US2535817A - Electrooptical dark trace storage tube - Google Patents

Description

A. M. SKELLETT ELECTROOPTICAL DARK TRACE STORAGE TUBE Dec. 26, 1950 4 Sheets-Sheet 1 Filed Oct. 23, 1948 SIGNAL SOURCE SAW- TOOT H SCANNING WAVES FIG. L

FIGZ;

SCANNING BEAM CURRENT SIGNAL 1' SOURCE SCANNING WAVES SAW TOOTH INVENTOR ALBERT M. SKELLETT Dec. 26, 1950 A. M. SKELLETT 2,535,817

v ELECTROOPTICAL DARK TRACE STORAGE TUBE Filed Oct. 23, 1948 4 Sheets-Sheet 2 SAW-TOOTH.

SCANNING S'GNAL 27 WAVES Z4ISOURCE 7 p H .5 2 20 3 5m 6 {L4 v j I l0 5 2 a c /9 r A 4- y I I I l l l l l l it [/8 /7 Z3 SIGNAL SOURCE d l lgl l h l l /& /7 23 INVENTOR ALBERT M. SKELLETT Dec. 26, 1950 Filed Oct. 25, 1948 FIG. 6.

Sheets-Sheet 3 /Z /5 SAW- TOOTH SCANNING SJGNAL WAVES 4 SOURCE /2 /f SAW- TOOTH SCANNING 532g: /3 WAVES 5141 O 6 43 E 2: /0 5 iii 26 7 up:

w.-- M FI 4 f- 11g l l l i |||l|||l| /c9 7 J-P Zi INVENTOR ALBERT M. SKELLETT reading and erasing arrangement for Patented Dec. 26, 1950 EL'E'CTROOPTICAL DARK'TRACE STORAGE TUBE Albert M. Skellett, Madison, N. J., assignor to National Union Radio Corporation, Orange, N. J., a corporation of Delaware 7 Application flctober 23, 1948, Serial No. 56,135

27 Claims.

This invention relates to electro-optical signalstoring and reproducing systems, andmore particularly it relates to s stems employing so-cal-led dark trace cathode-ray tubes.

It has been known for some time that certain materials, such for example as normally transparent ionic crystals, when impinged upon by a beam of electrons or cathode rays, develop localized opacity areas. Examples of such crystals are the halides of the alkaline metals or of the alkalineearth metals, and also certain salts of silver, for example silver bromide. For convenience, the expression dark trace tube is used herein asreferring to a cathode-ray tube wherein the image-producing or informationstoring screen is composed, at least in part, of suchionie crystal materialor materials. Likewise, for convenience, the express ioniccrystal refers to those crystals which have the property of developing localized opacity areas in response to impinging electrons or cathode-ray beams and includes those crystals containing positive and negative ionized or ionizable components which are held together mainly by electrostatic forces.

Accordingly, a principal object of this invention relates to anim-proved information storing, dark trace cathode-ray tubes.

According to the present invention, the dark trace tube is provided with a single electron gun which is used both for tracing and'storing the information on a dark trace storage screen and for reading the previously-stored information,

without affecting its storage.

nal-storing and reading arrangement for cathode-ray tubes of the dark trace type.

Afeature of the invention relatesv to a dark trace cathode-ray tube for storing signals or information in the form of opacity-controlled traces in an ionic crystal layer, and wherein the ionic crystal layer is applied to a metal backing plate. By means of a special switching arrangement, the said backing plate can be used in controlling the storage of information on the dark trace screen and also as a reading electrode in conjunction with a cooperating secondary emission collector electrode.

Another feature relates to a storage and reading screen for cathode-ray tubes of .the dark trace type employing a thin plate of light transparent material such as mica or glass having a coating of ionic crystals and a coating of light transparent met-a1 between the mica and crystal coatmg.

Another feature relates to an arrangement for producing-stored imagesof infra-red wave signals employing a tube with an electron emitter responsive to the infra-red radiation and a storage screen in the form of ionic crystals together with means for erasing the stored image on the ionic crystal screen at any desired time.

Another feature relates to an improved organization of apparatus and method for producing images under control of X-rays and the like.

Another feature relates to improved arrangements for controlling the erasure of previouslystored signals. on the screen of a dark trace cathode-ray tube.

A further feature relates to the novel organization, arrangement, and relative interconnection of parts which cooperate to provide an improved signal-storing, signal-reading, and signal-erasing system, employing a cathode-ray tube of the dark trace type.

