US2916661A

US2916661A - Storage cathode ray tube

Info

Publication number: US2916661A
Application number: US756155A
Authority: US
Inventors: Dean W Davis
Original assignee: Deutsche ITT Industries GmbH
Current assignee: TDK Micronas GmbH; International Telephone and Telegraph Corp
Priority date: 1958-08-20
Filing date: 1958-08-20
Publication date: 1959-12-08
Anticipated expiration: 1976-12-08

Description

Dec. 8, 1959 D. w. DAVlS STORAGE CATHODE RAY TUBE Filed Aug. 20, 1958 owl mmvrox. Bean M Dar/.

Atteneys United States Patent 2,916,661 STORAGE CATHODE RAY TUBE Dean W. Davis, Fort Wayne, Ind., assignor to International Telephone and Telegraph Corporation I Application August 20, 1958, Serial No. 756,155

8 Claims. (Cl. 315-11) This invention relates generally to storage cathode ray 2,916,661 P atented Dec. 8, 1959 age cathode ray tubes'is the relative locations of the'high velocity writing beam electron gun, and the low'velocity flood gun. It has commonly been found most desirable to mount the flood electron gun along the axis of the tube in order to avoid image shading in the visual display on the display screen. Paradoxically, however, the most tubes and more particularly to storage cathode ray tubes of thesignal-to-image type in which an electrical input signal is written into the storage system of the tube and thereafter read-out as a visual display.

' Conventional signal-to-image storage cathode ray tubes comprise an enclosing envelope with a phosphor display screen arranged at one end and with a conventional electron gun assembly arranged at the other end. A charge storage screen, commonly formed as a fine mesh metal screen withsecondary ernissive insulator material on one side thereof is disposed between the display screen and the electron gun with its insulator surface facing the electron gun. The electrical input signal is commonly connected to a control grid of the electron gun which is arranged to scan a high velocity electron beam over the insulator surface of the storage screen; impingement of the electron beam on the insulator surface of the charge storage screen results in secondary emission therefrom thus forming a charge pattern or image on the insulator surface corresponding to the electrical input signal. The charge image which has thus been written on the insulator surface of the storage screen is commonly fread-out by subsequently flooding the charge storage screen with a uniform flood beam of low velocity electrons, these low velocity electrons passing through the openings in the charge storage screen, being modulated by the incremental charges on the insulator surface thereof, and impinge upon the phosphor display screen thereby forming a visual image corresponding to the charge image on the insulator surface of the storage screen; the low velocity flood beam of electrons is'commonly provided by another conventional electron gun in the envelope.

In prior signal-to-image storage tubes known to the present applicant, the minimum spot size of the incremental elements of the visual display is limited to something greater than the size of the mesh openings in the storage screen because the mesh-formed low velocity electron beams spread after going through a cross-over close to the storage screen. It is possible to focus the low,velocity electron beams between the storage screen and the phosphor display screen with a magnetic focusing field thereby to reduce the minimum spot size of the display image and to provide accompanying increase in'the resolution thereof. However, to the best of the present applicants knowledge, no practical method has been found to restrict the magnetic focusing field to the region between the storage screen and' the phosphor display screen. In conventional signal-to-image tubes, the low velocity flood electron beam is emitted from a small area cathode and the effect of the magnetic focusing field which unavoidably enters the section of the tube containing the flood electron gun is to focus the low velocity electron beam from the cathode onto the storage screen instead of allowing the flood beam to spread so as to illuminate the entire area of the insulator surface of the storage screen as is required in tubes of this type.

desirable place for the high velocity writing electron gun is also along the axis of the tube in order to avoid distortion of the charge image onthe insulator surface of the storage screen. It has, however, been the general practice to mount the writing gun so that it is off-set from the axis of the tube.

Conventional signal-to-image storage tubes provide a visual displayed image having high brightness and high contrast capabilities. However, in tubes of this type known to the present applicant, the resolution of the displayed image deteriorates with increased brightness since, as the flood beam current is increased, the spot size in the displayed image also increases. It has therefore long been felt desirable to provide a signal-to-image storage tube in whichv the high contrastcapabilities of present tubes are retained and in which the low velocity electron beam forming the display image could be properly focused thereby to improve the resolution of the tube. It has further long been felt desirable to provide a signal-toimage storage tube construction in which the writing beam gun could be located on the axis of the tube thereby eliminating the distortion encountered with the writing gun off-set from the tube axis, but in which corresponding off-setting of the source of flood electrons was not necessary; while the use of ring-type flood guns concentrically surrounding the writing gun has been proposed, difficult problems in gun geometry are found in such constructions in order to insure uniform coverage of the charge storage screen with the flood beam.

