasing the width of stripes and reducing their number, '' possibly for some applications to a single electrode.
The "high velocity" tube discussed above has been 30 described with the usual raster dimensions of a conven-tional video tube. However, the principle of operation applies even better to a line sensor shape, as compared to an area sensor-raster shape. A line sensor for example, can be matched to an X-ray fan beam of radiation. The fan 35beam and line video sensor can then be translated in unison in a direction perpendicular to the plane defined by the fan beam and line sensor, to provide an image covering the area defined by the length oE the sensor and the distance it is moved. This tec'nnique is well 40established in diagnostic radiology, but suffers in part 2~ 37 from the nature of the sensors used in that application.
Both the "high and low" velocity beam operation can ?, operate with a long sensor, whose width is determined by resolution requirements and the number of raster lines 'I 5 required for an application ; The simplest case is a single raster scan line alo-ng the sensor length. In the low velocity case, the device will have low shunting capacity and target capacity limited by the usual requirement on the RC time constant 10 and preamplifier noise. In the high velocity case, the secondary electron redistribution effects become negligible by proper design of the collecting mesh and the device offers the opportunity to use a muck larger target capacity, for superior dynamic range. Additional raster 15 lines can be managed within the capacity limits imposed by an application. The advantages gained relate to decreased power requirements imposed on the radiation source (as for X-rays), and more speed in acquiring an image.
l`he line scanner also offers options in its shape.
20 It can be straig'nt or curved. The latter could have - applications, for example, in X-ray radiology to image a cylindrical object with a point X-ray source as in non-destructive testing. on example in medicine is to X-ray the breast with close proximity to the chest wall.
- 25 This approach to a line scanner applies with equal validity to versions described below for DELST WITH A HIGH
VELOCITY SCANNING BEAM and DELST WITH A LASER SCANNING
BEAM.

2. Operation with DELS'r and a HIGH
- 3~ Velocity Scanning Beam.

A device that can offer the advantages of the "high" and "low" velocity beam tubes without secondary electron re-listribution effects, is the displaced electron layer sensor-target. It san be designed to function with 35 either an electron beam or a laser beam.
Referring to Figure 6, a first embodiment dis-placed electron layer sensor-target (~ELST) 600 according to t'ne invention is illustrated. It includes a sandwich ;

29 ~2~

structure 602 formed of a first electrode 604, a first layer 606, a second layer 608, and a second electrode 610.
' The first layer 606 is sandwiched between the first elec-trode 604 and t'ne second layer 608. T'ne second layer 608 - 5 is sandwiched between the first layer 606 and the second : electrode 6]0. Between the -first layer 606 and the second '- layer 608 is an interface surface 612.
I, The first electrode 604 is connected to a conven-tional readout circuit 620 that includes a source 622 10 which positively biases the first electrode 604 relative to ground an'd a capacitor 623 which carries the vidao signal to a preamp. The second electrode 610 is connected to ground.
Also shown in Figure 6 are the exposure side of 15 the DELST indicated by imaging irradiation 630 directed to the electrode 604, and the charge-read side of the DELST
indicated by a scanning beam 650 directed to electrode ~610. The operation of DELST can be decribed in the following steps:
1. Use a scanning beam of particles or photons to penetrate electrode 610 and cause the generation of electrons in layer 608. Under the influence of the '; applied electric field due to the bias 622, the electrons flow to the interface surface where they are stopped and 25 stored.
2. Expose the DELST with imaging irradiation 630 directed at and transmitted through electrode 604, to be absorbed in layer 606. The absorption process leads to a conversion of the imaging irradiation into charge carriers 30 within layer 606 and under the influence of the applied electric field to a depletion of charge carriers stored at the interface surface 612.
3. Use the scanning beam to replace the missing electrons at the interface 61~, and in so doing generate a 35 video signal picked up by the preamplifier 620.
Of particular importance in the DELST structure is the addition of layer 608, whic'n makes possible over-coming the disadvantages of low velocity tubes. This occurs because this layer 606 permits operation with a 40 scanning beam of high velocity particles or an intense ..
!~
scanning beam of photons, without suffering the image degradation of the prior art. In effect the electron storage surface 612 is now buried between layer 606 and 608 and effectively isolated from the source of any t 5 scanner electrons, or of unwanted secondary electron emis-sion.
The layers 606 and 608 must have resistivities sufficiently high to permit cnarge storage, i.e. on the order of 1012 ohm cm, and is a requirement similar to t'nat 10 for vidicon type tubes. Various multiple layer structures with non-ohmic heterojunctions have evolved over the years to accommodate this requirement as found in the C'nalnicon, Saticon and Newvicon. Such junctions can readily be incorporated in layers 606 and 608 if desired.
Electrode 604 is transparent to the incident radiation comprising the image of some object. If the irradiation is light, then the electrode 604 must be 'I transparent to light. If it is X-rays, then electrode 604 !~ must be transparent to these X-rays. Similarly, for 20 energetic particles, such as alphas, betas and neutrons.
Layer 606 correspondingly must be responsivs to the nature of the incident radiation. If the irradiation is in the visible range, it must be a photoconductor ~3 responsive to light. Similarly, it must have a spectral 25 response matched to the spectra of the irradiation . throughout the spectra of interest, which can range from high energy gammas through the ultraviolet, visible and into the near infrared. As one moves to sufficiently long wavelengths in the infrared, this approach must be modi-30 fied to accommodate the sensors' lower resistivity by providing cooling as needed. Nevertheless, even infrared responsive devices are derivable from DELST sensor-targets. Appropriate materials for layer 606 could include, for example, properly doped germanium or silicon.
The scanner beam 650 as shown in Figure 6 can be derived from high velocity particles or photons. Parti-cles would be used in combination with a layer 608 selected to provide charge multiplication. This would permit a relatively low beam current of high velocity 40 particles to cause a large current scanning beam to flow ' ' 31 37 ., in the layer 608, suitable for charging surface 612 and s readout with low lag. Most commonly, the particles in the beam would be high velocity electrons with sufficient energy to cause the required charge multiplication in - 5 layer 608. however, energetic beams of other particles ; such as ions and alphas could also be used for scanning.
When the scanning beam is derived from photons, their source could be from devices such as flying spot scanners using incoherent sources of radiation, or 10 coherent laser raster scanners. The essential requirement is that the -flux of photons be sufficiently intense to ` generate sufficient numbers of electrons in layer 608 so that the charging and reading out functions can be managed properly. When using photon scanners, the layer 608 is 15 made of a photoconducting material whose spectral response is matched to the beam radiation spectrum.
It should be noted that the roles of the electrons and charge carriers can be reversed. Thus, if the polar-ity of electrodes 610 and 604 are reversed then the elec-20 trons are conducted through layer 60~ and the charge carriers are conducted through layer 608.
An example that can serve to illustrate a DELST
structure and its operation is shown in Figures 7, 8 and 9 for imaging with lignt irradiation and scanning with a 25 high velocity electron beam.
When the scanning beam projects energetic elec-trons through electrode 710, the layer 708 comprises a charge multiplication layer. Accordingly, the layer 708 generates more electrons than are incident thereupon.
30 Various materials and mechanisms are known which provide this charge multiplication effect. One mechanism i5 referred to as electron bombardment induced conduc-tivity (EBIC). Typical EBIC materials include semi-conducting glass, magnesium oxide, and silicon. When such 35 materials are struck, or bombarded, by electrons of high enough energy, electron-hole pairs are generated witch exceed the number of incident electrons. A second mechanism relates to seocndary electron conductivity (SEC). Potassium Chloride (KCl) is a material related to 40 this mechanism, which is embodied in known SEC tubes.

32 3'~
:
Another type of known charge multiplier is the channel multiplier commonly found in second generation image - intensifier tubes. Although different from each other, these various known mechanisms may be employed in the 5 second layer 708 to provide charge multiplication.
The various charge multipler mechanisms, it is ; noted, have been used in the image section of conventional low velocity scanner image tubes that incorporate gain.
nose tubes amplify or intensify the input image, as 10 described above in the background section.
-I The ideal multiplier layer 708 would simply provide a short circuit between electrode 710 and the - interface surface 712. In Figure 7 is shown the flow of multiplied electrons in an SEC type target, 708, which, 15 under the influence of the electric field, drift toward the interface where they are stored. Optimally, as the high velocity beam electrons 750 penetrate layer 708 by ; passing through electrode 710, they trigger an avalanche of electron flow, affecting a short circuit flow toward 20 the interface 712. Each picture element (pixel) on the interface 712 charges with electrons successively as the beam scans through a raster with each avalanche termi-nating as the voltage drop between 710 and 712 becomes too small to sustain breakdown. This would ensure the maximum number of electrons available in minimizing time for - storage and signal readout. However, such an abundance - might not be necessary for the dynamic range required in most imaging applications. Accordingly, EBIC and SEC type materials offer gain extending over a range from 2 to 3000 that might well cover the range needed in general. Figure