Other features and advantages not particularl enumerated, will be apparent after a consideration of thefollowing detailed descriptions and the appended claims.

In the drawing,

Fig. 1 shows one preferred arrangement for storing, reading and erasing signals employing a dark trace cathode-ray tube.

Fig. '2 is a graph explanatory of the operation of Fig. 1.

Fig. 3 represents a modification of the screen and electrode arrangements of Fig. 1.

Figs. 4-, 5, 6 and 7 represent further respective modifications of Fig. 1.

neck portion 2 terminating in an enlarged body portion 3, which is closed off by the usual end Wall 4. Suitably mounted in the neck portion 2 is an electron gun 5 of any construction wellknown in the cathode-ray tube art and comprising for example an electron-emitting cathode 6, an aperture electrode or beam intensit control electrode l, a first beam accelerating and focussing anode 8, and a second beam accelerating and focussing anode 9. Associated with the electron gun is any well-known beamdeflecting arrangement, comprising for example the vertical deflecting plates as, and the horizontal deflecting plates ll. These plates are connected in the conventional manner to any well-known source 2 for producing a saw-tooth deflecting voltage to effect the line scanning and frame frequency of the scanning, as is wellknown in the television scanning art. Mounted adjacent the end Wall iis a metal plate it which has. applied to the side facing the electron gun 5 r coating M of ionic crystals such as those mentioned hereinabove and of which potassium chloride is typical. In accordance with the wellknown operation of dark trace cathode-ray tubes, the coating 54 forms the screen upon which the signals or images are to be stored under control of the electron gun 5. For this purpose, the control grid or intensity control electrode 5 is connected to any well-known source 95 of signals which are to be set up or stored on the screen it. It will be understood, of course, that the beam from gun 5 is focussed in the conventional way as a minute scanning spot on the screen Hi and is caused to execute any predetermined scanning pattern under control of the plates at, H, and the source I2.

I have discovered that when the ionic screen i is darkened or varied in opacity under control of the modulated intensity beam from sun 5, thesecondary electron emission characteristics of the screen are correspondingly changed, and advantage is taken of this fact according to one phase of the present invention to facilitate the reading of the stored signals on the screen it without materially affecting their stored characteristic.

Since the screen M is made up of a multiplicity of ionic crystals such as potassium chloride and are located in an evacuated space, when they are subjected to bombardment by the cathoderay beam from gun 5, there are setup within the crystals, at the localized region corresponding to the instantaneous position of the oathode-ray beam, free electrons which tend to travel in the direction of the potential gradient across the crystal, namely towards the grid It which is positively biassed with respect to plate i3, e. g. 5!) or 100 volts by battery it in series with load resistor it which is coupled through condenser 2d to the output terminal iii. If desired, the load resistor instead of being connected between the positive terminal of battery its and grid Hi, can be connected between the negative terminal of battery is and plate 53. Because of the presence of positive ions within the crystal lattice, a certain percentage of these free electrons are captured and form so-called color centers within the crystal body. These color centers thereupon act as light absorbers for visible light. Thus,

there is set up on the screen M a substantially permanent record of the signal modulations applied to the grid 1, and the location of these stored signals on the screen M will be determined of course by the instantaneous correlation between the signal modulation applied to grid 2' and'the corresponding instantaneous position of the scanning cathode-ra beam from gun 5. For convenience of description, this function of. the cathode-ray beam and ionic stored therein.

4 screen will be referred to as the writing function of the tube. The record which is thus stored on the screen it will remain for an indefinite time, for example days or even months, unless it is erased.

In addition to the foregoing writing function, I have discovered that the secondary electron emission characteristics of the screen material I l vary in accordance with the opacity changes In other words, if the Writing beam from gun 5 has a current density between certain limits, e. g. between 50 micro-amperes and 109 micro-amperes, the corresponding 10- calized area of the screen It will set up a dark spot of the corresponding opacity, and this area will have a corresponding secondary electron emission characteristic. However at those 10- calized areas of the screen l4 wherethe scanning electron beam is cut off, the light transmission or absorption characteristics of the screen material i5 will be unchanged and it will have a different secondary electron emission characteristic. Thus for values of beam current between the predetermined limits above noted as examples, the scanned areas of the screen 14 will be correspondingly darkened and will have corresponding secondary electron emission prop erties.

It has been found that the best results for recording or writing, are obtained where the beam current from the electron gun 5 has a current density confined within certain limits, for example between 50 micro-amperes and microamperes. For this purpose, the control grid 1 is connected to cathode 6 through a D. C. bias source '23 and through switch 22 having three separate positions A, B and C.- In position A of the switch, the grid '5 is biassed with respect to the cathode 6, so as to produce a predetermined maximum current density in the electron beam as it strikes the screen 14 for writing or recording.