The storage image converter tube is an image-to-image tube well known in the art. In such tubes, a large area photocathode is arranged at one end of the tube with a phosphor display screen being arranged at the other end and a storage screen being arranged intermediate the photocathode and the display screen. Thus, a radiation image impinged upon the photocathode excites the same to emit photoelectrons in a pattern corresponding to the incident radiation image, these photoelectrons again by secondary emission forming a charge image on the insulator surface of the storage screen. This charge image in the case of the storage image converter tube is conventionally read-outv by uniformly illuminating the photocathode with a source of flood illumination thus exciting the photocathode to emit a uniform beam of low velocity electrons which, as in the case of the signal-toimage storagetube, pass through the storage screen, being modulated by the incremental charges thereon to provide a visual display on the display screen. In the storage image converter tube, the'. flooding photocathode is, itself, as large as theare'a it must flood on the insulator surface of the storage screen and thus, magnetic focusing of the flood beamimay be provided since the magnetic field does not, in this case, reduce thefdiameter of the flooded phosphor display screen and a planar target electrode spaced from the display screen and including a phosphor layer and a layer of 'photocathode material facing the display screen and arrangedn o be excited to emit photoelectrons by fluorescence of the phosphor layer. The writing electron gun is arranged to an electron beam over the phosphor layer of the target electrode thereby causing fluorescence of the same and in turn exciting the photocathode materiall ayer to emit photoelectsons toward the display screen. A perforate charge storage screen is arranged intermediate the target electrode and thedisplay screen. Means are provided for impressing an input signal on the electron gun and .-a suitable potential on the photocathode material layer so that scanning of t e P Q Ph layer w t the sls tmnb re n i [t the input signal causes the photocathode material layer to emit electrons which are then accelerated to high velocity to produce the charge infl age on the'storage screen .by secondary emission. Means are further provided for exciting the photoeathode material layer to emit a uniform flood beam of electronswhich pass through the openings in the storagescreen with low velocity, being modulated by the charge imagethereon, thereby to form the visual image on the display screen. In accordance with my invention, magnetic focusing means is arranged to focus the photoelectrons from the photocathode ma terial layer onto the charge storage screen, and to focus an electron image of the charge storage screen on to the display screen, the magnetic field provided by the magnetic focusing meanshaving negligible deleterious effect on the electron beam from theelectron gun scanning the phosphor layer since the magnetic field is weak in that region of the tube and the velocity of the gun electron beam is very high. In one embodiment of my invention, the scanned electron beam from the electron gunis used not only for writing the input signal upon the phosphor layer in turn exciting the photocathode to emit electrons accelerated at high velocity toward the storage screen, but is also used for scanning a uniform intensity beam over the phosphor layer with the potential impressed on the photocathode material layer being altered so that it emits a uniform beam of electrons accelerated with low velocity toward the storage screen for reading out the charge image stored thereon. In another embodiment of my invention, the low velocity 'flood photoelectrons from the photocathode material layer are provided by illuminating the phosphor layer with flooding source of light positioned outside of the tube envelope. The magnetic focusing means in one embodiment of my invention comprises an elongated annular coil surrounding the tube envelope and extending between the target electrode and the display screen. This focusing coil provides an axially polarized magnetic 'field with its flux lines extending parallel to the axis of the, tube in the image section thereof thus effectively focusing the photoelectrons emitted from the photocathode material layer onto the storage screen and the display screen.

i -It is accordingly an object of my invention to provide an improved storage cathode ray tube.

Another object of my'invention is to provide an improved signal-to image storage cathode ray tube.

A further object of my invention is to provide an improved signal-to-image storagecathode ray tube in which the resolution is'improved.

A still further object of my invention is to provide an improved signal-to-image storage cathode ray tube in which the writing electron gun is located along the axis of the tube thereby, elimina n distortion of the written image found in prior tubes having oif-set writing guns.

