US2368884A - Television transmitting apparatus - Google Patents

Description

Feb. 6, 1945. o. H. SCHADE TELEVISION TRANSMITTING APPARATUS Filed Aug. 2l, 1942 MTR MSN w en L w ww mw l ENTO o iz B MJ@ ATTORNEY Y across an impedance.

Patented Feb. 6, 1945 TELEVISION TRANSMITTING APPARATUS Otto H. Schade, West Caldwell, N. J., assigner to Radio Corporation oi America, a corporation of Delaware Application August 21, 1942, Serial No. 455,596

(Cl. P18-7.2)

19 Claims.

My invention relates to television apparatus and primarily to tubes for generating television signals representative of optical images. In its more specific applications, the invention is concerned primarily with providing ways and means for compensating for second order distortions which are introduced into the television signals produced by reason of the scanning of an electrostatic charge storage type of target or mosaic electrode in an electronic image transmitting tube which is subjected both to the light of an image from which the electro-optical image is to be Dro` duced, and to the action of a high velocity electron scanning beam used to discharge the mosaic electrode.

In a tube and system of the type to which the present invention is directed, it frequently happens that while it is possible and practical to generate and transmit television image signals in which there is substantially a complete absence of first order distortion, known systems are not entirely free from second order distortions. One objectionable second order distortion effect is that which is known in the art as "dark spot distortion. By dark spot distortion is meant the distortion appearing on the viewing plane at the points of reception of an image as a dark shading frequently distributed over random areas of the receiver observation screen and usually surrounded or bordered by lighter or ared areas.

In such television transmitting apparatus use is made of a cathode ray tube wherein there is provided means for developing and moving in desired co-ordinates an electronic scanning beam having relatively high velocity. The scanning beam is arranged to impinge upon and to traverse, under the control of suitable deecting voltages or currents, a target of the mosaic electrode type. In one type of transmitting tube the mosaic electrode includes a conducting signal plate which is connected in parallel with an output impedance to a first amplifying stage in such manner that the amplifier input circuit receives the signal energy in accordance with a varying voltage drop Supported directly upon or adjacent the signal plate is an insulating meinber or di-electric sheet, such as a sheet of mica, which supports photosensitive material capable of releasing primary photo-electrons in accordance with the light of the optical image impinging thereupon. The photosensitive material is formed as a series of minute-size particles, each electrically insulated one from the other, as well known in the art.

The optical image of which an electro-optical electrode and the release of the photo-electrons causes electrostatic charges to build up between the photo-electric particles and the signal plate. The stored electrostatic charges are neutralized or released when the cathode ray beam traverses and scans the photosensitive particles of the mosaic electrode. The charge accumulated and then released represents the light value of related elemental areas of the optical image and causes the production of signal modulations which constitute the video signals for transmission. However, during the time when the scanning electron beam is traversing the mosaic electrode, there develops a spurious or undesired signal which causes the above-mentioned phenomena of abnormally dark and light areas of the image on the reproducing screen of the receiver. Such dark spot signal is believed to result from the various electrical eld distributions occurring across the surface of the mosaic electrode, which cause the escape of photo-electrons and redistribution of secondary electrons released by the beam from the mosaic to be non-uniform.

I have' found that the sensitivity of target electrodes of the mosaic type is a function of the developed dark spotl signal and that the sensitivity is greatest over the areas where the dark spot is most intense. The collection of secondary electrons by these dark spot areas is greater than from flared or whiter areas and it has been proposed to reduce secondary electron emission to reduce dark spot. It is apparent, however, that this must be a selective variation in the flared or lighter areas only, which is diicult to accomplish. It has also been proposed to neutralize dark spot formation by other methods including use of compensating tubes, use of non-uniform mosaic formation, selectively sensitizing the mosaic, shaping the collector field by a frame surrounding the mosaic, and developing compensating shading signals either within the tube or by mixing such compensating shading signals with the image signals in the amplifying circuits. I have found that all of these compensating methods sacrifice the increased operating sensitivity obtainable over the dark spot area and reduce the sensitivity to that obtainable over the lighter or border areas.

Apparatus incorporating high velocity beam scanning tubes are also somewhat unstable in that during operation uctuations in response especially at low frequencies are produced causing streaking to appear in the recreated image replica. Thus immediately following and preceding dark image areas white streaks appear over the scanning line or lines following the dark image areas. Such instability occurs especially at high image brightness values incident on the target and incident on the envelope walls of the tube.

