US2288402A - Television transmitting tube - Google Patents

Description

June 3@ w42. H. A. lAMs 2,288,402

TELEVISION TRANSMITTING TUBE Filed Jan. 29, 1.9411,v 2 SheetsfSheet 1 Je .30, 19452. Y H. A MMS 2,288,402

TELEVISION TEANSMIT'EIVNG TUBE Filed Jan. 29. i541 2 sheets-sheet 2A ATTORNEY.

Patented June 30, 1942 Harley A. rams, summit, N. J., mignon nadio Corporation of America, a corporation of Delaware yApplitauon anuary 29, 1941, serial No. amiss isciaims.

tubes vutilizing low lvelocity electron scanning beams and incorporating electron multiplying arrangements.

In my U. S. Patent No. 2,213,177, issued August 27, 1940, I disclosed several forms of television transmitting tubes utilizing limited area secondary electron multiplying arrangements wherein the electron beam was projected toward a target of the electrostatic charge storage type, modulated in accordance with the electrostatic charges representative of an optical image, and returned to the electron multiplying structure along paths raised or lifted from the paths of the beam as. it approached the target. During the return of the beam itis inuenced by deecting elds operating to absorb the deection imparted to the beam in its passage toward the target. The arrangements disclosed in my above referenced patent require careful design and have certain inherent limitations as to size and useful areaY of the target and in addition the tube structure inherently introduces some distortion which I have found possible to eliminate by following the teaching of my present in-` vention. l

Objects of my invention are to provide a structure wherein more uniform secondary elecy tron amplification of electronic picture signals may be obtained, a structure wherein distortion produced by non-uniformity of secondary emitvting surfaces is minimized, a simplied symmetrical structure in combination with means for secondary electron amplification of electron picture signals, a tube wherein the image area is not limited by the necessity for widely diverging electron paths, a structure wherein electrodes of such form and shape as to minimize scanning distortion are utilized and a structure wherein the overall length of the tube may be reduced.

In accordance with my invention an electrostatic image corresponding in electrostatic en ergy distribution to an optical image is formed on a target electrode preferably of the photosensitive mosaic type which is scanned in one direction with a low velocity electron beam from an electron source, the scanning being accomplished by a combination magnetic-electrostatic field, the electrostatic eld generating means being slightly offset from the center of the useful target area, the electrons of the beam then being scanned in the other direction'and substantially simultaneously lifted to the center of the useful target area and returned in the gen- (Ol. Pls-7.2)

My invention relates to television transmitting `rtubes and is more particularly directed .to

eral direction of the electron source to a point intermediate the nrst scanning means and the target to an electron multiplying arrangement eiiective over the .width of the target in thedirection of the first scanning. More particularly, in accordance with my invention I provide a structure utilizing electron multiplying arrangements having an effective length at least equal to and a physical length preferably greater than the dimension of scanning in one direction as measured at the target surface. Y

The objects mentioned above and other lebjects, features and advantages will be apparent from the'following description taken in connec-Y tion with the accompanying drawings in which: Figure 1 is a longitudinal'cross-sectional view of the tube and circuit embodying tion;

Figure 2 is a cross-section of the tube shown in Figure 1 taken along` lines 2-,25 and Figure 3 shows a portion of the tube of Figure 1 and a circuit particularly suitable for use with sucha tube.

In general, the apparatus of my invention comprises an evacuated envelope having a target preferably` of the photosensitive mosaic type at one end thereof and an electron source and anode slightly oiiset from the effective center of the target at the opposite end of the envelope. The target, if of the mosaic type, is provided on its front surface with an extremely large number of small photosensitive particles and is so positioned lthat it may be scanned by an electron beam from the source and may also have focused thereon an optical image of the object of which a picture is to be transmitted. -The lected or rejected by the `trget in accordanceI with` the electrostatic chart s thereon. Thus those areas of the 4target whici are more highly illuminated acquire the more positive electrostatic charges with` respect to the nilluminated particles and these positive charges 'which represent an electrostatic image of a pic :re to be transmitted are neutralizedby a portion of the electrons comprising the beam, the remaining electrons being reflected from the target. In