The above-mentioned and other features and objects of invention andthe planner of attaining them a will m r ana ram. n the vention i e w b he waer taq b rr fsr a t thert low as des ripen ontak n con- Walnuts to-im-age storage cathode ray tube incorporating one embodiment of my invention, and

Fig. 2 is a fragmentary schematic cross-sectional view illustrating another embodiment of my invention.

Referring now to Fig. 1, my improved signal-to-image storage cathode ray tube, generally identified as 1, comprises an enclosing envelope2 having a conventional phosphor display screen 3 formed at one end thereof. A conventional high velocity writing electron gun assembly 4 is positioned within elongated neck 5 of envelope 2 and comprises a suitable cathode a. control grid 7 and conventional beam forming and accelerating elements 8, 9 and 10, as is well known in the ,Suitable potentials are applied to the accelerating and beam forming elements 8, 9 and '10 by means of external leads 11 and 12 and the high velocity electron writing beam 13 produced by the electron gun assembly 4 is scanned in the desired pattern by means of horizontal and vertical electrostatic deflection plates 14 and 15 connected rejspectively to external leads .16 and 17; it will be readily understood that while electrostatic deflection elements 14 and 15 are shown, conventional magnetic deflection of electron beam 13 may be equally advantageously employed. Cathode 6 of electron gun assembly 4 is connected to a suitable source of cathode potential -E by external lead 13 while the control grid7 is connected to switch 19 by means of external lead 20, Switch 19 has a first position 21 connected to a suitable source of control grid biasing potential E by means of suitable resistor 22 and also to a souroe of electrical input signals by means of coupling capacitor 23. Switch 19 has a second position 24 connected to external lead 18 of cathode 6, as shown. A conventional conductive oa n 25 i a ned on eimz r wa w enve o e 1 forwardly of electron gun assembly deflection elements 14 and 15, conductive coating 25 being connected to a suitable source of potential by external lead 26 and thus serves to accelerate electron beam 13, as is well known in the In accordance with my invention, I provide a target electrode assembly 27 within envelope 2 and spacedfrom display screen 3, target electron assembly comprising a phosphor layer 28 supported on a thin transparent film 29 which in turn supports a layer 30 of photocathode material. It will now be seen that electron beam 13 is caused to scan the phosphor layer 28 by means of deflection elements 14 and 15; impingement of electron beam 13 on the phosphor layer 28 causes it to fluoresce in turn exciting the photocathode material layer 30 causing it to emit photoelectrons toward display screen 3.

The phosphor layer 28 and photocathode material layer 30 are preferably connected together by means of

external leads

31 and 32 and in turn connected to switch'33. Switch 33, which may be ganged with switch 19, as shown by the dashed line 34, also has two positions, its first position 35 connecting phosphor layer 218 and photocathode material layer 30 to a suitable source of external pot n al, such a Q voltsby ex n ea 6, an its second position 37 connecting phosphor layer 28 and photocathode material layer 30 to another suitable source of external potential, such as ground 39, as shown.

A charge storage screen 40 is provided interposed be tween the target electrode 27 and display screen ,3, storage screen 40 preferably comprising a fine mesh metal screen 41 having secondary emissive insulator material 42 coated on its surface facing the photocathode material layer 30. Fine mesh metal screen has in external iead 43 connected to switch 44 whichlin turn has two positions 45 and 46 adaptedrespectively to connect the fine mesh metal screen 41 to two appropriate potentials, such as +15 .volis +18 voits respectively, as shown. A f ne mesh neial secondary electron collector screen .47 is providedclosely spaced from the insulator surface 42, of the storage screen 40 and is connected to a suitable source atrqtsiat a m means at ext rna lead 48- R n e e trodes 49, S0 and51 are provided between the target electrade 27 and collector screen 47 for establishing a uniform accelerating field for the photoelectrons emitted from photocathode material layer 30,

ring electrodes

49, 50 and 51 being connected to appropriate progressively higher voltagesrby means of

external leads

52, 53 and 54. Suitable n'ng electrodes 55 and 56 for the same purpose are disposed between storage screen 40 and display screen 3, being again connected to suitable progressively higher voltages by external leads 57 and 58.

In order to focus the photoelectrons emitted from photocathode material layer 30 'onto thestorage screen 40 and also onto the display screen 3, a suitable focusing coli 60 is provided surrounding envelope 2 and extending between the target electrode 27 and display screen 3', focusing coil 60 being adapted to be'connected to a suitable source of energizing potential by means of external leads 61.