7 is designed to show the beginning of electon deposition at the interface 712 as the raster scan is initiated.
Note that although layer 708 is responsive to the high energy beam of electrons 750, it is not responsive to input imaging irradiation 730 passing through electrode 704 and into layer 706. This scanning process results in a uniform deposition of electrons at the interface surface 712.
- The input layer 706 is similar to the photoconduc-; 40 tive sensor-target of the vidicon type tubes. It can have -33 ~Z,~3~
:C
!. any of the detailed structllres exemplified by the Vidicon, Saticon, Chalnicon, Newvicon, and Silicon Vidicon. On ; absorption of incident imaging radiation, electrons and :~ holes are created which drift in opposite directions due 5 to the internal electric field established by the bias voltage 622. This is illustrated in Figure 8. There is shown a c'narged interface 814 and the movement of holes drifting toward the interface 812. On arrival, they - remove stored electrons by recombination. This results in 10 the uniform distribution of electrodes at 812 being modi--I fied to form an electronic image reproduction of the optical image. The electrons generated within driEt to electrode 804 and are removed from layer 806. They do not in this process contribute to the video signal.
Readout of the electronic image 916 is il lus-- trated in Figure 9, where the high velocity electron beam 950, in collaboration with the multiplier layer 908, is shown replacing the interface layer of electrons removed in the imaging process. This is done on a pixel-by-pixel 20 basis during a raster scan, and results in a video signal being picXed up by the preamplifier circuitry 920.
Note that throug'nout the description of this example, the input electrode 904 is electrically conduc-tive while being transparent to the imaging light.
25 Furthermore, the scanned electrode 910 is also elec-trically conducting while permitting of the scanning electrons into layer 908.
In the description of this example, the process of imaging and readout has been described in sequence. In 30 fact, the process can be managed either in sequence or concurrently, similar to these possibilities for conven-tional video tubes. The forming of the electron layer may be thought of as charging or recharging the target uni-formly to a predefined equilibrium voltage.
Finally it is clear that electrons generated in layer 908 are deposited on the interface surface 912.
Ideally they do not penetrate layer 906, at least to the extent that significant video signal is lost. This is managed by the selection of materials in layers 906 and 40 908 ana their treatment during deposition so as to form a blocking layer of t'ne surface 912 preventing movement of electrons from layer 908 into layer 906. This includes the possibility of the blocking layer at 912 being formed of materials other than those found in layers 906 and 908.
` 5 It constitutes a separate, recognizable layer formed specifically to block the flow of electrons and optimize their recombination with holes generated in layer 906.
Such layers are well known in the art.
The electron density pattern is read out over line 10 924 as the beam 950 scans. This is shown in Figure 9.
-s Specifically, when the beam generates electrons which are - directe-l toward a portion (i.e. pixel) of the interface 912 where no electron-hole combination has occurred, there is no variation detected a-t electrode 904. The voltage at 15 t'ne line 924 does not change. When the generated elec-trons recharge a portion (or pixel) that has lost elec-trons due to recombination, a surye of current--depending on the level of recombination--is detected at line 924.
Further examination of the sandwich structure 902 20 reveals that the first layer 906 and the second layer 908 together represent the dielectric between two capacitor plates--namely the two electrodes 904 and 910. Interposed - between the two plates (i.e. the two electrodes 904 and 910) is the electronic image layer at the interface 912--25 displaced from the second electrode 910. Between the interface 912 and the second electrode 910 is a voltage drop av. By selecting the second layer 908 of a material that undergoes avalanche breakdown or other such elec-tronic multiplication when bombarded by electrons from a 30 beam, the voltage drop av approaches zero between the second electrode 910 and the interface 912 where breakdown has occurred. The minimum for TV is determined by the voltage at which avalanching or c'narge flow can no longer be sustained.
In accordance with the invention, the electronic breakdown is localized. Specifically, breakdown occurs where the beam 950 causes electrons to be generated at a particular time. Hence, as the beam 950 strikes t'ne second layer 908, avalanche breakdown occurs along a path 40 in the second layer 908 from the second electrode 910 to ~4~

the interface 912 along which electrons are generated and flow to the interface. Such a pat'n represents a sub-stantial short, and the localized resistance can momentarily approach zero.
With a low intensity, high velocity beam 950, successive portions of the interface 912--w~ereat target charge is or is not stored-- can be simultaneously read out and recharged as required.
In essence, the present invention displaces the 10 image forming electron layer from the equivalent of the second electrode 910--where it typically is positioned--to the interface 912 and achieves charging (or recharging) ' not with electrons from a low current beam subject to high beam resistance but rather with a larger flow of current 15 generated from layer 908 electrons that avoid the source of conventional beam resistance.
In that beam resistance combines with sensor ; capacity to determine lag, the reduction in beam rPsis-tance permits the sensor-target capacity to be increased 20 without adversely affecting lag. The increased sensor - capacity may be reflectea in a larger area sensor and/or a a thinner sensor. The larger area sensor permits use of the present invention in a broad range of applications.
The increased thinness permits (a) extended dynamic range 25 where the sensor can support increased charge density and (b) improved spatial resolution.
In either case, stripe width and stripe separation may be employed in further overcoming capacitance.
With the bias polarity shown in Figures 7, 8 and 9, electron flow is always from the target layer toward the sensor layer. Accordingly, the raster-caused, charge storage resides on the inner surface of the sensor at the interface, or in a special blocking layer. When charge is stored on the sensor's surface, the sensor material must have a high resistivity in the order of 1012 ohm-cm that is typical for video operation. In this mode of opera-tion, the sensor rnust verve to detect incident radiation and store charge. Because of space charge limitations, it cannot offer gain.

36 ~2~ l Reversing the polarity of the bias however, changes the situation dramatically. The high velocity beam scan now results in charge depletion since multiplied electrons now flow out of layer 908 through electrode 910.
-5 If the target performs as an SEC layer, for example, elec-tron flow induced by the "high velocity" electron heam flows away from the interface, causing the inner surface of t'ne target to become positively charged. If the sensor is an n type material such a CdS, the absorption of 10 radiation causes electrons to flow toward the interface and discharge the positive surface of the interlace. In this mode of operation, the sensor need not store charge and can have a somewhat reduced resistivity. It now has the opportunity to also provide gain, since space charge 15 limitations need not prevail.
Alternative, flexible schemes exist for applying stripes and their functions. In one possible arrangement, -biases can be applied to the stripes, with the video -signal picked-off the opposite surface electrode. This '20 offers the advantage of individual bias control, which can be important for optimization in obtaining uniform response, as well as unusual applications. Another arrangement has stripe electrodes on each side of t'ne DELST target, so that individual bias and preamplifier 25 stripes are paired to ensure optimum performance. In all arrangements described above, the stripes can be placed on -either the sensor or target surface, being required only to be transmissive to the irradiation at their surfaces.
Referring now to Figure 10, a modification is 30 applied to the D~LST in 602. Specifically, a light emitting X-ray sensor such as a cesium iodide layer 1000 is positioned between input X-radiation and the first electrode 1004 for another embodiment of DELST 1002. The cesium iodide layer 1000 converts X-ray photons into light 35 photons which pass t'nrough the first electrode 1004 to strike the first sensor-target layer 1006. As in the previously describe,1 electron scanner image tubes, the second layer 1008 is a charge multiplication layer that forms an interface region 1012 with the first layer 1006.

2~)3~

An electronic image forms at the interface 1012-- -displaced fron the second electrode 1010--as described in the previous emhodiments.
In a specific example of an electronic scanner -5 image tube according to the invention, the electron beam !has a comparatively low current of one microamp, and the charge multiplicaiton layer has a gain of 200. T'ne target is a conventional material having protions, or pixels, thereof which discharge within 10 7 seconds. With such parameters, the number of electrons n involved in current flow i over a time t can be calculated from the equation:

- i = n e (7) t That is, i n 1o-6 amp 6.7 x 1012 electrons/sec e t 1.6xlO-1 amp/electrons/sec (8) Over the dwell time td of 10-7 sec, the number of electrons imparted per beam diameter is:

Nb = n td = 6.7 x 105 electrons (9) With a gain of 200 in the charge multiplication layer, the number of electrons available to charge (or recharge) the interface is:

Nb = 6.7 X 105 X 200 = 13.4 X 107 = 1.34 X 108 g electrons/beam/diameter (10) It is essential in reducing lag that there be many more readout electrons available then the numher of elec-trons required to replace those electrons which combinewith holes at the interface, i.e. electrons lost in the charge storage surface. An estimate of the charge ratio can be obtained from the beam diameter, the dwell time of the beam, exposure, and gain. By way of example, the specific example is characterized by a beam diameter Ab of .016 mm; an input flux of radiation directed toward the 38 2Q~
., first electrode of 2 x 105 phontons/mm2; a sensor detec-tive quantum efficiency (DQ~) of 50%; and a pixel area of .02 mm2. The number of holes generated per pixel to remove stored electrons is calculated from the expression:

5 DQE X Input Flux X Pixel area = 2 X 103 holes/pixel (11) Thus, for perfect discharge of a pixel subject to maximum radiation exposure, 2 X 103 electrons are required.
one flux 2 X 105 photons/;nm2, it is noted, 10 corresponds to the flux available in a typical low light level condition. Also, this flux corresponds to the X-ray flux available in diagnostic radiology in an exposure of 1 mR of 50-60 KeV photons.
:; The number of electrons per pixel available for - 15 charging Nq is then determined from:
I, .
N = N Ap = 1.34 x 108 x .02 q bg A 2X10-4 1.34 x 101 electrons/pixel (12) The ratio of charging (or recharging) electrons to the nu~nber of holes generated in the above exposure is - 20 then:

1.34 x lolO = 67 x 107 (13) 2 x 103 As noted hereinbefore, when the radiation input is X-radiation rather than light, the scanner image tube includes a light scintillator in the form of a cesium iodide layer. Accordingly, the effect of the cesium iodide layer must be accounted for in the above calcula-tions. The cesium iodide layer is known to generate on the order of 1000 light photons for each absorbed X-ray photon. These light photons are absorbed by the sensor, e.g., the first layer 1006 of Figure 10, whereupon electron-hole pairs are generated. At a quantum effici-; 39 - ency of 100%, 1000 absorbed electron-hole pairs are gene-; rated in response to the 1000 absorbed light protons.
The number of electrons that combine with the holes con-veyed to t'ne interface 1012 through the first layer 1006 5 thus increases by a factor of 103. Thus, for a pixel, the number of electrons t'nat can combine with holes equals 2 X
106. The charge:di~scharge ratio, i.e. the ratio of (a) electrons directed to a pixel from scanning versus (b) electrons lost through the recombination, reduces to 6.7 x 10 103.
The high charge:discharge ratio result in rapid and total readout and recharge. It is further noted that the ratio can be further increased by increasing the charge density of the beam or increasing the gain in the 15 second layer. In this regard, silicon has been reported with gains as high as 3000.
As an alternative in the X-ray application, a suitable high resistivity photoconductive first layer 1006 - that is responsivQ directly to the X-rays may be 20 employed--thus obviating the need for the additional cesium iodide layer 1000.
A consequence of the high charge:recharge ratio is that the leading edge of the scanning beam is able to effectively cause discharge. The full beam diameter i5 : 25 not necessary for recharge, and the effective MTF of the beam's contribution to spatial resolution is improved.

