US2254140A - Image analyzing system - Google Patents

Description

Allg 26, 1941- P. T. FARNswoRTH 2,254,140

IMAGE ANALYZING SYSTEM Filed Sept. lO, 1938 Fgl.

\ una E/ Ecmo/v scANN/NG /MAGE BEAM /NsuL'AT/o/v METAL INVENTOR,

PH/LO 7T FARNS WORTH.

Patented Aug. 26, 1941 Ill/IAGE ANALYZIN G SYSTEM Philo T. Farnsworth, Springfield Township, Montgomery County, Pa., assignor, by mesne assignments, to Farnsworth Television & Radio Corporation, Dover, Del., a corporation of Dela- Application September 10, 1938, Serial No. 229,350

(Cl. TIS- 7.2)

7 Claims.

My invention relates to limage analyzing systems, and more particularly to a system employing image analysis electron tubes wherein an optical image may be analyzed by conversion and scansion into a train of electrical signals. Such eicient image analysis tube, system and method.

My invention possesses numerous other objects and features of advantage, some of which, together With the foregoing, will be set forth in the following description of specific apparatus embodying and utilizing my novel method. It is therefore to be understood that my method is applicable to other apparatus, and that I do not limit myself, in any way, to the apparatus of the present application, as I may adopt various other apparatus embodiments, utilizing the method, within the scope of the appended claims.

Referring to the drawing:

Fig. 1 is a longitudinal sectional View of one preferred form of tube involved in practicing the method of my invention, together with a schematic circuit .for operation of said tube. Y

Fig. 2 is a cross sectional View of a control element utilized in Fig. 1.

In the preferred form shown, an envelope I is provided at one end thereof With a transparent window 2 through which an optical image may be focused by means of lens 4 onto a translucent photoelectric cathodey 5 deposited on the inner wall of the envelope back of window 2. Obviously, a sensitized screen within the envelope is fully equivalent to the coating.

Within the tube, and spaced from cathode 5,'is a relativelyopen mesh accelerating electrode 6, this accelerating electrode preferably having a relatively large void-to-wire ratio. On the other side of accelerating electrode 6 away from photoelectric cathode 5 is a charge storage screen electrode] of relatively ne mesh, such as, for example, a perforated metal screen having approximately 400 meshes per inch. This screen is uniformly coated with insulating material 9 on the side facing the accelerating electrode 6 and photocathode 5. This insulating coating may be formed in any well known and convenient manner, such as, for example, the deposition by sputtering or the like, on the side to be insulated. of

an oxidizable vmaterial which, when oxidized, y

changes to an insulating oxide. Aluminum may be used for this purpose, as a specific example, or insulating oxides in nely divided condition may be sprayed, when mixed with a volatile liquid, on the side using the insulating coating. Other methods of deposiitng this insulating coating will be immediately apparent to those skilled in the art.

At the end of the envelope opposite the transparent-window 2, an electron gun is positioned, preferably to emit its undeflected beam axially of the envelope. One exampel of such gun is shown and comprises a unipotential thermionic cathode I0 heated by heater filament II. Electrons emitted from cathode I0 are accelerated through a central bore, not shown, in a gun anode I2, as is well known in the art. Surrounding the greater `portion of the path between the gun anode I2 and screens B and I is a conductive envelope wall coating I 4 electrically connected with anode I through link I5.

'Ihe above description covers all of the fundamental electrode structure necessary to practice my invention. One preferred circuit for operation of the tube is also shown, and in this circuit gun anode I2 and wall coating I4 `are grounded through ground connection IS. Heater filament I I is connected to the usual heater current source I1, and the thermionic cathode III is connected to the negative side of a D.C. anode source I8, the positive side of which is connected to ground. Photoelectric cathode 5 is connected to the negative end of a D.-C. cathode source I9, the positive end of which is grounded. Both anode source i8 and cathode source I9 may have voltages for a specific tube in the neighborhood of several hundred volts, but I prefer that the potential of thermionic cathode I0 be slightly more negative than that of photocathode 5 by an amount on the order of several Volts, say between 5 and l0 volts. Thus, in one specific tube it may be preferred to operate the device with thermionic cathode I0 at a potential of 405 volts and photocathode 5 at 400 volts.

Storagev screen 'I is connected directly to ground. The accelerating electrode 6 is connected through output resistor 20, to potentiometer circuit 2I, the negative end of the potentiometer source 22 being connected to ground. Thus, accelerating electrode 6 may be held at a potential positive with respect to ground by several volts. The output of the tube is taken from signal connection 24 connected directly to accelerating electrode 6.