When the signal intelligence has thus been stored on the screen Hi, it may be necessary to read it any desired number of times without substantially changing the stored signal condition within the various localized areas of the screen it. For this purpose, the switch 22 is moved to position B, wherein a higher negative bias potential is applied to the grid 1 and at the same time the source it is disconnected from the said grid by the switchZd. With the switches 22 and 24 in this position, the beam from gun 5 continues to scan the screen It, but because of the very much lower current density in the beam, it does not cause any change in the opacity conditions previously set up in the screen It as a result of the writing beam. However, since the various areas of the screen Ht have difierent secondary electron emission ratios and conduc tivities for the darkened portions as compared with the light portions, as the reading beam scans the screen it, there will be produced across the resistor i5 corresponding potential variations which appear at the output terminal 2 i. For this reason the grid or secondary electron collector It is positively biassed by 50 or 100 volts with respect to plate 53, thus "pulling out the said secondary electrons. Consequently, there will be produced at the output terminalZl, potential variations in timed coincidence with the scanning action of the electron beam from gun 5 and in magnitude correlation with the signal-modulated opacity conditions of the screen It as previously controlled by the signals from source l5.

asses-1:7

:One. explanation :oilthe. action oi .thereading electron. beam. as itsca-ns the pattern isas follows. m it crossesa :.light.or relatively nonopaque. area. there may .for example be asecondary electron emissicmratio of .two. there are ten imicro-amperes inthe beam, twenty micro-amperesgo to-the collector grid 16. Next, consider it crossing adark or relativelyopaque area. Here the secondary .emission ratio might be three, and therefore thirty micro-amperes will go to the collector grid H3. .d fifrerence in" current to the grid It, gives a -.difference of voltageacrossthe load resistance 1-9 connected to it, and this is. the output signal. It should be be observed, however, that secondary emission 7 does not play any role in thewriting function or in the erasing function, but only comes into play during the reading .function.

When it is desired to erase the previously, stored signals in the screen It, the switch .22 is moved to position C, wherein the current density in the beam from gun- 5 is increased well beyond that required for efiecting the. writing .as above described. In this position, the grid 1 is biassed with respect to. the cathode 6 to such an extent that the current density of the beam as it strikes the. screen it, heats-the plate i3 and the. screen t4 ,sumciently to cause the previously stored opacity conditions to be immediately erased. For example, when in position C, the scanning beam may have a current density of the order of 5 milliamperes for theerasing action.

Fig. 2 is a graph showing the relative magnitudes of .the respective beam currents for. writing or storingthe signalson screen [4; for reading these stored signals without affecting their stored condition; and for effecting erasure .of the previously stored opacity conditions.

Fig. 3 shows. av modification of Fig.v 1, wherein the signals are read ofi from the storage screen it by means of an additional grid .25 which. is located between the. grid ,l'fi andthescreen It. In this embodiment, the screen M can be applied directly to. the interior face of the end wall. '6 of the tube, thus. eliminating the metal backing plate l3 ofFig. 1;. The, grid-'25 is positively biassed with respect to thegrid it by means of a suitable D. 'C'. source such. as schematically represented by the battery 26.. As the screen M is scanned'by the reading beam as above described, the diiierent secondary electron emission characteristics of the areas thereof corresponding to the signals recorded thereon, appear as corresponding voltages across the resistor is which can be coupled through the condenser to the output terminal 21 as above described. The remaining elements of Fig. 3 can be identical with those of Fig. l and further d'escriptionthereof is not deemed necessary, and these corresponding elements are designated alike in both embodiments.

Fig. 4 shows another modification of Fig. 1, wherein the ioniccrystal screen l4 instead of being, applied to an opaque metal plate. such as the plate 13 (Fig. 1.) is. applied to a thin. sheet. of mica or glass 28 with an inter-leaved thin transparent metal layer 2?. Thus the side of mica sheet 28 nexttothecoating. is. has applied thereto a. coating 2.101. a conductive. metal such as aluminum of sufilcient. thinness. to. be light permeable, .and this. sheet. is. connected to the resistor i9 and coupling condenser 2d and thence, to the. output terminalZ i. Here again. the elements 21 and 28 are oi sufiicient thinness. andof such low heat capacity, so that when they are. bombarded by the. higncurrentdensity erasing beam theyheat :quickly. Theiremaining elements of t can guni, another electron. gun .28 having the elec.