3. Operation with DELST and an Optical Scanning Beam Figure 11 shows a further embodiment of the 30 invention. A laser scanner image DELST 1102 includes the first layer 1106 sandwiched between the first electrode 1104 and the second layer 110~. T'ne second layer 1108 is sandwiched between the first layer 1106 and thy second electrode 1110. Between the first layer 1106 and the 35 second layer 1108 i5 an interface region 1112 whereat an electronic image is formed.
i 3~7 Unlike the electron beam scanner image tubes, a laser beam 1150 scans the second electrode 1110 which is transparent to laser radiation.
The first layer 1106 and the second layer 1108 are 5 each pnotoconductive layers of high resistivity. The first layer 1106 is responsive to incident radiation, e.g., X-radiation, and does not respond to laser radia-tion. Similarly, the second layer 1108 is responsive to the laser input but is insensitive to the incident radia-10 tion. Alternatively, photoconductive layers 1106 and 1108 can be selected and their thicknesses adjusted to absorb ^~ the radiation passing through their adjacent electrodes 1104 and 1110, respectively. Such layers ensure that radiation entering a layer through an electrode blocks any 15 transmission to the opposite layer and thus each layer i exposed to only the radiation intended. Finally, it is also conceivable that a light blocking layer can be inter-posed at surface 612, wile maintaining the prerequisite - electronic properties prescribed or DELST operation.
As in the electron beam scanner image tubes, the interface 1112 represents an electron layer displaced from - the second electrode 1110. Electrons generated in the second layer 608 are blocked at the interface 1112. Elec-trons and holes are generated in the first layer 1106, the 25 electrons moving to the first electrode 1104 with the ; holes combining with electrons at the interface region ; 1112 to define a distribution of charges along the elec-tron layer. The second layer 110~3 may provide gain, if desired. However, the number of electrons generated by 30 the laser scanner 1150 may create a large enough charge:discharge ratio to enable readout and recharge following electron-hole combining at the interface 1112.
The incident radiation may be light, X-rays, or other radiation. In each case, the first layer 1106 is 35 selected to be responsive thereto--such materials being known in the art.
A sample laser scanner image DRLST operates as follows with specified parameters. A lOmw laser beam provides 105/h~ photons/sec. For = 6000 Angstroms, 40 this equals 3X1016 photons/sec. The number of photons available at a pixel with dwell time t of 10 7 sec is:
~
nt = 3 X 1016 X 10 7 = 3 X 109 photons (14) The number of photoelectrons created by photoconduction in the second layer 1008 is given by :
.
ne nt X I, where n is quantum efficiency of the layer.
For n = loo, ne = 3 X 109 electrons.
For n = 10%, ne = 3 x 10 electrons.
O
The number of electrons, ne, is compared to the 10 photon irradiation effects at the input to the imaging sensor. A typical low light level condition is irradia-: -tion with 105 photons/mm2. A pixel of .15mm by .lSmm I- would be exposed to about 103 photons/mm2. assuming 100%
: quantum efficiency for the Eirst layer 1106, the ratio of (a) electrons generated by the laser scanner 1150 to (b) the input generated charge for combining iS:3xlo9 t3X1o6 ; For 10% quantum efficiency the ratio becomes 103 3 x 105. In both cases, we have assumed conservatively that the laser beam and pixel diameters are essentially the same.
As an alternative, the laser scanner imager 1102 may include a cesium iodide layer (see Figure 10) to convert X-radiation into radiation matched to the first layer (if the first layer is not responsive to X radia-tion). In this cue, the CsI light photon gain provides afactor of 1000 which reduces the ratio to 3 x 103 or 3 x 102 for 100% and 10~ efficiency, respectively.
By way of comparison, a conventional low velocity electron beam operating with light and a similar pixel size and dwell time, and a beam current of 2 micoamps provides a charge-discharge ratio of 103. This ratio, it is observed, is quite small compared to present invention embodiments having light radiation input and is, in fact, comparable to the ratio for X-ray input.

3~' ;
, 42 . .
s The charge:discharge ratio may be enhanced as s desired by increasing the laser power, increasing the dwell time, and/or providing gain in the second layer 1108 as in the electron beam embodiments.
The term photoconductive layer, it is noted, is used in a generic sense to include one or more layers of material having structures such as intrinsic, p-n, and/or " p-i-n layers to provide a photoconductive effect with associated properties such as high impedance, good spatial 10 resolution, and good speed of response. It applies also j to heterojunctions from dissimiliar materials.
It is additionally noted that by employing elec-tron beam or laser beam raster scanning, the present invention achieves high spatial resolution and an enhanced 15 MTF. Moreover, the mixed potential electrodes combine with the high charging electron numbers to greatly reduce electron beam shot noise of prior art devices, 5~ nce depleted charges are completely replaced.
~^ Note that i the bias i5 reversed, and the laser 20 scanner is trained on an n type low hole mobility semi-is conducting target, electrons will flow away rom the j' interface through electrode 1110 leaving the target with a residual postive charge. When the charge is stored in the i; target, the sensor need not face space charge limited 25 performance and can provide gain as described earlier for I' the "high velocity" electron beam scanned DELST.
I, 4. Large Area DELST Video Imagers The large area DELST application is typified by diagnostic radiology, and in particular to X-ray imaging 30 of the adult chest or abdomen. The conventional X-ray film size for such applications is 14" x 17," or 238 square inches. This large area provides a formidable problem for imaging with a low or a high velocity image tube. In the low velocity tube, it is necessary to over-35 come large target and distributed capacities. In a con-ventional high velocity tube where bias is in the order of a hundred volts, distributed capacity is as much a prob-lem, and in addition charge redistribution effects must be . I,.

I:
., .

~Lf~ 2~3~

minimized if not eliminated. The structure of DELST is designed to eliminate the effects of lag from target i capacity and low sensitivity from secondary electron redistribution.
The effect of distributed capacity can be minimized when using "super high velocity" type electron beam scanning (kilovolt range), which makes use of display electron-optics. The field and suppressor meshes of low velocity tubes, which are close to the target surface and 10 are principal sources of interelectrode capacity and thus are a source of preamplifier noise, are eliminated. The remaining distributed capacity related to electrodes are associated with electrodes on the inner wall of the tube, and as such is small. Furthermore, its increase with tube - 15 dimensions approaches linearity with increasing sensor-target diameter, rather than its square when proportional to area.
; In the case of laser scanner operation, the electrodes associated with electron optics do not exist 20 and correspondingly that source of distributed capacity ; disappears.
The use of stripe electrodes was implemented as a solution to the problem of distributed capacity in dealing with conventional "low" and "high" velocity tubes. This 25 approach can also be applied to the DELST structure to the extent that capacity remains a problem in any particular application.
Consider the large area of 16" x 16" ~400 X 400, mm2) intended for diagnostic radiology. There are two 30 ways to provide a successful X-ray DELST imager. The first is to generate sufficient signal gain and the second is to minimize any DELST distributed capacity (and its associated preamplifier noise). If, for example, the gain can be increased by a factor of 200 or more, the signal 35 grows by a factor equivalent to the increase of noise due to distributed capacity in going form 5 to 1000 pica-farads. Correspondingly, the signal to the associated preamplifier noise ratio remains the same, which is a - design goal to provide a successful operational device.
40 Decreasing capacity can be managed to some extent by the two layer DELST, which offers some opportunity to control the capacity between electrodes 604 and 610 by selectiny materials for layers 606 and 608 with optimized dielectric constants and layer thicknesses.
,:
5. DELST with Increased Gain There are two principal ways in which to provide increased signal gain. The first is to use a photoconduc-tive 606, as in Figure 6 responsive to the incident photons or as layer 1006 in Figure 10 responsive to the 10 light from the X-ray sensor 1000, to provide substantial photoconductive gain. The second way to provide gain is to use an intensifier placed before the DELST structure.
For example, in the high or low velocity electron beam types of video tubes, forefront intensifiers have been 15 incorporated in the image orthicon, image isocon, SEC and - SIT tubes, with target gains ranging from as little as 2 or 3, to as much as 3000. The use of a DELST structure to replace these targets requires that layer 606 in Figure 6 be responsive to imaging energetic electrons, such as 20 materials used for EBIC and SEC targets. However, fore-front intensifiers can also include Generation I, II and III intensifiers optically or fiber optically coupled to the DELST target, wherein the layer 606 would be photoconductive.
Channel multipliers incorporated into proximity - focussed intensifiers are also applicable when coupled to the input layer 606. Channel multipliers have also been made to be directly responsive to incident radiation and can avoid in some cases the need for a photoemissive 30 photosensor arranged with proximity focussing before the input surface of the multiplier. The output surface would traditionally be placed for proximity focussing of exiting electrons to impinge on layer 606 through the electrode 604, where layer 606 would comprise an EBIC or an SEC
35 material.
For relatively low gain requirements, the channel multiplier can be eliminated and the simplier, less expen-sive one or two stage proximity focussed intensifier can -' 'l.q~ 3~

suffice. A simple one stage device would consist of a photoemitter placed before and in close proximity to elec-trode 604, which would be selected to permit electrons to pass through into ther EBIC or SEC layer 606.
The geometry of proximity focussed intensifier r devices shown in Figure 12 is particularly advantageous ; for application to large area devices, such as required < for diagnostic radiology. There a typical sensor would be - a layer of CsI, on which was deposited a photoemitter such I, 10 as CsSb. The emergent photoelectrons could then impact electrode 1204 directly or through a channel multiplier.
- The total structure Gould comprise a metal cap 1220 I; transparent to incident X-rays 1230, whose inner surface would support the CsI and CsSb layers 1222 and 1224 15 respectively and be proximity focussed through the vacuum space 1226 to the DELST structure as shown in Figure 12 --an electrode 1225 is located between the CsI and CsSb ; layers and is held at a negative potential relative to electrode 1204. With this approach, electrode 1204 would 20 permit photoelectrons to pass through an impact layer 1206, which could be either an EBIC or an SEC layer. The ; raster scanner 1240 could be a source of a high velocity electron beam 1242 which would pass through electrode 1210 and impact layer 1208 functioning as an electron multi-25 plier. The interface 1212 supports the electron layer for image reproduction as described earlier. In this arrange-, ment, the bias is reversed to expedite electron flow through layer 1206 to the interface.
Alternatively, the raster scanner 1240 could be a source of laser radiation 1242, which would pass throughelectrode 1210 and be absorbed by a photoconductor in layer 1208. Again an interface surface 1212 serves to support the electronic imaging layer.
In arrangements with the channel multiplier serving to provide gain of an imaging signal, the bias must be arranged so that the first electrode 1204 is biased negatively with respect to electrode 1210. This ensures that the photo-emitted and multiplying electrons drift toward the interface. Furthermore, holes generated in a semiconducting solid state EBIC layer 1208 created by ,.
,~