my invenl y and deflect the beam in another direction, an

`lm 6 to maintain this film at cathode potential termediate the electron source and the targef and preferably adjacent the source I provide means to electrostatically deflect the electron beam in combination' with a longitudinal magnetic field without introducing distortional effects, I have foundin accordance with my invention that I may place the electron multiplying structure intermediate the electrostatic deflection means and the target and thereby utilize a deflection structure which will not introduce defocusing or distortion of the electron beam. Likewise, intermediate the-electrostatic deflection means and target and preferably over the cusing coil Il wholly enclosing and preferably extending beyond the space between the cathode A Il and mosaic electrode 2. This focusing coil is so-designed as to provide a field strength in an axial direction of about 50 to 75 gausses which li` have found `sullicient to' 'maintain thev electron beam in a focused condition. 'Ihe eiIect of the magnetic field is to cause the electrons which are emitted by the cathode with initial transverse velocities to follow helical paths or trajectories of small amplitude so that while I have referred to the beam as being in focus throughout the length of the beam path, this is not literally same portion of the electron beam paths I provide means to lift the beam and simultaneously deflect the beam in a direction substantially normal to the direction of deection imparted to the beam bythe electrostatic means. Thus my new and improved television transmitting tube comprises structurewhich from` the electron source,- and anode end of the envelcpe comprises in the r following order, electrostatic means to deflect the beam in one direction auch as the direction of horizontal deection with enclosing shield electrodes if desired, electron multiplying means extending in the direction of vdeilection by a length at least as great as the deflection produced by said electrostatic means, separate means to simultaneously lift the electron beam electrode to control the electron beam in the vicinity of the target and thel target at the opposite end of the envelope from the electron source.

Referring particularly to -my tube structure as shown in Figures 1 and 2, the tube structure includes an elongated evacuated envelope I enclosing at one end a target or mosaic electrode 2 and at the opposite end an electrode assembly I adapted to project electrons toward the mosaic electrode such as along'the path I shown by the dashed line. The mosaic electrode 2 which faces the electrode structure 3 preferably comprises true, since the beam has a number of focal points along the path, the electrons actually following helical trajectories. The length of a single helix or pitch of vthe helical electron path may be varied by varying the strength of the magnetic field or by varying the velocity of the electron beam. The path of a beam under the influence of a magnetic field such as generated by the focusing coil I5 suddenly receiving large transverse velocity components from electrostatic deflection means is distorted and the electrons suddenly receiving such transverse-velocity components follow helical paths of great amplitude so that the electrons cannot be controlled or i'ocused at the target,

Tubes of the prior art, such as referred to in my above-mentioned patent and utilizing elecl to provide deflection means which avoid imparta substantially transparent sheet of insulation types such as photoconductive or photovoltaic targets may be utilized if desired.

The electrode structure 3 at the opposite end of the envelope I comprises a cathode I 0 from which electrons may be drawn, a control electrode II connected to the usual biasing battery and an anode I 2 having a beam defining aperture maintained positive with respect to the cathode I0 by a battery I4 to accelerate the electron beam such as along the path 4. The cathode I0 is connected to the mosaic electrode conductive to decelerate the electrom beam to substantially vzero velocity in the vicinity lof the target. 'I'he tron beam in a` focused condition between the electron source and the target I provide a foing to the beam large transverse velocity components. In accordance with my present invention, I provide the electron multiplying structure intermediate the electro-static deiection rr'vns and targets and can, therefore, use a preferable form of electrostatic deflection systerm.