In: operation and during writing of an input signal,

switches

19 and 33 are in their

first positions

21 and 35 respectively connecting control grid'7 of electron gun 4 to the source of input signals and to an appropriate grid bias potential and connecting phosphor material layer 28 and photocathode material layer 30 to a source of potential highly negative with respect to the potential impressed on the fine mesh metal screen 41'of the storage screen 40, i.e., +15 volts, with switch 44 in its position 45. The writing beam 13 from electron gun 4 is thus caused to scan the phosphor layer 28, the intensity of beam 13 being varied responsive to the input signal applied to control grid 7 of electron gun 4 by coupling capacitor 23. The spot caused by impingement of electron beam 13 on the phosphorvlayer 28 causes corresponding fluorescence of the phosphor layer, the light from this fluorescence being transmitted through the transparent film 29 thus exciting the photocathode material layer 30 to emit photoelectrons toward the storage screen 40. The photoelectron image emitted from the photocathode material layer 30 is focused onto the insulator layer 42 of the storage screen 40 by the magnetic field established by focusing coil 60, impingement of the photoelectrons on the insulator layer 42 producing secondary electrons which are collected by the secondary emission collector screen 47. T A charge pattern or image is thus formed on the insulator layer 42 of storage screen 40 proportional to the density of the photoelectron image and thus in turn corresponding to the electrical signal impressed on control grid 7 of electron gun 4; the charge image written onto the insulator layer 42 of storage screen 40 cannot easily leak off since the layer 42'is a good insulator.

In order to read-out the stored image on the insulator layer 42 of storage screen 40,

switches

19 and 33 are moved to their second positions 24 and 37 respectively disconnecting the input signal source and control grid biasing voltage from the control grid 7 and connecting it instead to the cathode 6, and impressing a potential, i.e., ground on phosphor layer 28 and photocathode material layer 30 which is only slightly negative with respect to the potential impressed on the fine mesh metal screen 41 of storage screen 40, i.e., +15 volts. The electron beam 13 continues to scan the phosphor layer 28, now having uniform intensity by virtue of connection of the control grid '7 to cathode 6. The resulting fluorescence of the phosphor layer 28 caused by impingement of the uniform intensity electron beam 13 thereon excites the photocathode material layer 30 to emit photoelectrons, however, by virtue of its only slightly negative potential with respect to the storage screen 40, the photoelectrons now emitted from the photocathode material layer 30 now approach the insulator layer 42 with very low energy. A uniform flood beam of photoelectrons is thus provided from the photocathode material layer 30 during the read-out operation, these low velocity photoelectrons passing through the openings'in the storage screen 40, being modulated responsive to the incremental charges on the insulator surface 42. Thelow velocity photoelectrons which pass'through the openings in the storage screen 40 are focused onto the phosphor display screen 3 by' the focusing coil 60, the resulting light image on the display screen 3 being a replica of the charge image which was'formed on the insulator layer 42.

The charge image on the insulator layerv 42 may be obliterated, i.e., erased, by momentarily increasing the potential of the entire surface by a few volts. This is accomplished by switching the potential of the fine mesh screen 41 of storage screen 40 by means of switch 44 from its original potential of +15 volts to a slightly higher potential of +18 volts; With

switches

19 and 33 still in their second positions, i.e., establishing the circuit connections for read-out with the signal input circuit and source of control grid bias potential removed from control grid 7 of electron gun 4 and with the phosphor layer 28 and photocathode material layer 30 at ground potential, the photoelectron beam emitted by the photocathode layer 30 will new approach the insulator surface 42 with slightly higher energy occasioned by the increase of the potential of the fine mesh metal screen 41 from +15 volts to +18 volts. The photoelectrons which do not pass through the openings in the storage screen 40 will strike the insulator surface 42 and will charge it in the negative sense since the secondary emission will be negligibly small for the low energy photoelectrons. This negative charge obliterates the former positive charge returning the insulator surface 42 to its original potential in readiness for receiving a new charge in the next writing operation by returning

switches

19 and 33 to their

original positions

21 and 35 and switch 44 to its original position 45.