the electron beam or laser beam from t'ne scanner, will also drift toward the interface, as required to discharge the imaging electrons in generating a video signal. The mechanism differs with the SEC type of layer, where the 5 interaction at the interface only involves electron flow discharging through the SEC layer.
The channel multiplier when operating with a relatively low gain requirement offers the potential for the very useful design advantage. For example, in appli-10 cations where the input window can be rigid and strong,the channel multiplier need not be self supporting. The input window can be used to support all device layers, and thereby offers more flexibility in acquiring an appro-priate multiplier structure. Multipliers can be designed 15 to function for example, without the difficult require-ments imposed in large area applications for rigidity and sturdiness. Since they would essentially lie against the - sensor which in turn would be supported by the input window, the multipliers need not be self supporting. They 20 could be optimized for spatial resolution and gain. This could be done with far less concern for mechanical properties the multiplier must possess as in proximity focussing or whether it be made of metal, glass or any other suitable substance. Fragility and rigidity simply 25become far less severe requirements with this design.
Examples of input windows that could apply here would be metal discs as used in X-ray imaging or glass discs as used in imaging with light.
The rear surface of the device could also provide 30a source oE structural support. In particular, w'nen using an optical scanner wherein photons are used for the scanning process, the rear surface need only be selected - for optical transmission matched to the spectrum of the scanning photons. Thus in a simple application, the rear 35window could be a disc of glass for transmission of the light beam from a scanning laser. In such a case, it is possible to design the device so that the front and rear windows both provide structural support and in effect permit the construction of a sturdy sandwich-like large 40area video sensor. Between the front and rear windows = would be placed in effect three layers: In succession from the inside surface of the front window to the inside surface o f the rear window, they would comprise the sensor it layer structure, the channel multiplier and the solid , 5 state layer. They would all be contiguous to one another, evacuated for the operation of the photoemitter and the multiplier, and sealed to maintain proper operation of all components. There are substantial advantages to this design such as in sturdiness, the replacement of a mal-10 functioning imager or a scanner without effecting the p other and the potential for reusing components when the imager fails.
A generalized arrangement that utilizes the channel multiplier is shown in Figure 12a. This figure 15 illustrates the channel multiplier arranged to be - separated from the scanned layer 1208' and the photo-emitter 1224' by a vacuum space 1225'. The separation on ? ' either side, if sufficiently wide, permits incorporation of electron optics as used in the Generation I image 20 intensifier, and if sufficiently thin, can function with proximity focussing. Furthermore, the photoemissive layer 1224' -an be deposited on the input side of the channel multiplier. Another option is to place the channel multi-plier in contact with layer 1208'. Clearly the device can 25 function with any of these arrangements, i.e., with or without separation on either side of the channel multi-plier relative to the appropriate layer 1224' or 1208', and thereby offers the opportunity to select the arrange-ment that best fits any one application. For example, in 30 large diameter X-ray imaging, the opportunity to support the channel multiplier against another surface reduces the need to make the channel plate rigid and inflexible.
The options available for layers 1206 and 1208 are desirable for maximum versatility, i.e., either could be a 35 photoconductor or a charge multiplying layer. Thus the designer, given the nature of the incident radiation, the cross-sectional area of the device and its imaging requirements can seek to optimize system performance by judicious choice of all components in Figure 12.

Gain achieved through the use of ,ohotoconductive gain in layer 1006 of Figure 10 is the simplest and most desirable method and shoula be used whenever possible. It ; reduces the number of components required for the device 5 and thereby its size and cost.
With reference to X-ray imaging, layer 1006 in Figure 10 can be managed with a material such as ZnCdS or even CdS, which have besn developed with high resistivi-ties. A specific example of a CdS sputtered film 10 developed by Bell Laboratories, was reported to have in a specific application, a gain of 750, a photoconductive rise and decay time of 150 and 50 sec, respectively, with dark resistivities in the range of 101-1011 ohm cm, and a ratio of dark to light resistivity of 5 X 104. These 15 characteristics are important for DELST operation, since even though reamining distributed capacity is reduced, any residual capacity induced preamplifier noise can be over-come by this magnitude of photoconductive gain. For example, if the noise is increased by a factor of 100-200 20 and a gain of 750 were incorporated in the DELST photo-conductor, the resultant performance would not suffer from any distributed capacity induced preamplifier noise. For the case of a device using display electron-optics with a "super high velocity" scanning beam, the distributed capa-25 city is expected to grow, at worst, approximately linearlywith diameter. Accordingly, an increase in diameter should see a 20-fold increase in noise associated with going from a 1 inch to a 20 inch tube. This could well be in the order of "state of the art" preamplifier noise, so 30 that only a small gain, if any, need be included. Never-theless, for unforseen circumstances, the order of gain available provides solutions to any distributed capacity problems that might arise.
In practice, gains as high as one million were 35achieved at the Bell Laboratories with CdS for resistivi-ties in the range of 107-108 ohm cm. As stated before, in the mode of operation for DELST where charge-storage resides on the target, it is possible for the sensor to have lower than customary resistivities. Since gain 40improve~ with reduced resistivities, this mode of DELST

operation becomes particularly attractive for possible low light level applications, and for reduced lighting requirements in conventional TV applications.
Other photoconductors and other gains are pos-5 sible, so that the characteristics for layer 606 can betailored to match the multiplier or pnotoconductor in layer 608.
Implications of a high-gain, sensor-target extend to a very favorable high level of signal current. Com-10 pared to a vidicon tube operating with hundreds ofnanoamperes, the DELST offers signal current in principal up to hundreds of micoramperres and with more gain to milliamperes. The result is that the preamplifiers attached to stripe electrodes become much less sophisti-15 cated and far less expensive. Furthermore, it permitsreduction in stripe numbers through the use of wider ; stripes and/or multiplexing. Finally, since there are no secondary electron redistribution effects, it makes the use of simultaneous multiple fan beam exposures easily 20 possible for diagnostic radiology, permits more scan lines per stripe as well as wider stripes. Using multiple fans results in reduction of the total time required to expose the full surface of the object and sensor-target and minimization of X-ray scatter. The number of fan beams is 25 thus determined by the sæacing required between fans to ensure that scanner from one fan does not fall on the sensor-target exposed by an adjacent fan; and also by the length of the sensor-target being scanned. Thus, for a length L for the sensor-target and a spacing S, the number 30 of fans become L/S.
A specific example illustrates the performance possible from a DELST designed for diagnostic radiology.
Assume a worst case with the following conditions where the distributed capacity is taken equal to a stripe 35 capacity:

Sensor-Target Layer 1006 Q E s = Quantum Efficiency = 100%
Gl Gain (Photoconductive) = 750 33~

E = Exposure 1 mR with Average Photon Energy Equal 60 keV = 3 x 105 photons/mm2 Gain in layer 1000 of Figure 10 CCsI) = 103 ' 5 Photoconductive Layer 608 in Figure 6 -Q.E.p = Quantum Efficiency = 100~
G3 = Gain - Unity L = Laser Scanner Beam Power - 10 mW
T = Dwell Time/Pixel _ 107 sec.
10 NpET Photoelectrons/pixel . within a dwell time T = 3 x 10 (see Equation 13) DELST

A = Surface Area = 400 x 400 mm2 " x 16"
15 a = Stripe Area = 4 x 400 mm2 Cs = Stripe Capacity = 3000 picofarads Q = Thickness between = microns Electrodes ; p = Average Resi.stivity = 1011 ohm-cm between Electrodes VB = Bias Voltage = 35 Volts Pr = Pixel Resolution = 0.1 x O.lmm NRL = Number of Raster Lines = 2000 TVL
LD = Raster Line Desity = 5 lines/mm 25 LS = No. of Raster Lines = 20 lines per Stripe Width With the above conditions, DELST performance can be predicted as follows:

1. Video Signal (Sv) -Iv = ExQEsxGlxG2xpr/T
s = 3 x 105x1750xlOOOx(O.lxO.1) / 10-7 = 3 x 7.5 x 1o5+2+3-2+7 = 2.25 x 10l6 electrons/sec = 2.25 x 10l6 x 1.6 x lO-l9 = 3.6 ma this current is unusually high and indicates the extent to which gain selection is flexible.

2. Noise in Signal (RMS) us ~ExQEsxpr GlG2/1 = 55 x 750 x 1000 x /10-7 electrons/sec = 5-5 x 7.5 x 1ol+2+3+7 = 4.125 x 1014 electrons/sec = 66 x 10-6 amps = 66~a 3. The Preamplifier Noise IN
associated with distributed capacity can be calculated from Equation 1 and found to be IN = 158.4 na d.cap.
This noise is much less than the X-ray noise and thus permits use of even larger DELST
capacity, which offers the opportunity to use wider and fewer stripes. The ratio of Ins/INd 400, and suggests for the selected operating conditions that the stripes might well be eliminated.

4. Power dissipated from bias Pt Vt2 (35)2 Pt = pQ 10 xlO /40x40 = ~3.5)2Xlo2~11+3xl.6x103 1 96Xlo2-ll+3+3+l = 1.g6xlo-2 Watt = .02 Watts 5. D~LST Exposure and Read-Out Time Conditions a. Use a single translating fan beam with thickness equal to a strip width.
b. Electrode stripes = 100 c. Stripe width = 4mm d. Raster Lines per stripe width = 20 e. Number of raster lines = 2000 f. Raster area = 400 x 400 mm2 g. Pixels per line (digital) = 400 s 10 h. Dwell time/pixel - 10 7 sec.
i. Readout time/line - 4x10 5 sec.
j. Exposure time/stripe = 3 msec.

A. TIME T to "Expose and Readout"
a stripe TStripe = 3x10 3 20x4x10 5 = 3xl0 3 + 8x10-4 = 3.8 msec B. TraSter to Expose and Readout"
entire area of 400x400 mm2 Traster = lXTstripe = 0.38 secs.
*Note that flyback time is not included in this calculation.

Note that time to readout a raster can be reduced further my multiple fan beams, with and without parallel, 25 simultaneous readout into designated memory. Further, sufficient design flexibility exists for a slower scan readout. This latter readout reduces noise rapidly when associated with distributed capacity since it is propor-tional to the bandwidth raised to the 1.5 power, 30 i.e-, (~f)3/2 ~2~

t C. Time to "Expose and Readout" Using Multiple Fan Beams and Simultaneous Read-Out of Exposed Stripes -It is readily possible to use two or more X-ray 5 fan beams for simultaneous exposure of a corresponding number of stripes. For example, two fan beams spaced 200 mm apart at the sensor surface, in simultaneous operation can reduce the time to expose the surface by one-half.
Furthermore, if each exposed stripe can be readout in parallel immediately after exposure, the readout time is - 10 also reduced by one-half. Since each stripe can have its own preamplifier or multiplexed to a pair of preampli-; fiers, simultaneous readout is easy to accomplish. Sig-- nal output from each preamplifier can then be fed to a line in digital memory, wherein the memory is designed to 15 accept one or more lines in parallel and simultaneously.
With multiple fan beams and simultaneous readout of exposed stripes, the time to expose and readout a , raster is given by 0.38 seconds divided by the number of fan beams (using the above conditions).