Accordingly, it is an object of my invention to provide a simplified apparatus and means for rendering second order distortion effects uniform in a reproduced television image. It is also an object to provide a single tube incorporating means for rendering the dark spot of substantially uniform and equal intensity and a tube and system which will not require the continuous readjustment of additional compensating tubes or circuits. Itis another object of my invention to eliminate fluctuations in the useful low frequency signal output orf high velocity pick-up tubes. It is another object to obtain a uniform and stable dark signal whereby the overall sensitivity of the tube and apparatus may be increased. A further object is to avoid the introduction of large shading signals in television apparatus and the variation of such signals with image intensity as well 25 as to increase the uniformity of the mosaic target in sensitivity and efciency. It is a still further object of my invention to permit the use of circuits and operating conditions most favorable for stable frequency response and high signal to noise ratio independently of image intensity and form variations.

These and other objects, features and advantages of my invention will become apparent when considered in view of the following description and the accompanying drawing wherein:

Figure 1 shows apparatus incorporating my invention and including a tube of the high velocity electron beam scanning type;

Figures 2 and 3 are sectional and plan views of the mosaic target structure shown in the tube of Figure 1, and

Figure 4 is a schematic circuit equivalent of the apparatus shown in Figure 1.

In the illustrative embodiment of apparatus made and operated in accordance with my invention and as shown in Figure 1, the tube comprises a highly evacuated envelope or bulb I of cylindrical shape having a neck section enclosing an electron gun 2 with an anode 3 to develop and project an electron beam on a target 4 o f the mosaic type more particularly described below. The target 4 is positioned to be illuminated by light representative of the image detail for which transmission is desired, the image being formed on the target by the lens system 5. I mask extraneous light from the object to prevent its reaching the target 4 or the wall of the tube, such as by a mask B interposed between the lens system and the envelope wall. The second anode 3 is supplied through a terminal 'I with operating potential by the battery or other potential source 8 and serves the dual purpose of -focusing streams of electrons from the electron gun as a beam on the mosaic target 4 as well as of collecting secondary electrons and photo-electrons from the target as well known in the art. Consequently, the second anode 3 may be termed a collector electrode and preferably includes a ring collector portion 9 surrounding a portion of the inner surface of the cylindrical section of the envelope.

In accordance with my invention, I obtain the above enumerated objects and advantages by utilizing a novel mosaic-type target electrode incorporating a. frame or guard band of photo-electric material and I so proportion this guard band with respect to the surrounding structure as to have a predetermined capacity thereto, and I then illuminate the guard band to produce thereon a predetermined potential in accordance with the image subject matter and image brilliance. Further in accordance with my invention, I expose my novel target electrode to portions of 10 the tube wall and I make these portions photoelectrically sensitive and illuminate them in a special manner to obtain a desired potential distribution, and I further control the spacing and size of the correlated target and wall structure with respect to each other and to a capacitively associated shield to provide a maximum and substantially uniform dark spot over the entire target area. and minimize or wholly eliminate fluctuations and deficiencies in the developed low frequency signal output of the apparatus.

Referring to Figures 2 and 3 which show a sectional and plan view of my novel mosaic target assembly, the foundation preferably comprises a sheet of mica I Il of approximately two to five thousandths inch in thickness. The sheet of mica I0 serves as a foundation for the mosaic electrode structure and is supported from its edges as well known in the art. The sheet of mica I0 is provided on one side by an opaque signal plate Il of predetermined area and extent with respect to a mosaic I2 of mutually insulated and separated silver particles which are `oxidized and photosensitively processed by introducing caesium within the envelope. The lmethod of manufacturing and sensitizing such photosensitive structure is well known in the art, although I prefer to use the method described by :Hickok in U. S. Patent No. 2,178,232 wherein a. vsource of silver or other metal is provided within 4Q the envelope for evaporation and condensation 'upon the mosaic. This source may be utilized in providing a foundation for other photosensi- .tive structure described hereinafter.