tron-emitting cathode .30, beam intensity. control grid 3|, ifirst beam. focussing and accelerating anode 32, and thesecondlbeam focussing and accelerating anode 33.... In addition, if desired, the gun 29 may be provided withthe usual beam defiector'plates. 34, 35, which are supplied with the usual saw-tooth voltages for causing the electronsbeam to scan the ionic crystal screen 14 in any desired scanning pattern. However, for erasing purposes, the control grid 3| is arranged to be directly grounded by means of a switch 36, and the gun .29 is designed so that when the switch 36. is. closed, it. projects on to the screen It an electron beam of yery great current density as compared with the beam from gun 5, thus enabling the previously stored record onscreen M to be erased. In Fig. 5, the gun 5 and itsassociated electrodes function in th same manner as the corresponding electrodes and parts of Fig. 1, to effect the original-writing and reading operations. .Since the erasing action ofgun 29 is to bombard the screen with a very highintensity beam, it is not necessary that the'beam fromv gun 29 be moved in a pointsby-poi-nt fashion overscreen. IA. If desired, the deflecting plates .34, 35, can be omitted or not used, and the beam from gun 29. can be produced so that when it is turnedcon .by the closure of switch 36, it completely covers the screen M with the desired high intensity .beam. If, however, the erasing beam covers. the. screen. [4 in a point-by-point fashion, the. plates; 34, 3.5, can be excited from the same saw-tooth scanning voltage source (l2, Fig. 1

butthrough; asuitable delay network (not-shown) so that. the erasingibeam'can follow the writing beam'at any. desired time interval.

mall the :foregoing embodiments, reliance is placed upon. the heating of the. ionic crystal screen :by the .electron-..bombardment to efiect the erasure. If desired, this erasing action can be supplemented byzapplyine. othe screen a separate heating current. .Thus, as shown in Fig. 6, the metal-backing plate 1.3 canbe made of high resistance material, and opposite sides. of the plate can be connected by the conductors 31, 3-8., and

through a-switch 39, toe-current supply 40, for passing a heating current through the plate 13 when the previously-stored signals are to be erased. The remaining elements of Fig. 6 which aredesignated the same as those of Fig. 1, function in the same w y, and detailed description thereof is not necessary. If desired, the switch arm of switch 39 canbe mechanically linked to the switch arm of switch 22, so that when switch 22' is. in theerasing position 0, the heating source 40 is also connected in circuit with'the backing plate-l3.v

Fig. 7 shows a further modification of Fig. 1, wherein the erasing action is controlled in part by the-heating. of the backing plate l3 as a result of the impingement thereon of the electron beam as-above described, and'in part by the development of an electric field across the crystal screen I 4. For this purpose, there is applied to the crystal screen on the side facing the electron gun, a thin layer M of metal which is of sufficient thinness to be transparent to the electrons from the gun. The plate 53 is negatively biassed with'respect to the coating and collector grid l6 so as to develop a potential gradient across the crystal screen it. of sufiicient magnitude when subjected to the erasing beam, so that this po-v tential gradient augments the erasing action produced by the heating of the plate M as a result of the electron bombardment. If desired, an additional potential may be impressed across the elements I3 and M by means of the switch 42 and battery 43. The switch 42 is arranged to be closed when the switch 22 is in position C, and for this purpose the switch arm of switch 42 may be mechanically ganged to the switch arm '0 switch 22. While certain embodiments have been described herein, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention. Furthermore, while the invention finds its primary utility where the ionic crystal screen has its opacity or light transmission characteristics changed by a signal-controlled cathode-ray or electron beam, it will be understood that the screen may have its opacity conditions varied by signal-controlled beams of other electromagnetic radiations, such for example as X- rays, cosmic rays, or other high energy particles. In other words, the invention comprises within its scope, a dark trace screen of the ionic crystal type which can be scanned by any beam of electrons, electromagnetic radiation or high energy particles, to vary the localized opacity conditions of the screen and concomitantly therewith the secondary electron-emission characteristics, for. the purpose of enabling the stored signals to be read.