For two fan beams, TR.2 = .19 secs ,- For four fan beams, TR.4 = .095 secs I, Correspondingly, the imaging rate is the reciprocal of the above, and with four fan beams, approaches 10 images per second.
Most of diagnostic radiology is carried out below 7 7.5 frames/sec. Only for studies of the heart and coronary arteries is there a need for rear time imaging.
I'hus with only four fan beams, one exceeds the requirement in speed for at least 90% of diagnostic radiology. In 30addition, because of fan beam projection and the operating sequence, one can eliminate X-ray scattering radiation from outside the fan beam. This involves erasure of previously recorded scattered X-rays and accomplished by scanning the lines in each stripe before its exposure to a 35~an beam. Since such a scan involves 0.8X10 3 sec per stripe, the additional time required for 100 stripes equal .08 secs.
Thus, with scatter rejection, the exposure and readout times required for a full image are One Fan Beam = .46 secs.
Two Fan Beams = .23 secs.
Four Fan Beams = .125 secs.

Clearly with four fan heams, the imaging, raster rate can i approach 8 frames/second and include rejection of 1 10 scattered X-rays.

; 6. DELST With seduced Capacity The layers 606 and 608 in Figure 6 can be thought of as two capacitors in series. Thus the capacity between electrode 604 and 610 is less than t'nat of the sensor-target, the extent depending upon the thicXness and dielectric constant of layer 608. Reduced capacity means reduced noise associated with the preamplifier. The extent to which this is possible depends upon the extent of increase in bias 622 required to maintain the proper 20 internal electric fields in layers 606 and 608; the ability to maintain a well defined electron beam of small diameter in 608 to hold spatial resolution, maintaining ! sufficient charge storage for the desired dynamic range and maintaining prerequisite performance as an electron multiplier. For example, ZnCdS and/or CdS can serve asthe photoconductive layer 606, and have dielectric con-stants that are in the range of 8.37-9.4. The multiplying layer 608 used with an electron beam might comprise KCl with a dielectric constant of 4.64. The combination with thicknesses adjusted for p'notoconductive bias and multi-plier requirements, offer the opportunity to reduce the net capacity, while holding the density of charge in the interface surface 612 to desired levels required for imaging. In the case of the DELST used with a laser beam scanner, an example of materials selection could be a ZnCdS or CdS sensor-target of 1-2 microns thick used for layer 606, to be used in conjunction with a porous anti-mony trisulfide film as thick as ten microns or more in the layer 608. An advantage to using KCl as an electron multiplying layer or Sb2S3 as the photoconductor respon-5 sive to the laser radiation is that they are relatively - insensitive to X-rays. Accordingly, they would not be activated by incident X-rays 630 as would be used in diagnostic radiology. On the other hand, ZnCdS and CdS
have been used as X-ray sensors in the past. Accordingly 10 any X-rays penetrating the X-ray sensor layer of CsI 1000 in Figure 10, and absorbed in the sensor-target layer ; would only have a favorable effect in total photoresponse.
Since the layer 606 here, however, is only a few microns thick, X-ray absorption will be small and any effect on 15 overall device performance will be small.
Unusually high target charge storage requirements can be met with the ASOS photoconductors used by O.
! Schade, which were capable of 400 times the capacity used in conventional vidicon type sensor-targets.
The effect of capacity reduction by adjusting the individual capacities of layers 606 and 608, and using electrode stripes combine to reduce any potential capa-city problem. Nevertheless, it is conceivable for some applications that reduced photoconductive gain in layer 25 606 and reduced signal current are desirable. This could result in the requirement for substantially reduced capacity beyond that possible by adjusting the dielectric properties of layers 606 and 608, or by further reducing stripe widths. An alternative way to achieve capacity 30 reduction is by insertion of an intensifier such as the channel multiplier already described as a means to achieve gain in lieu of photoconductive gain.
Reference to Figures 13 and 14 reveal two alterna-tive approaches, both using channel multipliers. In Figure 35 13, the signal electrode is 1304 and the ground electrode is 1310 as described heretofore. Layer 1306 is a photo-conductor responsive to the incident radiation 1330 pas-sing through 1304. However, layer 1308 is now a channel multiplier placed between layer 1306 and electrode 1310.
40 A raster scanner 1340 sends a high velocity beam of elec-~6 trons through electrode 1310 and with proper bias voltageapplied to 1304 (not shown) causes a displaced charge to be deposited at the interface 1312. On imaging exposure, the scanner and channel multiplier replace the lost charge and in the process generate a video signal which is picked up by the preamplifier 1322 through the coupling capacitor 1320. The channel plate can now be millimeters thick versus microns thick for designs described earlier, and correspondingly cause a drop in DELST capacity ranging from a few hundreds to a thousand or more. The multiplier need not operate with dramatically high gain, and the range of 1,000 to 10,000 should prove adequate for most cases.
Figure 14 shows the same basic scheme making use of the channel multiplier, but incorporating a laser raster scanner 1440 and a scanning laser beam 1442 passing through electrode 1414 to be absorbed by a photoemitter 1410 deposited on the channel multiplier 1408. Photo-electrons emitted from 1410 enter the channels of 1408, are multiplied and deposited on the interface 1412. On exposure by imaging radiation 1430, the video signal generated by scanning is picked up by preamplifier 1422 through the coupling capacitor 1420. The photocondutor 1406 is made of a material responsive to the spectrum of the incident radiation 1430.
The option to use the channel multiplier on the imaging side ox the interface 1312 and 1412 for increased electronic image gain has been described in section 5.
When used there, the capacity is reduced in the same manner as described above. It can be reduced further by adding the vacuum separation 1226 with proximity focussing for example.

DELST WITH CONTROLLABLE GAIN

In the various embodiments of the DRLST structure described heretofore, stress was placed on the potential need for gain particularly for large area devices such as X-ray devices and low light level applications. This gain was a requirement on the input, imaging side of the DELST

to overcome the large distributed capacity induced elec-tronic noise in the video preamplifier. Obtaining gain was described for example using photoconductor in one version of DELST, and a channel multiplier in another. It should be noted that the magnitude of the gain is depen-dent, in either case, on a voltage drop across the layer, i.e., the photoconductor or the channel multiplier depending upon which is being used. Accordingly, gain can be controlled by the magnitude of the potential difference applied across the DELST electrodes.
Applications can exist where different gains are required to overcome different imaging conditions. For example, diagnostic radiology requires that for certain procedures, fluoroscopy be carried out prior to obtaining a diagnostic quality radiograph. The ormer is managed with a low X-ray exposure to minimize dose to the patient.
It serves the dual purposes of providing the radiologist wtih a preliminary viewing of the body anatomy and the positioning of the X-ray imaging apparatus. A radiograph is then obtained with a much larger exposure, which is essential to obtaining a diagnostic quality image.
Accordingly, fluoroscopy requires a much higher level of gain sufficient to ensure that the resultant image is of sufficient quality to meet its purposes. The diagnostic exposure, on the other hand, being much larger requires less gain in DELST to provide a suitable radiograph.
The design of the DELST structure as shown in Figure lO, leads to a selection of resistivities for layers 1006 and 1008. This is to ensure the appropriate voltage drop across 1006 during exposure and 1008 during readout of the electronic image of the interface 1012.
In effect, the potential difference applied across elec-trodes 1004 and 1010 combined with the resistance of layers 1006 and 1008, plus that associated with the inter-face layer 1012, determines the voltage drop across each of the layers and the interface. Accordingly, the poten-tial differences across the electrodes 1004 and 1010 are selected to provide the necessary gains in 1006 during fluoroscopy and diagnostic imaging, and a third potential difference for the appropriate voltage drop across 1008 ~2~33~

during raster scanning for readout. For example, the layer 1008 require a much larger voltage drop to provide an optimum scan readout, than would exist during exposure conditions set-up to optimize gain in 1006. Thus, after exposure and the formation of the electronic image at 1012, the electrodes 1004 and 1010 potential differencP
could be switched to a higher value to accommodate the requirements imposed by layer 1008 for optimum readout.
A substantial benefit in using the channel multi-plier to substantially reduce capacity is in theopportunity to use fewer and wider stripe electrodes.
This leads to more scan lines per stripe, fewer preampli-fiers and the opportunity to readout and erase scattered events before readout within one stripe width. These ! 15 factors lead to overall improved imaging frame rates and operational simplicity. Even real time imaging can be designed into a DELST system given all the options avail-able to incorporate into a system.
In the structure described heretofore, biases have been arranged so that the first electrode is either posi-tive or negative with respect to the second electrode taken as ground. This results in a potential difference across the DELST structure such that it causes charge carriers to move in a preferred direction compatible with a specific DELST structure. It is clear that this description is presented for simplicity; that, in fact, the grounded electrode could be the first electrode and that the second electrode could be at a postive or nega-tive potential relative to ground; that, in fact, neither electrode need be grounded to support a preferred poten-tial difference, and that any bias arrangement need only be accompanied by proper electronic circuitry to transmit the video signal to preamplifier.
Other improvements, modifications and embodiments will become apparent to one of ordinary skill in the art upon review of this disclosure. Such improvements, modi-fications, and embodiments are considered to be within the scope of this invention as defined by the following claims.

Claims (122)

I CLAIM:

1. A scanner image tube comprising:
a first electrode and a line connected thereto for carrying a video signal from said first electrode;
a second electrode and a line connected thereto to establish a potential relative to said first electrode such as to cause charge carries to move in a predetermined direction;
a first layer;
a second layer; and means for providing a beam of irradiation which raster scans said second electrode at a distance therefrom;
wherein said first layer is sandwiched between said first electrode and said second layer; and wherein said second layer is sandwiched between said second electrode and said first layer; and wherein said first electrode comprises a first material which passes therethrough and into said first layer a pattern of image defining irradiation in a given spectral band: and wherein said second electrode comprises a second material which passes therethrough and into said second layer irradiation from said beam; and wherein said first layer and said second layer lie against each other at an interface region; and wherein said second layer comprises means for transporting and generating electrons to irradiation from said beam passing through said second electrode and striking said second layer, said generated charge carriers travelling toward the interface region whereat the carriers are blocked so as to become a uniformly charged surface layer; and wherein said first layer comprises means for generating charge carriers of opposite sign to the carriers generated in the second layer, in a pattern corresponding to the pattern of image-defining irradia-tion that passes through said first electrode and strikes said first layer, the pattern of these carrier travelling toward the interface to combine with stored carriers thereat to form an electronic charge image at the inter-face region; and wherein said charge carrier transporting and generating means provides sufficient carriers in response to the scanning of said second layer by said beam to recharge the interface and with charge balance maintained through the line connected to the second electrode, the video signal on said lines connected to either the first or second electrodes varying in amplitude over time the magnitude of recharging required for the portion.
of the interface subject to the scan.