In accordance with my invention, I make the larea of the mosaic I2A substantially equal to the area of the opaque signal plate Il, and further- ;more, I position the mosaic I2 directly opposite the signal plate II. Further in accordance with my invention, I provide surrounding the mosaic and substantially coplanar therewith a guard or frame I3 of electrically conducting substantially transparent photosensitized material. The frame may be electrically continuous or divided "into a number of frame portions, for example, 5s four portions, one along each edge, as shown. It will be noted in the showing of Figures 2 and 3 that the guard or frame I3 is opposite an area of the [foundation mica I0 which is clear of the signal plate II so that it may be illuminated (m from beyond the side of the signal plate such as by a bias light source I4 external to the tube, as shown in Figure 1. Further in accordance with my invention, I provide a photosensitive wall coating I5 on the internal surface of the c.; envelope I immediately in front of the mosaic I2 and between the mosaic I2 and the ring collecy tor 9, and I utilize the external adjustable intensity light source I4 to illuminate a certain area of the wall coating I5 as determined by the To spatial opacity of the filter I6. In addition, I

provide a conductive member capacitively associated with the photosensitive wall coating I 5 connected to ground as shown. Such a conductive capacitive member may be either internally of the tube, in which case it is separated from the photosensitized wall coating by a thin film of di-electric, although this member may comprise a conductive coating I1 externally of the envelope directly opposite the photosensitive wall I5. The photo-emissive frame and wall coating may be made by evaporating silver on the wall surface followed by oxidation and sensitization with caesium at the time the mosaic is oxidized and sensitized.

In operation the signal plate II is connected to a tube terminal 'la which is inturn connected to ground through an external impedance I8 and to a translating device such as an amplier I9 from which the signals developed across the output impedance I8 may be further amplified and applied to a transmission network as well known in the art.

The operation of my improved apparatus will be fully understood by reference to Figure 4 which shows the approximate circuit equivalent of the target and associated structure wherein parts corresponding to those of the preceding figures are similarly referenced. It will be noted that the inherent capacitance between the mosaic l2 and signal plate II is represented by C-I. This capacitance for representative tubes which I have made utilizing a mosaic and signal plate area of seven square inches with an aspect ratio of 4/3 and for a mica'thickness of 0.002 inch is approximately 5000 micro micro-- farads (auf). As described above, the mosaic I2.and signal plate I I are preferably opposite one another and limited to coextensive areas, whereas the photosensitive frame I3 extends beyond and is clear of the signal plate and is coplanar with the mosaic I2. The frame I3, in accordance with my invention, has a very small total capacitance shown as C-2 which for my construction is less than 0.001 C-I for a frame width of onequarter inch surrounding the above dimensioned mosaic. As previously described, the ring co1'- lector 9 is grounded through a connection to the tube as shown at 1, whereas the wall coating I5 which I make photosensitive is not connected to any potential source either within or without the tube. The wall area has a capacitance C-3 to the collector ring 9 and ground and a capacitance C-ll to the mosaic. The capacitance C-3 may vary from ten to several hundred auf., although in accordance with my invention I prefer to mainalso shown the capacitance C-4 which is the i.'

capacitance between the mosaic I2 and the photosensitive wall coating I5, although the value oi this capacitance is exceedingly low, being from 3 to 6 auf. and consequently may be neglected in the following analysis.

As indicated above, the electron beam incident on the mosaic is made to scan the entire surface thereof and inherently develop secondary electrons. The principal portion of this secondary emission as well as photo-emission from the mosaic flows internally of the tube to the ring collector 9. These currents as well as photo-electric currents between the mosaic and photo-electric wall coating, from the Wall coating to the photosensitive frame I3 and from the frame to the mosaic cause conductances between these elements represented in Figure 4 by internal resistances Rpi, Rpz and R103. The photocurrent paths represented by R272 and R103 vary from iniinity to values as low as one megohm as functions of image illumination on the mosaic, illumination from the scene through the lens and by light from the light bias source I4 on the wall coating I5, and illumination on the frame I3 from the scene light and the bias light source I4. The photocurrent path Rm depends largely on the photo-emission from the mosaic and is therefore a function of image illumination on the mosaic, although Rpi likewise depends somewhat on secondary emission developed by the beam incident on the mosaic. Consequently, in the above example Rpi varies from ten megohms to flve megohms approximately as a function of image illumination on the mosaic,

As the uniformity of mosaic potential is obtainable only by means of selective lighting from the source I4 on the wall area coating I5, the illumination on the photosensitive wall should be a fixed parameter in tube operation. Therefore, in accordance with my invention, I adjust the illumination on the wall coating by adjusting the intensity of the light source I4, and the light from the object is confined to the image on the scanned area of the mosaic. It is therefore essential that the light shield or mask I6 limiting the viewing or projection angle of the lens 5 be provided to prevent extraneous light from the image source becoming incident on the wall coating I5. In addition, the distribution of the b-ias lighting from source I4 is controlled by the lter I6 so that the illumination of Wall areas opposite the .end of the horizontal scanning direction and the end of the vertical scanning direction is considerably stronger than on opposite sides, such as by a ratio of at least 20 to 1 de-v pending upon the variation in photosensitivity over the surface of the wall coating I5. The intensity and distribution of the light are thus adjusted to spread the dark spot uniformly over the mosaic. This adjustment may be made in the absence of image light or an average image intensity on the mosaic. By gradation in bias lighting described above, substantially uniform darkv spot in conjunction with high sensitivity is f obtained and small corrections for dark signal response may be introduced in addition by methods well known in the art.