With the phosphor layer 28 and photocathode material layer 30 connected together by

external leads

31 and 32, as shown, when the potential applied thereto is increased by moving switch 33 from its first position 35 to its second position 37, the electron beam 13 will no longer be focused on the phosphor layer 28. This fact is, however, of no consequence, and may even be desirable since the purpose of the electron beam during the read-out operation is to excite the phosphor layer 28 uniformly over its entire area thereby uniformly to excite the photocathode material layer 30 to provide a uniform low velocity beam of photoelectrons during viewing and erasing.

Turning now to Fig. 2 in which like elements are illustrated by like reference numerals, the uniform low velocity flood beam electrons from the photocathode material layer 30 may be obtained by flooding the layer 30 with light. Thus, in the illustrated embodiment, electric lamps 63 and 64 are provided outside of the envelope 2 and arranged to illuminate the surface of the transparent phosphor layer 28; this requires that the wall section 65 of envelope 2 be transparent, however, this is readily ac complished by means of the use of a transparent conductive coating 25 as is well known in the art. Lamps 63 and 64 are energized through leads 66 and 67 connected respectively to a suitable source of power by means of switch 68. Switch 68 may be ganged with

switches

19 and 33 so that it is closed in the second positions of

switches

19 and 33. Thus, during read-out and erasing, when switches 19 and 33 are in their second positions, as hereinabove described, switch 68 will be closed thus illuminating lamps 63 and 64 in turn uniformly illuminating photocathode material layer 30 through transparent phosphor layer 28, uniformly and instantaneously exciting all of photocathode material layer 30 thereby to provide the low velocity flood beam of photoelectrons directed toward and through the storage screen 40 and onto 7 stances he preferred since no external lamps are required, theembodimentof Fig. 2 may .be advantageous since the entire photocathode material layer 30 is instantaneously illuminated during viewing and erasing rather than being illuminatediby the writing spot exciting a raster as in the embodiment of 'Fig. .1. Thus, it is to be expected that with the embodiment of Fig. 2, higher brightness and faster erasing will result since the charge per element of cathode area emitted from the photocathode material layer 30. in the time T required to scan a raster would be if whereas .itwould be only r z A in the embodiment of Fig. 1 with the scanned/raster where A isthe area of the raster and .(a) the area ,of the light spot which traces the raster on the phosphor layergs thusprovidinga corresponding emission of photo electrons from the photocathode material layer 30 in raster form. I I

It will be readily seen that with both embodiments of my invention, the magnetic focusing coil 60 is axially polarized, as indicated in Fig. .1, thus providing amagnetic field with its lines of flux extending between target electrode 27 and display screen 23 being essentially parallel to the axis of the tube, thus uniformly focusing the photoelectrons from the photocathode material layer to emit high .velocityphotoelectrons toward said storage screen whcrebyia chargeimageaistformedon said'storage screen correspondingtto. said .:input. signals; and means arranged .for at zotheri times .excitingasaid photocathode material layer to. emit' a uniform (flood beam of low' velocity photoelectronstoward saidstorage screen whereby said low velocity photoelectrons pass through sa1d storage screen. being modulated by the.charge image facing said display screen andarrangedtto be excited by fluorescence of said phosphor layer; a perforate-charge storage screen positioned intermediate said target elec- 'trode andtsaididisplay screen; anelectron gun arranged to scan an electron beam over said phosphor layer thereby causing fluorescence of the same; circuit con- -onto and through the storage screen and onto the I display screen 3 and in turn providing a focuseddisplay image withthe resolution of prior storage image converter tubes and with the brightness and .contrastcapa bilities of prior signal-to-image, storage tubes. It will furtherbeseen that with my improved storag cathode ray. tube construction, the writing electron gun:

is disposed along the axis of the tube thereby eliminating the distortion encounteredin prior signaleto-image storage tubes in which the writing gun is off-set with respect to the axis of'the tube. I

7 While I have described above the principles ofmy invention in connection with specific apparatus; it is to be clearly understood that this description is made only by way of example and not as a limitation .to the scope of my invention.