The cathode ray beam emitted from electron gun anode l2 is scanned across storage screen 1 in two directions by means of scanning coils 25 and 26 connected to scanning current generators .21 and 28, respectively. Obviously, however, electrostatic scansion may be utilized if desired.

In operation, the optical image of the scene to be analyzed into a train of signals is focused upon photocathode 5, thereby causing photoelectrons to be emitted. It is then desired to produce an electron image in the plane of the storage screen 1. As is well known in the art, this may be accomplished in different ways; iirst, lby spacing electrodes 5, S; and 1 at'dis'- tances on the order of between 1/2A to 1 inch apart 'in specific tubes, and then providing a magnetic focusing eld generated by a focusing coil surrounding that portion of the tube adjacent the periphery of electrodes 5, 5 'and 1, 'or these same electrodes Ican be spaced atdistances on the order of le inch apart andthe electrostatic iields alone will focus the electron image. Such expedients are well known in, the art and are deemed fully equivalent.

The photoelectrons emitted from photoelectric cathode 5 under the inuence of the optical image are therefore accelerated by the higher potential on accelerating screen E, pass through this relatively open screen, and travel toward storage screen 1. In the plane of storage screen 1 an electron image is formed, as described above, corresponding to the optical image. 'This electron image will arrive at the insulating surface 9 of storage screen 1 with a velocity corresponding to the potential difference between photocathode 5 and the metallic portion of storage screen 1 which may be, as above described, 400 volts. This electron velocity is sufcient to produce from the insulating materials which may be utilized to form the insulating coating, secondary electrons at a ratio higher than unity. These emitted secondary electrons are then accelerated toward accelerating electrode 6 which is at a voltage higher than storage screen 1 by the amount determined by the adjustment of potentiometer circuit 2|, and these secondary electrons are therefore collected by accelerating electrode 6. This collected electron current constitutes the so-called D.C. component of the picture signals, and corresponds to the average brightness of the optical image.

The secondary electrons leaving the insulating portion 9 of the storage screen leave-elementary areas of the insulating portion positively charged by an amount comparable to the number of secondary electrons emitted, which latter are in turn dependent upon the nurrrber of photoelectrons impacting this insulating material. This impacting number is again a function ofI the brilliancy of the optical image on differentelementary areas of the photoelectric cathode. Thus, a positive charge image is produced on the insulating surface 9 corresponding in all respects with the optical image.

The metallic side of the storage screen 1 facing the gun anode l2 is then scanned by-the beam issuingfrom the gun anode l2, which is confined to have a cross section of elemental area, and electrons inthis beam, under the voltages used, have sufficient velocity also to produce secondary emission at aratio greater than unity upon irnpact with the metallic side of screen 1. In this respect it is to be distinctly'understood that if it appears desirable, the metallic side of screen 1 may be subjected to special treatments `well known in the art in order to increase the ratio of secondary emission. Under normal circumstances, however, materials can be chosen for the metal of screen 1 which will have a suiciently high ratio within the voltage ranges used.

The secondary electrons emitted on the metallic side of. the4 storage screen 1 upon impact of the scanning beam issuingK from` the anode l2 may be said to form a small cloud of electrons about the point of impact. The secondary electrons will of course have low emission velocities, and are therefore subjected to, and come under, the influence of the positive charges on the insulating side 9, the latter charges providing a potential 'gradient pulling secondary electrons through the apertures in the screen, and the number of secondary electrons of the cloud pulled through is of course proportional to the positive charge on the insulating side 9 at the point of secondary production on the metallic side. Some` of the secondary electronsv pulled through will neutralize the. positiv/eY charges on the insulating material, while the majority of them will be accelerated toward screen 6 and be collected thereby, Thus, in addition to the so-called D.C'. component` already described, there will be collected on accelerating electrode 6 a varying electron current constituting, through output lead; 24, a train of picture signals.

Aportion ofthe scanning beam will of course pass throughthe openings in storage` screen 1 without impacting it. The majority of these beam primary electrons, however, pass directly through accelerating electrode 6 and are collected by photocathode 5, because, as was above stated, this photocathode is held at a potential a Yfew volts higher thanA gun anode I2. Therefore, except for the minute number of primary beam electrons which may be actually intercepted by the wirev of accelerating electrode 6, noneof theprimary electrons from gun anode l2y will enter the picture signal. Consequently, it is possible to reduce the picture current. substantially to "zero `in absence of lightin the` optical image. A

It has been found in practice that a positive charge on the order of 1 volt on the insulating surface 9-of the storage screen maybe sufficient to'saturate the secondary emission from the metallic side. It will therefore be preferable to choose the potential of accelerating electrode 6 so that equilibrium, i. e., the number of arriving photoelectrons, to Abe equal to that portionof the secondaries produced-by photoelectrons (which portion is collected, of course, by electrode 6), is reached when a positive charge of l` volt` has accumulated on the insulating side 9? for the brightestspots of the. image. The-time in which this equilibrium can be. "reached" is made lto ap proximate the time between scansions.