Referring again to Figure l, I provide a pair of electrostatic deection plates cvomprising the plates 2li-2| directly adjacent the electrode structure 3 and aligned with the offset electron beam path 4, the plates 20 and ZI being flared outwardly from the electron path at each end thereof as described by Albert Rose and me in our U. S. Patent 2,213,175. Such a deflection structure is not wholly suitable for use in the tube shown in my first-mentioned patent because the electrons not reaching the target are redirected through a similar deflection plate structure to an electron multiplier so that if the plates such as the plates 20 and 2| are flared outwardly at each end, the returning electrons would impinge on the flared portions of these paths. The eil'ect of the flared plates is to provide an electrostatic deflecting field which first increases and then decreases in intensity along the path of the electron beam through the plates. Further in accordance with my invention and as shown best by Figure 2, I provide magnetic deflection means such as the coils 22--23 to deflect the electron beam in a direction perpendicular to the deflection produced by the plates 20-2I,v

and over the same portion of the beam path I provide means such as the beam lifting electrodes or "plates 24-25 to lift or raise the center of the scanning pattern to the center of the effective area of the target 2. The lifting or raising of the beam is imparted simultaneously with the deflection producedby the deection coils 22-23 and I have found that when the electrodes 2I-25 are formed as cylindrical surfaces a minimum of distortion is produced and the tube is easily constructed. Thus in operation the electron beam is rst deflected in a plane parallel with a plane midway between the deection plates 2li-2|; as along the path 4. The deiiected electrons are lifted, however, along the path 30 to a plane displaced from the plane of deflection and simultaneously deflected in a direction normal to that produced within the plates 2li-2l by the deflection coils 22-23. Electrons not reaching the target are similarly lifted or raised to a third plane while' returning in the general direction of the electron source as shown by the path 3l. The returning electrons are then intercepted by a secondary electron emitter 32 which extends in a direction normal to the drawing of Figure l for a length at least equivalent to the deflection produced by and within the deection plates 2li-2l. flection is equivalent to the effective width of the target and the beam is not displaced in this direction of deflection after passing through the deflection plates 2li-2|. I further provide secondary electron emitting electrodes 33 and 34, each of these electrodes likewise being of a length at least equal to the said length of deflection or target width, and an electron collecting electrode 35 of similar length beyound the electrode 34 to collect the secondary emission from the preceding electrode. Each of the electrodes 32-36 may be of silver which during theprocessing of the mosaic electrode 2 are oxidized and upon introduction of alkali metal such as caesium for purposes of photosensitizing the mosaic these electrodes become highly secondary electron emissive. All of the electrodes 32-35 are inl the form of elongated cylindrical surfaces. To maintain the electrodes 32-35 at proper operating potentials I provide a potential source or battery di! and potentiometer di to maintain the electrodes at progressively increasing positive potentials, the electrode 35 being connected to the positive end of the battery d through an output impedance d2 and to the input circuit of a thermionic amplifier d3 For optimum operation of the tube shown in Figures 1 and 2 I enclose the deection plates 2li-2l in a substantially field-free space and I therefore provide apertured electrodes 50-5I, one at either end of the deflection plates 2li-2l, the aperture 52 of the electrode 5D preferably being slightly larger than the maximum diameter of the electron beam and the` aperture 53 being elongated and of a length greater than the maximum deflection produced within the plates 2li-2l. The electrode `5I likewise serves as one shield member for the space occupied by the electrodes 32-35, the apertured shield member 54 forming the other shield member. I have found that the use of the shields as the lmember 5l and 54 is desirable to shield the electrodes 32-35 from the deflection plates and also to limit the collection .of photoelectrons by these electrodes. Electrostatic fields generated by the plates 2li-2l would otherwise produce non-unlform collection of beam electrons returning from the target. The shield member 54 may be provided with a single aperture to pass the electron beam as well as to pass the returning beam electrons, although this electrode may have two apertures as shown, the aperture 55 for the electron beam deflected in one direction by the plates 2li- 2| and the aperture 55 to pass the returning electrons to the secondary emitting electrode 32. Each of the apertures 55-55, as well as the This deaperture 53 as noted above, is elongated, the

length being at least as great as the effective target width which is equal to the deflection produced within thedeection plates 2li-2|.