2. A scanner image tube according to claim 1, wherein said first layer can be selected to provide a positive charge flow (holes) or a negative charge flow (electrons) to deplete carriers stored at the interface and to form a charge image.

3. A scanner image tube according to claim 1, wherein said second layer can be selected to provide a positive charge carrier flow (holes) or a negative charge flow (electrons) to store charge at the interface.

4. A scanner image tube according to claim 3, wherein the interface surface can store either a positive charge or a negative charge dependant upon the sign of the transported charge carrier and the direction of carrier flow, determined by said second layer selection.

5. A scanner image tube according to claim 1, wherein said second electrode is biased through a resistor via a connection to a selected source of potential, including ground.

6. A scanner image tube according to claim 1 wherein said beam providing means generates a high velo-city electron beam; and wherein said electron generating means comprises a charge multiplication layer offering gain of unity or more, such that one or more electrons are generated for each electron from said electron beam that strikes said second layer.

7. A scanner image tube according to claim 6 wherein said beam providing means generates an electron beam of at least sufficient energy to cause charge multiplication, determined by the magnitude of gain required.

8. A scanner image tube according to claim 6 wherein said charge multiplication layer comprises an electron bombardment induced conductivity (EBIC) layer;
and wherein said electron beam has sufficient energy to cause the magnitude of charge multiplication required in said EBIC layer.

9. A scanner image tube according to claim 6 wherein said charge multiplication comprises a material that multiplies charge by a gain factor sufficiently large to charge and recharge the interface layer.

10. A scanner image tube according to claim 6 wherein said charge multiplication layer comprises a channel multiplier layer.

11. A scanner image tube according to claim 6 wherein said charge multiplication layer comprises a secondary electron conductivity (SEC) material.

12. A scanner image tube according to claim 6 wherein said charge multiplication layer comprises semi-conducting silicon.

13. A scanner image tube according to claim 1, wherein said second electrode is subject to a floating bias;
wherein the floating bias is established through the joint action of low velocity flood gun electrons, photoconductive current flow through the first layer induced with bias lighting, bias voltage on the first electrode and the super high velocity scanning beam;
wherein the bias lighting can be transmitted to the photoconductor through the first transparent electrode or through the second electrode and the second layer when in combination they are made either translu-cent or transparent.

14. A scanner image tube according to claim 1, wherein said second conducting surface electrode is removed and a floating potential established on the exposed second surface layer;
wherein the floating potential is established through the joint action of low velocity flood gun electrons, photoconductive current flow through the first layer induced with bias lighting, bias voltage on the first electrode and the super high velocity scanning beam;
wherein the bias lighting can be transmitted to the photoconductor through the first transparent electrode or through the second layer when it is made either translucent or transparent.

15. A scanner image tube according to claim 1 further comprising:
a third layer, said first electrode being sandwiched between said first layer and said third layer;
wherein said third layer converts an incident pattern in one form of radiation into the image-defining pattern of another form of radiation which passes through said first electrode to (a) strike said first layer and (b) generate holes in a pattern corresponding to the incident pattern.

16. A scanner image tube according to claim 15 wherein the incident pattern is in the form of X-radiation; and wherein said third layer comprises an X-ray sensor such as cesium iodide which (a) absorbs the X-radiation and (b) generates light photons to form the pattern of image-defining radiation in said given spectral band.

17. A scanner image tube according to claim 7 or claim 16 wherein said first electrode comprises a plurality of electrode stripes disposed substantially parallel to the direction of electron beam line scan;
said electrode stripes being substantially parallel to one another and placed side by side, said first electrode stripes thereby limiting the effective readout capacity in the scanner image tube.

18. A scanner image tube according to claim 1 wherein said electrode comprises at least one electrode stripe said first electrode stripe thereby limiting the effective readout capacity in the scanner image tube.

19. A scanner image tube according to claim 18 wherein said first electrode is positioned parallel to and displaced from said second electrode; and means for biasing said first electrode relative to said second electrode such that electronic carriers migrate from said one toward the other.

20. A scanner image tube according to claim 19 wherein said first electrode is at a positive voltage relative to said second electrode whereby the scanned charge is stored on said first layer at the interface.

21. A scanner image tube according to claim 19 wherein said first electrode is at a negative voltage relative to said second electrode.

22. A scanner image tube according to claim 18 or wherein said stripe electrode or electrodes can be one raster line thick.

23. A scanner image tube according to claim 18 wherein said stripe signal electrode is a straight line.

24. A scanner image tube according to claim 1 wherein the center of the scan of said beam lies in the simplest example, at approximately a 0° angle relative to the direction of incident radiation.

25. A scanner image tube according to claim 1 or 17 wherein a single scanning beam is employed.

26. A scanner image tube according to claim 1 wherein multiple scanning beams are employed each scanning a different stripe area of said first electrode, and a distinct amplifier associated with each said stripe.

27. A scanner image tube according to claim 1 wherein one of said electrodes comprises a plurality of electrode stripes deposited parallel to the raster lines.

28. A scanner image tube according to claim 27 wherein said first electrode comprises a plurality of parallel stripes.

29. A scanner image tube according to claim 27 wherein said second electrode comprises a plurality of parallel stripes also parallel to the raster lines.

30. A scanner image tube according to claim 27 wherein both said electrodes comprise a plurality of said stripes.

31. A scanner image tube according to claim 17 wherein said each electrode stripe comprises for example a stripe of approximately 24 microns in width, said stripes being separated by 1 micron.

32. A scanner image tube according to claim 31 wherein said first electrode extends over an area of approximately 16 inches by 16 inches, each stripe being 16 inches long.

33. A scanner image tube according to claim 1 wherein said beam comprises a laser beam; and wherein said second layer comprises a photo-conductive layer which generates at least electrons when struck by irradiation from said laser beam.

34, A scanner image tube comprising:
a first electrode;
a second electrode at a negative potential relative to said first electrode;
a solid-state first layer of high resistivity;
a charge multiplication second layer of high resistivity;
means for raster scanning an electron beam of sufficient energy to cause charge multiplication in the second layer;
wherein said first layer is sandwiched between said first electrode and said second layer; and wherein said second layer is sandwiched between said first layer and said second electrode; and wherein said raster scanning means directs high velocity electrons into said second layer through said second electrode, said second layer generating and conveying a greater number of electrons toward the inter-face between said first layer and said second layer than the number of electrons from said scanning means that strike said second layer; and wherein when said first electrode is exposed to a pattern of image-defining radiation which strikes said first layer, said first layer conveys a pattern of holes to the interface region of said first layer and said second layer analogous to the pattern of image-defining radiation.

35. A scanner image tube according to claim 34 wherein said charge multiplication layer comprises means for reducing beam resistance and lag.

36. A scanner image tube according to claim 35 wherein said charge multiplication layer comprises a material subject to avalanche breakdown, where struck by the high velocity electrons, said means providing enhanced reduction of beam resistance and lag reduction in response to avalanche breakdown of said charge multi-plication layer between said second electrode and the interface region.

37. A scanner image tube according to claim 35 wherein the holes conveyed to the interface combine with the electrons conveyed thereto through said second layer;
and wherein a recharge ratio of (a) the number of electrons conveyed to the interface through said charge multiplication layer to (b) the number of electrons which combine with holes at the interface is substantially greater than one.

38. A scanner image tube according to claim 25 wherein said charge multiplication layer when struck by electrons from the scanning means conveys sufficient electrons resulting from a recharge ratio to be broadly in the range of unity to in excess of 10,000.

39. A scanner image tube according to claim 38 further comprising:
a third layer, said first electrode being sandwiched between said first layer and said third layer;
said third layer converting an input image-representing pattern of X-radiation which impinges thereon into a corresponding incident pattern of photons which pass through said first electrode to strike said first layer:
said first layer generating holes which travel to said interface and electrons which travel to said first electrode in response to photons striking said first layer.

40. A scanner image tube comprising:
a first electrode;
a second electrode, said first electrode being at a predetermined potential relative thereto;
a first solid state layer which generates electrons and holes when struck by radiation;
wherein said first layer is sandwiched between said first electrode and said second layer; and wherein said second layer is sandwiched between said first layer and said second electrode, said first layer lying against said second layer to form an inter-face region; and raster scanning means for directing a radiation beam into said second layer through said second electrode, said second layer being struck by the beam electrons and causing current flow in an amount depending on imaging storage requirements, with electrons conveyed toward the interface region for storage thereat and with charge neutrality maintained in the second layer through the second electrode and connecting resistor;
the electrons stored at the interface region forming an electron layer displaced from said second electrode, the electrons of said electron layer being combinable with holes generated in said first layer in response to radiation passing through said first electrode into said first layer;
the combining of holes from said first layer with electrons from said second layer at the interface region forming an electronic image thereat.

41. A scanner image tube according to claim 40 wherein said second layer is subject to local electronic breakdown, where struck by the beam, and wherein the resistance provided by said second layer to electrons being conveyed to a given pixel of the interface reduces with increased electron generation between said second electrode and the given pixel;
the resistance provided by said second layer approaching zero along direct paths between said second electrode and a pixel which have undergone total electronic breakdown.

42. A scanner image tube according to claim 41 wherein (a) the number of electrons conveyed to any pixel at the interface region when said second layer is struck by the beam and (b) the maximum number of electrons combinable with holes at said any pixel at the interface region are in a recharge ratio of at least unity to in excess of 10,000.

43. A scanner image tube according to claim 41 further comprising:
a third layer, said first electrode being sandwiched between said first layer and third layer;
said third layer converting an incident pattern of X-radiation which impinges thereon into a corresponding image-defining pattern of photons which pass through said first electrode to strike said first layer, said first layer generating holes which travel to the interface region and electrons which travel to said first electrode in response to photons striking said first layer.

44. A scanner image tube according to claim 41 wherein the scanner image tube responds to irradiation from an object, which irradiation passes through said first electrode and strikes said first layer; and wherein the scanner image tube further comprises:
intensifier means, interposed between the object and the interface region, for increasing the number of generated holes combinable with electrons at the inter-face for a given level of radiation from the object;
said recharge ratio being at least unity to in excess of 10,000 for each pixel of the interface.