Therefore, under these circumstancesr I- may adjust. the potential differencefbetwenf photocathode 5' andstorage screen 1 to! a'valu'e. for which equilibrium may be reached at a positive charge of 1 volt onV the `insulating Y coating', and then choose the lightintensity of rthebrilghtest portions of the optical imagesto be analyzed to have such a value thatfthe time required. to accumulate thispositivecharge of .1 volt-on the insulating side Slis approximately` equal to the time between scansions.

It should be distinctly.A understood, however. that the value of 1 volt'herewith given is alvalue arrived atinconjunction Withlthe other voltage` values herein` mentioned, and has been arrived at by observation. Therefore, I do not Wish to limit the scope of my invention in any way to the volt-age values herewith disclosed, inasmuch as these values may be raised and lowered, and the best operating conditions with changed voltages arrived at, by observation during operation, which procedure is well known to all those skilled in the operation of image analysis tubes.

I claim:

l. The method of generating a television picture signal which comprises forming a charge image in accordance with an optical image to be transmitted, successively creating a plurality of secondary electrons adjacent successive elemental areas of said charge image, modulating the stream of said secondary electrons in accordance with the charges of said elemental areas, utilizing a portion of said modul-ated stream of secondary electrons to neutralize thecharges of successive element-al areas of said charge image, and collecting the remainder of said modulated stream of lsecondary electrons to develop a picture signal.

2. The method of generating a television picture signal which comprises producing a cloudv of secondary electrons representing a virtual cathode in space, moving said cloud in a plane in accordance with a scanning pattern, creating a charge image representative of an optical image in a plane closely adjacent the plane of said moving cloud, drawing electrons from said cloud in accordance with the charges of said charge image, and collecting the electrons drawn from said cloud to develop a picture signal.

3. The method of generating a television picture signal which comprises generating a beam of primary electrons, utilizing said beam to produce a cloud of secondary electrons representing a virtual cathode in space, moving said cloud in a plane by scanningsaid beam in accordance With a scanning pattern, creating a charge image representative of an optical image in a plane closely adjacent the plane of said moving c loud, drawing electrons from said cloud in accordance with the charges of said charge image, and collecting the electrons drawn from said cloud to develop a picture signal.

4. 'I'he method of generating a television picture signal which comprises producing a cloud of secondary electrons representing a virtual cathode in space, moving said cloud in a plane in accordance with a scanning pattern, creating a charge image representative of an optical image in a plane closely Iadjacent the plane of said moving cathode, drawing electrons from said cloud in accordance with the charges of said charge image, utilizing a portion of the electrons drawn from said cloud to neutralize said charges, and collecting the remainder of the electrons drawn from said cloud to develop a picture signal.

5. A television picture signal generator comprising an envelope containing an apertured screen, one side of said screen being adapted to store electrical charges, means for scanning the other side of said screen with an electron beam to liberate secondary electrons upon impact therewith, means including a photoelectric cathode for producing a charge image on said rstnamed side of said screen representative of an optical image to draw a portion of said secondary electrons through the apertures of said screen in accordance with the charges of said charge image, and means for collecting said portion of electrons to develop a picture signal.

6. A television picture signal generator, comprising an envelope containing an apertured screen, one side of said screen being adapted to store electrical charges, means for scanning the other side of said screen with an electron beam to liberate secondary electrons upon impact therewith, a photoelectrilc cathode adapted to receive an optical image and to transmit a corresponding electron image of such velocity upon said first-named side of said screen to produce by secondary emission a charge image representative of said optical image to draw a portion of said secondary electrons through the apertures of said screen in accordance with the charges of said pharge image, and means for collecting said portion of electrons to develop a picture signal.

7. A television picture signal generator comprising an envelope containing an apertured screen, one side of said screen being adapted to store electrical charges, means for scanning the other side of said screen with an electron beam to liberate secondary electrons upon impact therewith, a photoelectric cathode adapted to receive an optical image and to transmit a corresponding electron image oi such velocity upon said first-named side of said screen to produce by secondary emission a charge image representative of an optical image to draw a portion of said secondary electrons through the apertures of said screen in accordance with the charges of said charge image, and means disposed between said photoelelctric cathode and said apertured screen for collecting said portion of electrons to develop a picture signal.

PHILO T. FARNSWQRTH.

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