^ The electrodes 50, 5I and 54 are preferably operated at the mean potential of the deection plates 2li-2|, such as at ground, which is the potential applied to the anode I2 so that a fieldfree space is formed within the deflection chamber in the absence of deflection fields, that is, between the electrodes .S0-5I and likewise within the electron multiplying chamber between the electrodes-'il and 54. Photoelectrons liberated from the mosaic electrode 2 under the influence of lightincident thereon are collected by the shield member 54 and to minimize photoelectron electron multiplication I provide the aperture 55, or the single aperture combining the function of both apertures 55 and 55, with a height equal to or less than 10% of the effective target height, these dimensions being in the plane of the drawing of Figure 1.

To provide a more uniform electrostatic field in the vicinity of the lifting plates or electrodes 24-25 I provide additional electrodes 60--5 likewise within the space partially enclosed by the deflection coils 22--23 and forming with the lifter plates or electrodes 24-25 a discontinuous circular cylindrical surface wherein the electrodes 2& 25 and SII-5| completely surround a portion of the electron beam paths except for adequate space between these electrodes provided for purposes of insulation.` These electrodes may be formed as conductive wall coatings as shown or may be metal plates'formed with cylindrical surfaces. The electrodes Sil-,5l are likewise operated at ground potential to maintain, in the absence of lifter plate potentials,.a field-free space within the lifting plate region.

The lifting plates or electrodes 24-25 are operated respectively above and below ground potentials such as by a. potential source or battery S5 across which I provide a potential divider 56. While I have shown the polarity of the battery 55, reversal of the magnetic iield generated by the coil i5 will necessitate a reversal of this polarity which is chosen in accordance with the field direction to lift the electron beam from the offset electron source and deflection plate structure to the effective center of the target area. this lifting being accomplished over that portion of the path designated withthe numeral '30. It is also desirable that the electron beam pass between the lifter plates in a region of constant potential, even though the beam is deflected horizontally by the deflection plates. I therefore prefer to apply a small -amount of the horizontal deflection potential across the lifter plates 24-25 so that the beam as well as the returning electrons while passing between the lifter plates passes through a zero potential gradient with respect to ground.

To accomplish this mode of operation I-connect the center of the potential divider 65 to a point on a second potential divider 51 connected across the horizontal deflection supply applied to the deflection plates 2li-2|. AWhile I have shown this connection as being madeA to the left of the grounded center tap of the divider 51, the connection is made to the opposite side of the center tap if the direction of the axial magnetic iield produced by the coil I5 is reversed. I have found that when the electron beam is in sharp focus at the first electrode, such as the electrode 32 of the series 32-35, any small variations in the secondary electron emitting properties of the electrode 32 over the extended length thereof produce blemishesfin the reproduced television picture. In the processing of secondary electron emitting surfaces it is very difficult to obtain surfaces which have uniform secondary electron emitting characteristics over the entire area thereof and I have found such inequalities produce a minimum of difficulty when the electron beam is in focus at the target, but the returning electrons are out of focus in the plane of the first multiplying electrode 32. I have already pointed out that the effect of a longitudinal magnetic field produced by the focusing coil I5 is to cause certain electrons to follow helical paths of small amplitude so that the beam has a number of focal points along the beam path, the electrons actually following helical trajectories. I have likewise pointed out that` the length of a single helix or the pitch of the helical electron path may be varied by varying the.

strength of the magnetic field or by varying the velocity of the electrons. I therefore prefer to locate the electrode 2 with respect to the other electrodes of the tube and'especially with respect to the defining aperture of the anode I2 to minimize the effect of variations in the secondary emission properties of the' electrode 32. I therefore locate the electrode 32 so that its secondary electron emitting surface is positioned to intercept that portion of the returning electron beam when it is out of focus with respect to the focused condition at the target. Thus the beam is out of focus at the surface of the electrode 32 when the distance measured in centimeters along an axial direction between the dening aperture in the anode I2 to this surface is expressed by the following equation:

where n is an integer, n is a fraction of n (preferably equal to the fraction 1/), V is the axial velocity of the electrons measured in volts (potential of battery I4), and H is the magnetic field strength measured in gausses. 'Ihus if the distancebetween' the anode I2 and the emitting /surface of the emitter 32 is made unequal to the length of an integral number of pitch revolutions of the beam along said axis the electrons are defocused upon reaching the emitter. Preferably this distance is equivalent to an integral number of pitch revolutions plus one-half of a pitch revolution along said axis so that said beam is in a maximum .out-of-focus condition when it impinges on the emitter 32. The beam may be focused upon the target irrespective of the out-of-focus condition in the plane of the emitter 32 parallel with the target without causing refocus of the returning electrons upon the emitter because the fields between the emitter and target are effective to the same degree on both the electrons ofthe beam approaching the target and on the electrons reaching the emitter 32. The net effect of the elds is thus to cancel any focusing action between the emitter and the target planes leaving the returning electrons in a defocused condition upon reaching the emitter 32. The operating parameters are therefore chosen to provide an out-of-focus condition of the electron beam at the secondary electron emitting surface of the electrode 32 and the beam may be brought to focus at the target by adjusting the potentials applied to the electrodes between the electrode 32 and the target. For this purby the alternating current voltage applied to the the electrode 32. The electrode II may be in the A form of a narrow coating of electrically conducting material deposited on the tube wall adjacent the front surface of the target. This electrode serves the additional purpose of providing a substantially uniform electrostatic field over the scanned area of the target. Such "a uniform field controls the rotation of the entire scanning pattern which rotation may be produced by the magnetic eld of the coil I5.

It is very desirable in the construction and operation-of the tube shown in Figures 1 and 2 to provide minimium collection of photoelectrons by the secondary electron emitting electrodes and especially by the electrode 32 inasmuch as the photoelectrons which may be collected at any instant of time are not representative of the desired picture signal. While serious difficulty from this cause may be avoided by the usev of an-en trance aperture such as thev width of the aperture 56 in the shield electrode 54 equal to or less than 10% of the target height, the arrangement shown in Figure 3 is suitable for eliminating the effect of photoelectron collection by the secondary electron emitting electrode 32. In accordance with this teaching of my invention, I separate the signal due to varying collection of photoelectrons from the signal due to the electrons of the beam returning from the target. In accordance with this teaching, I apply a, high frequency alternating potential to the metal film electrode 6 of the target electrode assembly such as by interposing an alternating potential source 'I0 in the connection between the electrode 6 and the cathode I0. The frequency of the alternating current potential source 10 is chosen to be greater than the maximum signal frequency pro- Y tions of the alternating current wave, but not on the negative portions. Thus, that portion of the electron beam not reaching the target but returning to the secondary emitter 32 is unmodulated during the time the beam is scanning unilluminated portions of the target, but this portion of the beam varies in amplitude at the impressed alternating current frequency during the time the beam is directed upon lighted portions of the target.- Meanwhile7 the photoelectron emission from the target is not interrupted electrode 6. It is obvious from the foregoing that the output such as applied across the output impedance 42 consists of three components, one with a modulated carrier wherein the amplitude of modulation is representative of the picture signal desired, another due to the remaining unmodulated beam current, and third component representa-tive of collection of photoelectrons, the signal for which suppression is desired. The third component has no carrier. To suppress the components without a carrier I provide a band pass filter 1I preferably interposed between the electron-collecting electrode 35 and the in- V *electron collection are eliminated.

While I have indicated the preferred embodiments of my invention with particular reference to a tube and system adapted tothe generation and control of signals representative` of images for which television is desired, it will be apparent l that my invention is not limited to the various embodiments herein set forth, but that many variations may be made in the particular structure used and the associated system as well as for the purpose for which my invention is employed without departing from the scope thereof, as set forth in the appended claims.