What is claimed is:

1 A direct viewing storage cathode ray tube comprising: a phosphor display screen; a planar target .electrode spaced from said display screen and including ,a phosphor layer and a layer of photocathode material fs iss vsai di a sc nda r ge t be excited by fluorescence of said phosphor layer; a perforate charge storage screen arranged intermediate said target electrode and said display screen; electrongun means arranged toscanan electron beam over said phosphor layer thereby causing fluorescence of the same and exciting said p tqcat o m e l s t em h h ve i y pho electrons toward said storage screen; means for causing said .photocathode material layer .to emit low velocity photdelectrons toward said storage screen and magnetic focusing means arranged to focus said photoelectrons onto said charge storage screen and to focus an electron image of the charge storage screen onto said display screen. i a

2. A direct viewing storagecathode ray tube comprising: ,a phosphor display screen; a planar target electrode spaced from said display screen and including a phosphor layer and a layer of photocathode material facing said display screen and arranged to be excited by fluorescence of said phosphor layer; a perforate charge storage screen positioned intermediate said target electrode and said display screen; electron gun means including a signal input circuit adaptedto be connected to a source of input signals, said electron gun means being arranged at times to scan an electron beam responsive .to said input signals over said phosphor layer thereby causing fluorescence of the same and exciting said hotocathode material layer nections arranged during first intervals to impress an input signal on said electron .gun .and 'to impress a first predetermined potential. on said target electrodeso that saidelectron gun scans said phosphorlayer with anelectron beam responsive to said input signal thereby exciting said photocathode material layer toernit high velocity photoelectrons toward saidstorage screen to form a charge image thereon responsive to said input signal;

said circuit connections being arranged during second intervals to disconnect said input signal from said electron gun and to excite said photocathode material layer and impressanother predetermined potential thereon so that said photocathode material layer .emits a'uniform' flood beam of low velocity photoelectrons toward said storage screen whereby said low velocity photoelectrons pass through said storage screen being modulat'ed bythe charge image thereon to provide a visual image on said display screencorresponding to said charge image; and magnetic focusing meansextending :between said target electrode and said display screen for focusing said high and low velocity photoelectronsonto-said storage screen and said low velocity photoelectronsronto said display screen. i

4. The combination of claim3 :in which said first predetermined potential .is highly negative with respect to the potential of said storagerscreen and said other predetermined potential is slightly negative with respect to the potential of said storage screen. 1 I

'5. A direct viewing storage cathode ray tube comprising: a phosphor displayscreen; apla nar t argetelectrodeflspaced from said display screen and including a phosphor layer and a layer of ,photocathode material facing said display screen and arranged to be excited by fluorescence of said .phosphorlayer; a perforate charge storage screen positioned intermediate said target electrode and said display screen; an electron gun arranged to scan an electron beam over said phosphor layer thereby causing fluorescence .of thesaine; circuit connections arranged during first intervals to impress an input signal on said electron gun .and to impress a predetermined potential on said target electrode so that said electron gun scans said phosphor layer with an electron beam responsive tosaid input signal thereby exciting saidphotocathode material layer to emit high. velocity photoelectrons toward said storage screen to form a charge image thereonresponsive to said input signal; said circuit connections being arranged during second intervals to disconnect said input signal from said electron gun and to impress another predetermined potential on said photocathode material layer so that said electron gun scans said phosphor layer with aluniform intensity electron beam whereby said photocathode material layer emits a uniform flood beam of low velocity photoelectrons toward said storage screen whereby said low velocity photoelectrons pass through said storage screen being modulated by the charge image thereon to provide a visual image on said display screen corresponding to said charge image; and magnetic focusing means extending between said target electrode and said display screen for focusing said high velocity photoelectrons onto said storage screen and said low velocity photoelectrons onto said display screen.

6. A direct viewing storage cathode ray tube comprising: an enclosing envelope; a phosphor display screen at one end of said envelope; a planar target electrode in said envelope spaced from said display screen and including a phosphor layer and a layer of photocathode material facing said display screen and arranged to be excited responsive to fluorescence of said phosphor layer; a charge storage screen in said envelope intermediate said target electrode and said display screen and including an insulator layer facing said photocathode material layer; an electron gun in said envelope at the other end thereof arranged to scan an electron beam over said phosphor layer thereby causing fluorescence thereof; a signal input circuit; switching means arranged in a first position to connect said signal input circuit to said electron gun and to impress a potential on said photocathode material layer highly negative with respect to the potential of said charge storage screen so that said electron gun scans said phosphor layer with an electron beam responsive to said input signal thereby exciting said photocathode material layer to emit high velocity photoelectrons toward said storage screen forming a charge image on said insulator layer corresponding to said input signal; said switching means being arranged in a second position to disconnect said signal input circuit from said electron gun and to impress a potential on said photocathode material layer slightly negative with respect to the potential of said charge storage screen so that said electron gun scans said phosphor layer with a uniform intensity electron beam thereby exciting said photocathode material layer to emit a uniform flood beam of low velocity photoelectrons toward said storage screen which pass through said storage screen being modulated by the charge image thereon to provide a visual image on said display screen corresponding to said charge image; and an annular magnetic focusing coil surrounding said envelope and extending between said target electrode and said display screen for focusing said high and low velocity photoelectrons onto said storage screen and said low velocity photoelectrons onto said display screen.