Referring to Fig. 8, there is shown another modification wherein the source id may consist of infra-red radiations. 'For example, it may consist of the infra-red radiations reflected from an object which is to be viewed under adverse visibility conditions, in which event the original infra-red radiations are projected upon the object and the reflected radiations, represented 'by the arrows in Fig. 8, are used to produce a visible image. The image-producing means comprises an evacuated enclosing envelope 45 preferably of infra-red transmitting glass having an inclined side arm 155 within which is located an electron gun 41, comprising the usual electron-emitting cathode 48, the beam intensity control grid or electrode 49, first accelerating and focussing anode 50, second accelerating and focussing anode 5i, and the usual coordinate beam deflector ele-' ments comprising for example the horizontal beam deflector plates 52 and the vertical beam deflector plates 53. Suitably applied to the right hand inner wall of the tube 45 is an ionic crystal screen which may be similar to the screen l-i of FigsQl to '7. This screen may consist of a transparent backing layer of metal 54 applied to the inner face of thetube wall and with the ionic crystal material 55 applied to the trans: parent metal backing. Applied to the innensurface of the left-hand end wall of the tube 45 is a transparent backing of metal 55 such as transparent aluminum, and deposited on this metal backing is any well-known photo-emissive material such as is conventionally employed in photoelectric cells andthe like, and which emits photoelectrons when excited by infra-red rays. This constitutes an electron-emissive cathode and the quantity of photoelectrons emitted from each elemental area corresponds to the intensity of the infra-red radiation striking it. Suitably mounted within the tube 45 between the photoemissive cathode and the ionic screen 55, is any well-known electro-optical system for forming an electron image of the infra-red illumination of cathode 57 on to the screen 55. Merely by way of example, this electro-optical system may comprise a pair of cylindrical ring- like electrodes 51, 58, which are connected to suitable respective positive potentials with respect to the cathode 5'! so as to accelerate the photoelectrons with the required velocity to the screen 55, to project thereon a corresponding electron image wherein the electrons havesufiicient velocity to produce corresponding color centers in the screen as above mentioned. Preferably, the screen 55 through its metal backing 54, is connected to the positive terminal 55 of the'D. 0. power supply which also supplies to electrodes 51 and 58. Consequently, there is set up on the screen 55 a stored image of the infra-red radiation from the source 55. During the setting up of the image in the ionic screen 55, the electron beam from gun 41 has a sufficiently low beam intensity so that it does not affect the stored image in the screen 55. When it is desired to erase the stored image, the bias on control grid 50 with respect to cathode es, is changed so that the intensity of the beam from gun (a? is of sufficient magnitude as above pointed out, to effect erasure of the stored image in screen 55. It will be understood that the defiector plates 52 and 53 of gun 41 are energized by suitable scanning supply voltage so as to subject the screen 55 to the erasing action of the beam in any desired scanning pattern or in any desired region. If desired, the plates 52 and 53 can be omitted and the screen 55 can be subjected to an erasing beam of electrons of the required intensity which beam covers the entire or any other desired portion of the surface of screen 55. Instead of using a high intensity beam of electrons from a separate electron gun to effect erasure, the photo-emissive cathode 51 can be subjected to a very intense light to emit electrons of sufficient number and/or velocity to effect the erasure. Likewise, a very high electron accelcrating potential can be applied to the photoelectrons to impart to them sufficient velocity for erasure purposes. For this object, a separate high voltage electron accelerating electrode (not shown) can be located between the electrode 58 and the screen 55.

Referring to Fig. 9, there is shown how the invention can be used to record X-ray pictures and the like. In this figure, the block 60 represents any well-known source of X-rays which are to pass through the object 6| of which an X-ray picture is to be produced. For reasons well-known in the X-ray art, it is desirable to employ under certain conditons, X-ray radiations of extremely 1 high intensity but for physiological reasons it is also desirable to have the excitation interval of extremely short duration. Fig. 9 shows an arrangement whereby an optically visible image of the X-ray radiation can be produced regardless of its duration. For this purpose, there is provided an evacuated glass tube 62 having applied to the enlarged end 63 and interiorly thereof, an ionic crystal screen 64 which may be similar to the screen [4 of Figs. 1 to '7. The ionic crystal material can be deposited on a thin transparent 9 layer' of" metal 65 which has been previously applied to the inner surface of wall 63. Mounted in the neck portion 86 of tube 62 is an electron gun 61 which may be similar to the gun of Figs. 1 to '7. It has been found that when the X-ray radiation from the source 60 after passing through the object 61, strikes the ionic crystal screen 64, there is produced a color center image of the X-ray radiation. During this recording of the X-ray image on thescreen 64, the electron beam from gun '6'! is either cut off or is below the above-mentioned minimum intensity so that it does not aifect the stored image on the screen 64. When it is desired to reproduce the image,