I claim:

1. A television transmitting tube comprising an evacuated envelope, an electron source, means adjacent the source to accelerate electrons in a direction longitudinally of said envelope, an extended area target having its eiective center oilset from the initial path of said beam, means adjacent said target to decelerate said electrons to a longitudinal velocity of substantially zero adjacent said target, a magnetic coil extending over the entire path between said source and target to generate a longitudinal magnetic field, a pair of deection plates adjacent said source and said means to deflect electrons from said source in one direction over the width of said target, means between said plates and said target to deflect the said electrons in a direction perpendicular to said target width, electron lifting means to lift the said electrons to scan the electrons over the effective height of said target in said perpendicular direction, a secondary electron emitter having a length at least equal to the eective width of said target positioned between said deflection plates and said target to liberate secondary electrons when impinged by a portion of said electrons not reaching said target and an electron collecting electrode likewise having a length at least equal to the effective width of said target and adjacent said secondary electron emitter.

2. A tube as claimed in claim 1 including a plurality of secondary electron emitters between said rst-mentioned secondary electron emitter and said collecting electrode.

the beam path between said electrostatic means and said target, means to simultaneously deilect the beam over the same said Vportion of the beam path in a direction normal to that produced by said electrostatic means, means to substantially prevent said electron beam reaching said target in the absence of light projected thereon, elongated secondary electron emitting i means positioned in a direction transverse to said center line, in a direction parallel to the direction of scanning across the width of said target, and between said electrostatic means and said beam lifting means to receive and multiply electrons of said beam not reaching said target and a collecting electrode adjacent said emitting means to collect secondary emission therefrom. l

4. A television transmitting tube as claimed in claim 3 including means between said/ electrostatic means and said emitterv to shield said emitter from the electrostatic eld generated by said electrostatic means.

5. Atelevision transmitting tube comprising an evacuated envelope', a cathode and anode to develop an electron beam, alight sensitive target of extended area oppositely disposed from said cathode and anode. means substantially coextensive with said target to prevent the electron beam reaching said target in the absence of light projected thereon. magnetic means to constrain the electron beam and direct it along a path normal to the extended area of said target, a pair of electrostatic denection plates associated with said magnetic means to deflect the beam from said path in a plane parallel with said plates, a pair of oppositely disposed electrodes partially enclosing a portion of the beam path between said plates and said target each of said electrodes having cylindrical surfaces with their vconcave surfaces facing each other, said electrodes when energized being adapted to lift the electron beam to a plane parallel with but spaced from said rst-mentioned plane to scan the target over the width thereof, a second pair of oppositely disposed electrodes having cylindrical surfaces adapted to be maintained at the same potential,

l said first-mentioned and second pair of elec- 3. A television transmitting tube comprising erating means wholly immersed in said eld and adjacent said anode to generateu a. deection eld which increases uniformly in intensity with increased distance from said anode over a portion oi' the beam path and decreases uniformly in intensity over another portion of the beam path to deflect the beam over the width of said target, beam lifting means to lift the beam to the eiective center oi said target over a portion of trodes forming a substantially closed cylindrical electrode system around said portion of the beam path, means effective over said portion of the beam path to deiiect the beam in a direction,

normal to that produced by said deflection plates and a plurality of secondary electron emitters having a length at least equal to the width of said target scanned by said beam in the iirst direction of deflection, said emitters being located between said plates and said substantially closed cylindrical electrode system whereby electrons of said beam not reaching said target are lifted to a plane displaced from said firstand secondmentioned planes and intercepting one 'of said secondary emitters. I

6. A television transmitting tube comprising an evacuated envelope, a photosensitive mosaic target of extended area in said envelope to liberate electrons in response to light, an electron source and anode oppositely disposed and offset from the effective center of said target to develop a electrostatic field of said plates being eilective.l

termediate said target and said anode to deflect said beamin one direction over the width of said target, a pair of electrodes intermediate said plates and target to lift electrons of said beam to impinge on said target across the eiective center thereof, a secondary electron emitter between said plates and said electrodes and at a predetermined distance from said anode to receive electrons of said beam not reaching said target, an auxiliary electrode adjacent said target to provide a substantially uniform electrostatic field over the surface of said target, means to generate a magnetic field extending in a direction parallel with said axis and over the space between said cathode and target to cause electrons emitted from said cathode with velocities transverse to said axis to follow helical paths toward said target, and sources of potential connected between said cathode and anode and between said cathode and auxiliary electrode oi such values that electrons of the beam describe an integral number of pitch revolutions along the helical paths while passing between the predetermined distance separating said anode and target and that the electrons of the beam describe a number of pitch revolutions equal to (n-l-n) while passing between the predetermined distance separating said anode and secondary emitter where n is an integer and n' a fraction of n, each of said predetermined distances being measured along the said axis.