7. A direct viewing storage cathode ray tube comprising: an enclosing envelope; a phosphor display screen at one end of said envelope; a planar target electrode in said envelope spaced from said display screen and having a transparent film with a phosphor layer on one side thereof and a layer of photocathode material on the other side facing said display screen and arranged to be excited responsive to fluorescence of said phosphor layer, said phosphor and photocathode material layers being electrically connected together; a charge storage screen in said envelope intermediate said target electrode and said display screen and having a fine mesh metal screen with secondary emissive insulator material deposited thereon facing said photocathode material layer; a secondary electron collector electrode in said envelope intermediate said target electrode and said storage screen; an electron gun in said envelope at the other end thereof and including a cathode and a control grid, said electron gun being arranged to scan an electron beam over said phosphor layer; a signal input circuit, switching means arranged in a first position to connect said control grid to said signal lnput circuit and to a source of grid bias potential and to connect said phosphor and photocathode material layers to a source of potential highly negative with respect to the potential of said storage screen so that said electron gun scans said phosphor layer with an electron beam responsive to said input signal thereby exciting said photocathode layer to emit high velocity photoelectrons toward said storage screen forming a charge image on said insulator material corresponding to said input signal; said switching means being arranged in a second position thereof to connect said electron gun control grid to said cathode and said phosphor and photocathode material layers to a source of potential slightly negative with respect to said storage screen so that said electron gun scans said phosphor layer with a uniform intensity electron beam thereby exciting said photocathode material layer to emit a uniform flood beam of low velocity photoelectrons toward said storage screen which pass through said storage screen being modulated by the charge image thereon to provide a visual image on said display screen corresponding to said charge image; and an annular magnetic focusing coil surrounding said envelope and extending between said target electrode and said display screen for focusing said high and low velocity photoelectrons onto said storage screen and said low velocity photoelectrons onto said display screen.

8. A direct viewing storage cathode ray tube comprising: an enclosing envelope; a phosphor display screen at one end of said envelope; a planar target electrode in said envelope spaced from said display screen and including a transparent phosphor layer and a layer of photocathode material facing said display screen and arranged to be excited responsive to fluorescence of said phosphor layer; a charge storage screen in said envelope intermediate said target electrode and said display screen and including an insulator layer facing said photocathode material layer; an electron gun in said envelope at the other end thereof arranged to scan an electron beam over said phosphor layer thereby causing fluorescence thereof; a signal input circuit; switching means arranged in a first position to connect said signal input circuit to said electron gun and to impress a potential on said photocathode material layer highly negative with respect to the potential of said charge storage screen so that said electron gun scans said phosphor layer with an electron beam responsive to said input signal thereby exciting said photocathode material layer to emit high velocity photoelectrons toward said storage screen forming a charge image on said insulator layer corresponding to said input signal; a flood light source disposed outside of said envelope and arranged uniformly to illuminate through a transparent wall of said envelope the side of said phosphor layer remote from said photocathode material layer; said switching means being arranged in a second position to disconnect said signal input circuit from said electron gun and to impress a potential on said photocathode material layer slightly negative with respect to the potential of said storage screen and to illuminate said flood light source whereby said photocathode material layer is excited to emit a uniform flood beam of low velocity electrons toward said storage screen which pass through said storage screen being modulated by the charge image thereon to provide a visual image on said display screen corresponding to said charge image; and an annular magnetrc focusing coil surrounding said envelope and extending between said target electrode and said display screen for focusing said high and low velocity photoelectrons onto said storage screen and said low velocity photoelectrons onto said display screen.

OTHER REFERENCES Knoll: Storage Tubes and Their Basic Principles, John Wiley and Sons, Inc., New York, 1952, pp. 78-81,