the control grid 68 of gun 61 is biassed with respect to the cathode 69 so as to produce a beam intensity corresponding to the point B (Fig. 2). Located adjacent the screen 64 is a collector grid electrode which is positively biassed with respect to the metal backing plate 65 by means of a suitable D. C. source H connected in circuit with the resistor I2. The grid I0 is connected through a coupling condenser 73 and thence through a suitable amplifier 14 to the control grid 15 of a television image reproducing tube 16, having the usual electron gun 11 and the usual fluorescent screen 18. The deflecting voltages applied to the deflecting system 19 of gun 51 are synchronized with the deflectin voltages applied to the deflecting system 80 of gun 11, in the manner well-known in the televsion art. As explained above, the grid Ill acts as a collector of secondary electrons emitted from the screen 64 when it is subjected to the reading beam from gun 61. The magnitude of this secondary electron current, I have found, is correlated with the condition of the color centers set up at the various elemental areas of screen 64 as a result of the impinging X-ray radiation, and as pointed out above, the intensity of this reading beam is sufficiently low so that it does not affect these color centers whi e the beam is undergoing its scanning or reading action on the screen 64. Th re is thus set up on the fluorescent screen 18 a visible picture of the stored image on the ionic crystal scren 64.

It will be understood that the expression light transmi sion as employed in the claims, is not to be limited to an arrangement wher in light waves strike one side of the ionic crystal screen, and are viewed from the opposite side. This expression is used in a generic sense to cover any arrangement. wherein the incident light waves are viewed either as a result of passage entirely through the body of the ionic crystal screen or by reflection therefrom, since it has been found that the color centers also act as light absorbers for incident li ht waves.

While reference has be n made herein to the controlling of the intensity of the elect on beam to effect the writing, reading and erasing functions, it will be understood that the te m intensity is used in a generic sense. The various functions of writing, readin and erasure can also be controlled by correspondingly varying the velocity of the electrons in the beam. In other words, instead of applying the reading and erasing potentials to the control grid 1, the potentials of the accelerating anodes 8 and 9 can be lowered to the required value to efiect reading and can be raised to the required value to effect erasure. It is understood, therefore, that the term intensity as used in the claims, is intended to cover the controlling of the various functions either by controlling the bias on the intensity control electrode or grid, or by controlling the potential on the electron accelerating electrodes, or by a combination of both grid control and accelerating control, whereby the number of electrons arriving per unit area, per unit time, at the ionic crystal screen can be controlled.

What is claimed is:

1. Electric signal storage apparatus, comprising, a signal storage surface composed at least in part of a stratum of material which changes its light transmission properties in localized areas when impinged upon by a signal-controlled beam f electromagnetic radiation, means to project said radiation on said surface within predetermined limits of radiation intensity corresponding to said signals to translate said signals into corresponding opacity conditions in said surface, and means to increase the intensity of said radiations to restore said surface to its original light transmission properties.

2. Electric signal storage apparatus, comprising, a signal storage surface composed at least in part of a stratum of ionic crystals whose opacity varies in accordance with the intensity between certain limits of an impinging electron beam, means to develop a signal-controlled electron beam, means to project said beam on said surface to store said signals therein in the form of opacity changes, and means to increase the intensity of said beam to erase said stored signals.

3. Electric signal storage apparatus, comprising, a cathode-ray tube having an electron gun for developing an electron beam, a screen upon which said beam impinges, said screen being composed at least in part of a layer of ionic crystals which change their light transmission properties when impinged upon by said beam when A between certain limits of beam intensity, means to control the intensity of said beam within predetermined lower and upper limits and corresponding to signals to be stored on said screen, and means to increase the intensity of said beam beyond said upper limit to erase said stored signals from said screen.

4. Electric signal storage apparatus, comprising, a cathode-ray tube having an electron gun for developing an electron beam, a screen upon which said beam impinges, said screen bein composed at least in part of material which changes its light opacity conditions when impinged upon by said beam, means to control the intensity of said beam within predetermined lower and up-- per limits and corresponding to signals to be stored on said screen, means to decrease the intensity of said beam below said lower limit to produce a reading current corresponding to said stored signals, and means to increase the intensity of said beam beyond said upper limit to erase said stored signals from said screen.

5. Electric signal storage apparatus, comprising, a cathode-ray tube having an electron gunfor developing an electron beam, a screen upon which said beam impinges, said screen being of the dark trace type for storing opacity conditions in response to a signal-controlled impinging electron beam, means to control the beam current between predetermined lower and upper limits and corresponding to signals to be stored as opacity conditions in said screen, means to decrease said beam current below said lower limit to produce a reading current corresponding to said stored signals but without substantially affecting their stored character, and means to increase the intensity 'of said beam current beyond said upper limit to erase said stored signals.