45. A scanner image tube according to claim 41 wherein said first layer is a thin crystalline layer.

46. A scanner image tube according to claim 41 wherein first layer generates electron-hole pairs, the electrons from which drift to said first electrode; and wherein said second layer generates electron-hole pairs, the holes from which drift to said second electrode.

47. A method of producing with a scanner image tube a video signal corresponding to the image of an object, the method comprising the steps of:
sandwiching a first layer between a first electrode and a second layer;
sandwiching the second layer between the first layer and a second electrode;
applying an electric field between the two electrodes:
exposing the first electrode to irradiation limited to a first spectral band corresponding to the image of the object wherein the first electrode is transparent to irradiation in the first spectral band but not in a second spectral band;
scanning the second electrode with an optical beam of irradiation limited to the second spectral band distinct from the first spectral band wherein the second electrode is transparent to irradiation in the second spectral band but not in the first spectral band; and forming an electronic image at the interface region between the first layer and the second layer, said forming step comprising the steps of:

selecting the second layer of a photoconductive material which generates electrons in the second layer when scanned with the beam, the electrons drifting to the interface under the influence of the electric field to promote a uniformly charged electron layer at the inter-face; and selecting the first layer of a material which generates holes therein when irradiation in the first spectral band impinges thereon through the first electrode, the holes drifting to the interface under the influence of the electrical field to combine with electrons at the interface.

48. A method according to claim 47 comprising the further step of:
reading out surges of electron flow as the optical beam is scanned to provide a video signal output.

49. A method according to claim 47 comprising the further step of:
supplying electrons from said second layer to recharge the interface region with electrons, responsive to said scanning of the beam and the generating of electrons in said second layer.

50. A method according to claim 49 comprising the further step of:
draining excess electrons out of the second layer through the line connected to the second electrode.

51. A method according to claim 49 or 50 wherein said supplying step includes the step of supplying electrons to the interface region to promote a fixed uniform charge density and a fixed equilibrium voltage thereat.

52. A method according to claim 47 wherein said selection of material for said second layer includes the step of:
selecting a material that undergoes avalanche breakdown locally where subjected to the scanning beam.

53. A method according to claim 47 wherein selecting the second layer material comprises the step.
of:
selecting a material of high resistivity subject to local avalanche breakdown;
wherein the resistance to the flow of generated electrons along a path through the second layer to the interface, where the path has undergone avalanche break-down, approaches zero; and wherein the high resistivity of the second layer inhibits lateral spread of charge.

54. A scanner image tube comprising:
a vidicon-type tube including:
(a) means for providing a low velocity beam;
(b) a sensor-target having a surface of a given area; and (c) a plurality of stripe signal electrodes transparent to incident radiation arranged side-by-side spanning the area of the sensor-target surface, said stripe signal electrodes being substantially aligned with the direction of raster line scan, the width of the stripe signal electrodes being at least equal to the width of a raster line, said stripe signal electrodes having narrow widths which represent low distributed and target capacitance relative to single element electrodes;
a plurality of preamplifiers, each preamplifier being connected to a respective one of said stripe signal electrodes; and a plurality of storage elements, each storage element being connected to receive input from a respective one of said preamplifiers and each storage element including means for separately storing in memory inputs corresponding to each raster line scanned along a given stripe signal electrode.

55. A scanner image tube according to claim 54 further comprising:
analog signal multiplexing to reduce the number of preamplifiers necessary.

56. A scanner image tube according to claim 54 further comprising:
means for reading out simultaneously in parallel the stored inputs corresponding to all stripe signal electrodes when the entire target-sensor surface is exposed to beam radiation at one time.

57. A scanner image tube according to claim 54 wherein said incident radiation comprises at least one fan beam oriented to project radiation parallel to the electrode stripes.

58. A scanner image tube according to claim 57 wherein at least two fan beams scan simultaneously, reducing the time required to scan a raster.

59. A scanner image tube according to claim 57 further comprising:
means for translating said fan beam from one stripe signal electrode to another; and means for serially reading out the stored inputs for successive stripe signal electrodes as the fan beam is translated.

60. A scanner image tube according to claim 54 wherein the stripe signal electrodes are of equal width.

61. A scanner image tube according to claim 57 further comprising:
means for erasing residual imagery and scatter from prior exposures, said erasing means including means for scanning raster lines before fan beam exposure.

62. A scanner image tube according to claim 54 or 57 or 61 further comprising:
means for selectively bias switching each of said stripe signal electrodes, thereby limiting sensitivity to stripes exposed to fan beam readiation while all others are not responsive and do not record any direct or scattered radiation.

63. A scanner image tube comprising:
a displaced electron layer sensor-target (DELST) which includes stripe signal electrodes, transparent to incident radiation, said stripe signal electrodes being arranged side by side to span the sensor-target surface area, said stripes being sufficiently wide to be scanned by at least one raster line whose length is parallel to said stripe signal electrodes, each stripe width being defined so that the area of each stripe signal electrode is sufficiently small to minimize excessive operational capacity;
a high velocity beam for scanning said DELST, said high velocity beam and said DELST cooperating to avoid large beam impedance associated with low velocity beam, and charge redistribution effects associated with high velocity vidicon type tubes.

64. A scanner image tube according to claim 63 wherein the DELST includes a photoconductive layer for the sensor-target which is responsive to incident radia-tion said photoconductive layer providing substantial photoconductive gain so as to boost the signal layer to as much as hundreds of micoramperes.

65. A scanner image tube according to claim 64 further comprising:
forefront intensifier means for adding gain to the signal level.

66. A scanner image tube according to claim 63 further comprising:

electron multiplier means for producing high values of beam current within the layer while using a relatively small current scanner beam outside the layer.

67. A scanner image tube according to claim 63 further comprising:
a plurality of preamplifiers, each preamplifier connected to receive a video signal from a respective one of said stripe signal electrodes.

68. A scanner image tube according to claim 67 further comprising:
a plurality of storage elements, each storage element being connected to receive input from a respective one of said preamplifiers and each storage element including means for separately storing, in memory, inputs corresponding to each raster line scanned along a given stripe signal electrode.

69. A scanner image tube according to claim 63 further comprising:
means for reading out simultaneously in parallel the stored inputs corresponding to all stripe signal electrodes when the entire target-sensor surface is exposed to beam radiation at one time.

70. A scanner image tube according to claim 63 further comprising:
at least one fan beam;
means for translating said at least one fan beam from one stripe signal electrode to another; and means for serially reading out the stored inputs for successive stripe signal electrodes as the fan beam is translated.

71. A scanner image tube according to claim 63 wherein said displaced electron layer stores a formed , image thereon from which a video signal is generated during scanning by said beam.

72. A scanner image tube according to claim 63 further comprising:
means for erasing residual imagery and scatter from prior exposures, said erasing means including means for scanning raster lines before fan beam exposure.

73. A scanner image tube according to claim ,2 further comprising:
means for bias switching so that only the DELST
stripe exposed directly by the fan beam is responsive to the incident radiation.

74. A scanner image tube comprising:
a displaced electron layer sensor-target (DELST) which includes at least one stripe signal electrode, transparent to incident radiation, said s ripe signal electrodes being arranged side by side to span the sensor-target surface area, said stripes being suffi-ciently wide to be scanned by at least one raster line whose length is parallel to said stripe signal electrodes, each stripe width being defined so that the area of each stripe signal electrode is sufficiently small to minimize excessive operational capacity;
an optical beam for scanning said DELST, said optical beam and said DELST cooperating to avoid large beam impedance associated with low velocity beam and charge redistribution effects associated with high velocity beam vidicon type tubes.

75. A scanner image tube according to claim 74 wherein said optical beam is a laser beam raster scanner.

76. A scanner image tube according to claim 74 further comprising:
a plurality of preamplifiers, each preamplifier being connected to receive a video signal from a respective one of said stripe signal electrodes.

77. A scanner image tube according to claim 76 further comprising:
a plurality of storage elements, each storage element being connected to receive input from a respec-tive one of said preamplifiers and each storage element including means for separately storing in memory inputs corresponding to each reaster line scanned along a given stripe signal electrode.

78. A scanner image tube according to claim 74 further comprising:
means for reading out simultaneously in parallel the stored inputs corresponding to all stripe signal electrodes when the entire target-sensor surface is exposed to beam radiation at the same time.

79. A scanner image tube according to claim 74 further comprising:
at least one fan beam, means for translating said at least one fan beam from one stripe signal electrode to another; and means for serially reading out the stored inputs for successive stripe signal electrodes responsive to the fan beam being translated.

80. A scanner image tube according to claim 74 further comprising:
means for erasing residual imagery and scatter from prior exposures, said erasing means including means for scanning raster lines before fan beam exposure.

81. A scanner image tube according to claim 80 further comprising:
means for bias switching so that only the DELST
stripe exposed directly by the fan beam is responsive to the fan beams radiation.

82. A scanner image tube according to claim 74 said DELST further comprising:

a photoconductive layer which transforms the optical beam into a high value electron current beam within said photoconductive layer;
the scanning by said optical beam resulting in the generation of a video signal corresponding to the charges stored at the displaced electron layer which represent a formed image.

83. A scanner image tube comprising:
a video-type tube including:
(a) means for providing a high velocity beam;
(b) a sensor-target having a surface of a given area; and (c) a plurality of stripe signal electrodes transparent to incident radiation arranged side-by-side spanning the area of the sensor-target surface, said stripe signal electrodes being substantially aligned with the direction of raster line scan, the width of the stripe signal electrodes being at least equal to the width of a raster line, said stripe signal electrodes having narrow widths which represent low distributed and target capacitance relative to single element electrodes;
a plurality of preamplifiers, each preamplifier being connected to a respective one of said stripe signal electrodes; and a plurality of storage elements, each storage element being connected to receive input from a respec-tive one of said preamplifiers and each storage element including means for separately storing in memory inputs corresponding to each raster line scanned along a given stripe signal electrode.

84. A scanner image tube according to claim 80 or 83 further comprising:
means for reducing the potential of a stripe being scanned relative to adjacent stripes whereby some secondary charge carriers are diverted to said adjacent stripes.

85. A scanner image tube according to claim 84 wherein said tube has a collector grid, said collector grid being located and having a potential relative to said scanned stripe and that some of said secondary charge carriers are diverted to said grid.

86. A scanner image tube according to claim 83 further comprising:
at least one fan beam;
means for translating said fan beam from one stripe signal electrode to another; and means for serially reading out the stored inputs for successive stripe signal electrodes as the fan beam is translated.

87. A scanner image tube according to claim 85 further comprising:
means for scanning raster lines before fan beam exposure.