l5. In a cathode ray tube system a tube having a cathode to emit electrons, an anode to accelerate the electrons along an axis and dene an electron beam cross-section, a light sensitive target oppositely disposed from said anode to receive electrons of the beam, an electrode in capacitive relationship with said light sensitive target, means to maintain said cathode and said electrode at substantially the same potential, deilection means to deflect the electron beam to scan said beam in one direction over said target, means to deflect the electron beam in a second direction over said target and a secondary electron emitter between said iirst deflection means and said target spaced from said anode in the.

direction of said axis by a predetermined distance to receive electrons of saidbeam not reaching said target, a magnetic coil surrounding said tube and extending over the space separating said anode and said target to direct electrons of said beam alon ghelical paths, means to maintain a difference of potential between said cathode and anode and means to energize said coil to produce a magnetic field measured in gausses substantially equal to where n is the pitch length of said helical paths, n' is a fraction of n, V is the potential difference in volts applied between said cathode and anode and d is the predetermined distance between said anode and said secondary electron emitter measured in centimeters along said axis.

16. In a cathode ray tube system a tube having a cathode and anode to develop an electron beam, an electron emitting photosensitive target in the path of said beam, means' to scan said beam over said target, means to -liberate lphotoelectrons from and develop electrostatic charges on said target representative of a picture to be transmitted to collect electrons in accordance with the magnitude of said charges, means to maintain said target at a potential with respect to said cathode insuicient to allow impingement of said beam upon said target in the absence of light thereon, means exposed to said target to receive electrons of said beam not reaching said target said electrons constituting a current representative of elemental charge areas of said electrostatic image, means to modulate the said current at a relatively high frequency without modulating the photoelectric current comprising the photoelectrons liberated from said target means to develop signals representative of said currents, and means to separate the signals developed from the current representative of said elemental charge areas from those representative of said photoelectric current.

17. In a cathode ray tube system a tube having a cathode and anode to develop an electron beam, a photo-emissive target to intercept and collect a portion of said beam, means to deflect said beam over the width of said target, means to prevent electrons of said beam reaching said target in the absence of light projected thereon electrode means to collect photoelectrons liberated by said target in response to light projected thereon and electrons of said beam not reaching said target, said means comprising an electrode having a width substantially equal to the width of said target scanned by said beam, means to deflect said beam in a direction normal to said first-mentioned deflection, means to modulate at a relatively high frequency the electron current consisting of the electrons of said beam not reaching said target without modulating the electron current consisting of said photoelectrons, means to develop signals representative of said currents and means to separate the signals developed from each of said currents.

18. In a cathode ray tube system a tube having a cathode and anode to develop an electron beam, a photosensitive mosaic to liberate photoelectrons in response to light representative of an optical image, an electrode in capacitive relation with said mosaic, an electrical connection between said electrode and said cathode to maintain said electrode at substantially cathode potential, means to deflect said beam in two mutually perpendicular directions over said mosaic, a secondary electron emitter exposed to said target to receive photoelectrons liberated from said mosaic and electrons of said beam not reaching said mosaic, means to maintain said cathode and said electrode at substantially the same directeurrent potential, means to apply a high frequency alternating potential between said cathode and said electrode to modulate thte electron current consisting of electrons of said beam not reaching said mosaic without modulating the electron current consisting of photoelectrons intercepted by said emitter, means to develop signals representative of said electron currents, and means to derive from said signals signals representative of only said first-mentioned current.

HARLEY A. IAMS.

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