6. Electric signal storage apparatus according to claim 5, in which the electron gun is provided with means to move the said beam in a predetermined scanning pattern over said screen 7. In combination, a signal storage screen of opacity controllable ionic crystals, an electron gun for developing a beam of electrons, signal responsive means to control between predetermined upper and lower limits the beam current from said gun reaching said screen to effect corresponding changes in said opacity and thereby to store the signals, means to control the intensity of said beam current below said lower limit to read said stored signals, and means to control the intensity of the beam current above said upper limit to erase the stored signals.

, 8. Incombination, a signal storage screen of opacity controllable ionic crystals, an electron gun for developing a focused beam of eectrons and projecting it on to said screen, said gun having means responsive to received electric signals for controlling the beam current between predetermined lower and upper limits of beam current to effect signal storage in the form of corresponding opacity conditions in said screen, means to bias said controlling means to cause the beam current to fall below said lower limit and thereby to produce a reading current representing said opacity conditions, and means to bias said controlling means to cause the beam current to rise above said upper limit and thereby to eiTect erasure of said stored signas in said screen.

9. The combination according to claim 8, in which the said control means includes a beam intensity control electrode, and said biassing means includes a source of biassing potential and switch means for selecting said reading bias potential and said erasing bias potential.

10. In combination, a signal storage screen of opacity controllable material, an electron gun for developing a focussed beam of electrons and projecting it on to said screen, means to vary the beam intensity between predetermined lower and upper limits and corresponding to signals to be stored as opacity conditions in said screen, and means to produce reading currents corresponding to said stored signals without substantially affecting the said stored opacity conditions; the

last-mentioned means comprisingmeans to project on to'said screen an electron beam of an intensity below said lower limit, electrode means 7 located in spaced relation to said screen, and circuit means biassing said electrode means with respect to said screen to produce a reading current controlled by the secondary electron emission from the areas of said screen at which said signa s are stored.

11. In combination, a signal storage screen which has the property of changing its opacity conditions in localized areas in response to an electron beam impinging thereon between predetermined lower and upper limits of beam intensity and which also changes its secondary electron emission properties in correspondence with said opacity changes, means to develop a beam of electrons and to project said beam on selected areas of said screen, means to vary the beam intensity between predetermined lower and upper limits and in correspondence with signals to be stored in said screen, means to derive a reading current from the signals as stored in said screen; the

last-mentioned means including a pair of electrodes mounted in spaced relation adjacent said screen, means to project on to said screen a reading electron beam of an intensity below said lower limit, and means to bias said pair of electrodes relatively to each other to produce under istics of the said localized areas of saidscreen.

12. The combination according to claim 10, in which said electrode means comprises a conductive plate to which said screen material is attached, and an electron permeable electrode, said permeable electrode being positively or negatively biassed with respect to said plate.

13. The combination according to claim 10, in which said electrode means comprises a pair of spaced electron permeable electrodes located between the gun and screen, the permeable electrode nearest the screen being positively or negatively biassed with respect to the other permeable electrode. r

14; The combination according to claim 10, in which said screen material is applied to a plate of light transparent electric insulation material, and said electrode means includes a layer of metal on the face of said insulating plate next to said screen material, a grid mounted in spaced relation to said screen, material, and means to bias said layer of metal negatively with respect to said grid, and a signal output circuit capacitively coupled to said layer of metal. 7

15. An electric signal storage tube, comprising,

an evacuated enclosing envelope, means to develop within said tube a beamof electrons, a layer of material upon which said beam impinges, said material having the property of changing its opacity in response to impinging electrons, a conductive plate and an electron permeable electrode between which said layer is located,

said material having the property of changing its opacity in response to impinging electrons, a pair of electron permeable electrodes located in the path of said beam before it impinges on said layer, means to bias the electrode adjacent said layer negatively with respect to the other electrode, means to control the intensity of said beam at different levels of beam current corresponding respectively to the recording of said signals on said layer in the form of opacity changes therein, for reading the stored signals, and for erasing the stored signals. 7 V

17. An electric signal storage. tube, comprising, an evacuated enclosing envelope, means to develop within said tube a beam of electrons, a layer of material upon which said beam impinges, said material having the property of changing its opacity in response to impinging electrons, a backingplate of insulating material to which said layer is attached, a backing of metal on the opposite face of said backing plate, a grid electrode mounted in spaced relation to said layer, means to bias said metal backing negatively with respect to said grid, means to control the intensity of said beam between predetermined lower and upper limits to effect signal recording in said layer, means to control the intensity of said beam below said lower limit to control the reading of the signals stored in said screen, and a reading signal responsive circuit capacitively coupled to said metal coating.