88. A scanner image tube according to claim 87 further comprising:
means for making adjacent stripe sensors insensitive through removal of the potential difference across a sensor when both its electrodes comprise stripes.

89. A scanner imager comprising:
a first electrode;
a second electrode, said first electrode being at a specified potential relative to;
a third photoemissive-sensor layer for generating electrons when struck by radiation;
a first layer comprising a channel multiplier having an input surface in contact with the third photoemissive-sensor layer, said third layer being adjacent to and in electri-cal contact with the first electrode, said third layer being sandwiched between said fist layer and said first electrode, said first layer being sandwiched between said third layer and said second layer, said second layer being sandwiched between said first layer and said second electrode, with said first layer lying against said second layer to form an inter-face region;
said raster scanning means directing the radiation beam into said second layer through said second electrode, said second layer being struck by the beam generating charge carriers which migrate to the interface whereby a positive charge is formed at the interface and is displaced from the second electrode, and in position to be discharged by electrons, generated in said third and first layers respectively, in response to imaging radiation passing through the first electrode into said third layer, and causing the formation of an electronic image at the interface, said raster scanning means generating a video signal on recharging the interface.

90. The image scanner according to claim 1 further comprising a channel multiplier located between one of said layers and one of said electrodes.

91. The image scanner according to claim 1 or claim go further comprising means for varying the gain of said image scanner by selectively varying the potential across said electrodes.

92. A scanner image tube comprising:
a DELST structure, a first electrode transparent to imaging radiation and a charge multiplying second layer devoid of any second transparent conducting electrode, a super high velocity beam for scanning said DELST;
a flood gun to provide a flow of electrons for charging the exposed surface of the second layer and to establish a floating potential;
a bias light positioned in front to illuminate the photoconductive first layer through the first electrode or in the rear to illuminate the first layer through a transparent or translucent multiplying layer;
a means for the scanning beam, bias light and flood gun to operate in combination to establish the storage charge at the interface surface and an equilibrium potential difference between the first and second layers;
a means for minimizing noise inducing capacity to the preamplifier, wherein the charge stored on any pixel of the second layer surface is available for signal generation during the high velocity beam scan;
a means for sequencing the process of charge storage, exposure and video signal generation.

93. A scanner image tube according to claim 92 operating in a pulsed mode, wherein the exposure, bias light, flood gun and scanner are pulsed to operate in any sequence needed for a particular imaging requirement.

94. A scanner image tube according to claim 92 operating in a continuous mode of operation, wherein the flood beam, bias light, scanning beam and exposure are in operation simultaneously and continuously.

95. A scanner image tube according to claim 92 structured with the first electrode divided into stripes to further minimize capacity when applications warrent.

96. A scanner image tube comprising:
a DELST structure, a first electrode transparent to imaging radiation and a charge multiplying second layer with a second transparent conducting electrode, a high velocity beam for scanning said DELST;
a flood gun to provide a flow of electrons for charging the second electrode and to establish a floating potential;
a bias light positioned in front to illuminate the photoconductive first layer with light passing through the transparent first electrode or positioned in the rear to illuminate the photoconductive first layer with light passing through a transparent or translucent second electrode and multiplying layer;
a means for the scanning beam, bias light and the flood gun to operate in combination to establish the storage charge at the interface surface;
a means for the scanning beam, bias light and the flood gun to operate in combination to establish an equilibrium potential difference between the first and second electrodes;
a means for reducing noise inducing capacity to the preamplifier while permitting charge stored on the second electrode to be shared by pixels during scanning for signal generation.

97. A DELST image tube according to claim 96 operating in a pulsed mode wherein the scanner, bias light and flood gun can operate in any sequence as required by the imaging procedure.

98. A DELST image tube according to claim 96 operating in a continuous mode for real time imaging wherein the scanner beam, bias light and flood gun are in operation simultaneously and continuously.

99. A scanner image tube according to claim 96 structured with the first electrode divided into stripes to further minimize capacity when applications warent.

100. A scanner image tube according to claim 96, where the conducting electrode is extended outside the scanned raster area for non-iteractive flood beam and scanner beam operation, where the flood beam electrons are trained on the conducting electrode without intruding into sphere of electron optics governing the performance of the scanning electron beam.

101. A scanner image tube comprising:
a DELST structure, a first electrode transparent to imaging radiation, a photoconductive first layer responsive to imaging radiation, a photocondutive second layer devoid of any second transparent electrode and responsive to a laser beam for scanning said DELST;
a flood gun to provide a flow of electrons for charging the exposed surface of the second layer;
a bias light positioned to illuminate the first layer through the transparent first electrode or positioned to illuminate the first layer with selected radiation able to pass through the photoconductive second layer and absorbed by the first layer;
a means for scanning beam, bias light and flood beam to operate in combination to establish the storage charge at the interface surface and the equilibrium potential difference between the first electrode and the exposed surface of the second layer;
a means for minimizing noise inducing capacity to the preamplifier.

102. A DELST vacuum tube according to claim 101 where the tube is designed to include the flood gun and to contain windows transparent to the laser and bias light radiation, and positioned to place the flood gun so as not to interfere with the optical path of the laser and flood beam radiation coming from outside the vacuum tube.

103. A DELST image tube according to claim 101 operating in a pulsed mode, wherein the flood gun, bias light, exposure and the scanner are pulsed in a sequence as needed for an imaging requirement.

104. A DELST image tube according to claim 101 operating in a continuous mode, wherein the flood beam, bias light, scanning beam and exposure are in operation simultaneously and continuously.

105. A DELST image tube according to claim 101 structured with the first electrode divided into stripes to further minimize capacity when applications warrent.

106. A scanner image tube comprising:
a DELST structure, a first electrode transparent to imaging radiation, a photoconductive first layer responsive to imaging radiation, a second photoconductive layer responsive to laser beam radiation and supporting a second conducting electrode transparent to LASER
radiation, and a LASER beam for scanning said DELST;
a flood gun to provide a flow of electrons for charing the second electrode;
a bias light which when positioned for its radiation to pass through the first electrode causes a photoconductive response in the first layer, and when positioned for its radiation to pass through the second electrode, is able also to pass through the second layer photoconductor and to cause a photoconductive response in the first layer;
a means for the scanning beam, bias light and the flood gun to operate in combination to establish the storage charge at the interface surface and the second electrode as well as the equilibrium potential across the first and second electrodes;
a means for reducing noise inducing capacity to the preamplifier, a means for sequencing the process of charge storage, exposure and video signal generation.

107, A DELST image vacuum tube according to claim 106 where the image tube is designed to include windows transparent to the laser radiation and to the bias light radiation located so that the optical paths of the laser scanning beam and the bias light irradiating the second electrode are not obstructed by the flood gun.

l08. A scanner image tube according to claim 106 operating in d pulsed mode, wherein the flood gun, bias light, laser scanner and exposure are pulsed in a manner designed to meet the needs of an imaging requirement.

l09. A scanner image tube according to claim 106, operating in a continuous mode of operation, wherein the flood beam, the scanning beam, bias light and exposure are in operation simultaneously and continuously.

110. A scanner image sensor according to claim 106, structured so that the second electrode has an extension which comprises an appendage placed so that the flood gun is off to the side of the second electrode and permits more direct placement of the transparent windows relative to the laser scanner, bias light and the scanned raster.

111. A scanner image tube according to claim 110 structured with the first electrode divided into stripes to further minimize capacity when applications warrent.

112. A scanner image tube comprising:
a DELST structure, a first electrode transparent to imaging radiation and a charge multiplying second layer with a second transparent conducting electrode;
a channel multiplier whose output is in proximity focusing to the second electrode and whose input surface is raster scanned by a high velocity electron beam, whose output can have acceleration potentials adjusted to provide an amplified scanning high velocity electron current or a low velocity electron current trained on the second electrode;
a means for the scanning high velocity beam and the low velocity beam to operate in combination to establish the storage at the interface;

a means for the low velocity beam operation to charge the second electrode;
a means for the high velocity beam to generate a video signal during the scan after exposure;
a means for the electron optics governing scanning electron beam input to the channel multiplier to be designed at least similar to a flat panel type display;
a means for a bias light positioned to expose the first layer from the front through the first electrode or from the rear through the second electrode and the multi-plying layer for the purpose of maintaining an equilibrium potential across the first and second electrodes.

113. A scanner image tube according to claim 112, where the second electrode is extended beyond the area covered by the raster, and where a flood gun can be added to provide low velocity electrons for deposition on the second electrode, and where separate scanning and flood guns permit the choice of their simultaneous or pulsed sequence operation.

114. A scanner image tube according to claim 112, where the second electrode is removed.

115. A scanner image tube according to claim 113 or 114 structured with the first electrode divided into stripes to further minimize noise inducing capacity when applications warrent.

116. A scanner image tube comprising:
a DELST structure, a first electrode transparent to imaging radiation and a photoconductive second layer with a second transparent electrode;
a channel multiplier positioned in a manner resembling a third generation low light level intensifier, whose output is in proximity focusing to the second electrode and whose input surface is in proximity focusing to a photoelectron emitter deposited on a transparent electrode surface:
a laser scanner trained on the photoemitter surface to generate an electron beam scan at the input to the channel multiplier;
a control of the accelerating potential at the output of the channel multiplier to permit high velocity or low velocity electrons to reach the second electrode;
a means for the high and low velocity beams to operate in combination to establish the storage charge at the interface;
a means for the low velocity beam operation on to charge the second electrode;
a means for the high velocity beam to generate a video signal during the scan after exposure:
a means for a bias light to be positioned so that its radiation can pass through the first electrode and expose the first layer or pass through the second electrode and the multiplying layer to expose the first layer, for the purpose of maintaining an equilibrium potential across the first and second electrodes.

117. A scanner image tube according to claim 116, where the second electrode is extended beyond the area covered by the raster as an appendage, and a separate flood gun is positioned to train flood beam electrons on to the second electrode extension, and where the sources of the scanning beam and flood beam permit their simultaneous operation.

118. A scanner image tube according to claim 116, where the second electrode is removed.

119. A scanner image tube according to claim 116, structured with the first electrode divided into stripes to further minimize noise inducing capacity when applications warrent.

120. A scanter image tube according to claim 1, wherein said first electrode is adapted for coupling to a video preamplifier by the attachment of a resistor through which said preamplifier would be coupled.

121. A scanner image tube according to claim 1, wherein said second electrode is adapted for coupling to a video preamplifier by the attachment of a resistor through which said preamplifier would be coupled.

122. A scanner image tube according to claim 20 or 21 wherein said stripe electrode or electrodes can be one raster line thick.