18. Electric signal storage apparatus, comprising, an evacuated cathode-ray tube, a conductive backing plate within said tube on which is applied a layer of material which stores opacity changes in response to an impinging electron beam, an electron permeable electrode mounted adjacent said layer, means biassing said backing plate negatively with respect to said permeable electrode, a first electron gun for developing an electron beam having a beam current intensity between predetermined lower and upper limits, means to modulate the intensity of the beam from said first gun to effect stored opacity changes in said layer, means to lower the intensity of said beam below said lower limit to produce reading currents corresponding to said stored opacity changes, a reading current circuit capacitively coupled to said plate, a second electron gun having means to develop an electron beam of an intensity much higher than the first-mentioned beam to effect erasure of said stored opacity changes.

19. Electric signal storage apparatus according to claim 18 in which switch means are provided for rendering said two guns selectively efiective on said layer.

20. Electric signal Storage apparatus, comprising, a signal storage surface composed of a material which stores opacity changes in localized areas when impinged upon by a signal-controlled beam of electromagnetic radiation, a conductive backing plate for said material, means to project said radiation on said storage surface within predetermined limits of radiation intensity corresponding to said signals to translate said signals into corresponding stored opacity conditions in said surface, means to increase the intensity of said radiations to erase said stored opacity conditions, and means to pass a heating current through said plate to assist said radiations in erasing said stored opacity conditions.

21. Electric signal storage apparatus, comprising, a signal storage surface composed of a material which stores opacity changes in localized areas when impinged upon by a signal-controlled beam of electromagnetic radiation, means to project said radiation on said surface within predetermined limits of radiation intensity and corresponding to said signals to translate said signals into corresponding stored opacity conditions in said surface, a pair of conductive electrodes between which said surface is located, means to increase the intensity of said radiations to erase said stored opacity conditions, and means to apply a potential gradient across said electrodes in timed relation with said increase of intensity to supplement said radiations in erasing said stored opacity conditions.

22. The method of electric signal storage, signal reading, and signal erasure, which comprises, applying the received signals to modulate an electron beam, applying said beam with a beam current intensity between predetermined lower and upper limits to produce corresponding opacity changes in an ionic crystal layer, lowering the intensity of said beam below said lower limit to translate said opacity changes into corresponding reading currents, and then increasing the intensity of said beam above said upper limit to erase said stored opacity changes.

23. Apparatus the type described, comprising a source of infra-red radiations, an evacuated tube having a photoelectromemissive cathode upon which said radiation impinges, a screen Within said tube and including a layer of ionic crystals which develop color centers in response to impinging photoelectrons, an electron optical system in said tube for acting on said photoelectrons to project them on to said screen to produce a stored image of the inira-red radiations thereon, and an electron gun for projecting on to said screen an electron beam above a predetermined minimum beam current density to erase the said stored image.

24. Apparatus or" the type described, comprising a source oi X-rays, an evacuated tube having therein a screen including a layer of ionic crystals which develop color centers in response to the X-rays irom said source to produce a stored image of said X-rays, and an electron gun within said tube for projecting on to said screen an electron beam above a predetermined minimum beam current density to erase the said stored image.

25. Apparatus of the type described, comprising a source of Y-rays, an evacuated tube having therein a screen including a layer of ionic crystals which develop color centers in response to the X-rays from said source to produce on said screen a st red image of said X-rays, and means to develop within said tube an electron beam having a beam current between predetermined minimum and maximum intensities to produce reading curre' ts corresponding to said stored image but without effecting the stored character thereof. I

26. Apparatus according to claim 25 in which said tube is provided with means to develop a beam of electrons of an intensity much higher than said maximum intensity to effect erasure of said stored image from said ionic screen.

27. Electric signal storage apparatus, comprising, a signal storage surface composed at least in part of a stratum of material which changes its light transmission properties in localized areas when impinged upon by a signal-controlled beam of electromagnetic radiation, means to project said radiation on said suriace within predetermined limits of radiation intensity corresponding to said signals to translate said signals into corresponding opacity conditions in said surface, and means to increase the intensity of said radiations for controlling the restoration of said surface to its original light transmission properties.

ALBERT M. SKELLETT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Ross Sept. 21, 1948

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