As the electron beam is scanned over the mosaic in the presence of image illumination a varying potential on the signal plate II is developed depending upon the time and charge sequence of scanning, and signal currents will flow in all internal impedances and in the external load impedance I8 represented in Figure 4 as R12-Cp. Thus While the main signal current flows in the tube over C-I and Rpi, branch currents also flow from the connector 'Ia over C--2, Rpz, C-3 and C--I R113, C-3-to ground and back over the common external load impedance I8. These branch currents cover frequency bands depending on the resolution of their source, that is, video currents flow from the mosaic I2 to the wall coating I5 but also a photocurrent without video information may flow during parts of the scanning cycle in the opposite direction from the wall coating I5 to the mosaic I2, varying mainly with the average potentials of the mosaic areas. The impedance branches discriminate against low frequencies at high illumination levels because of the small Values C-Z and C-3. The low frequency loss in the photocurrents over C-2 and C-3 which depend on the average photocharge of the scannedline appears as streaking in the picture. Extended black objects are thus followed and preceded in the scanning line direction by areas which are whiter than the general background.

Therefore in view of the above analysis it will be appreciated that in accordance with my invention I maintain the values of capacitances C-2 and C--3 at a very low and a very high value respectively so that they are such as to give a time constant in combination with values of Rp and load resistance which will not cause noticeable phase shift at the lowest useful frequency which corresponds to the fleld or vertical frequency of scanning. In accordance with this teaching and as indicated above, I construct the mosaic or target electrode in such a manner that the signal plate does not extend beyond the mosaic I2 nor under the frame I3 to minimize C2.

I further provide in accordance with my invention in combination with the substantially zero value of C-2 a high capacitance C-3 between the photosensitive wall coating I and ground.

While I have referred to the frame I3 as being a semitransparent conductive coating surrounding and coplanar with the mosaic, the operation of the tube may be made somewhat more flexible by subdividing the frame I3 into a number of conducting sections insulated one from the other such as a separate section along each of the four edges of the mica sheet I0. Consequently, each of the individual frame areas may be made to assume a different potential in accordance with the intensity of light projected thereon by the external bias light source I4 and at different times of the scanning cycle. Furthermore, portions of the light projected through the semi-transparent conductive frame may be masked off so that certain desired areas are more highly illuminated than others or that some areas are allowed to remain unilluminated. In operation the frame area or areas acquire negative equilibrium potentials because collection of secondaries from the mosaic is balanced by loss of electrons due to illumination, the value of this potential being controllable by illumination from the bias source I4. Alternatively, the photo-emission effect from the frame may be intensied to render the flared areas more negative, that is, to darken the flared areas by slightly overscanning the mosaic upon the frame area. This overscanning produces secondary electrons on the scanned frame area which, because of their high emission velocities, reach the ared mosaic portions, driving'these portions more negative and thereby increasing the uniformity of the dark signal, thereby increasing the eflciency over these areas. A portion of the photo-emission from the frame area or areas becomes incident upon the adjacent edge portions of the mosaic, thereby driving these edge portions more negative and increasing the dark signal from the edges. The dark signal area on the central portion of the mosaic can thus be extended to the mosaic edges rendering the dark area more uniform, not by a decrease but by extending the dark area over the originally ared areas. Consequently, a greater difference in potential exists between all areas and especially between the edge areas of the mosaic and the ring collector, thereby increasing the operating efficiency of the tube. These potential dilerences may be controlled, as indicated above, by more intense illumination of the frame and wall coating areas adjacent the end of the horizontal and vertical scanning directions. Preferably, however, because the frame areas supply electrons only to adjacent mosaic areas, the frame areas may be uniformly illuminated provided the said adjacent wall coating areas are more highly illuminated than the areas adjacent the edges at which scanning is initiated. Consequently, more electrons may be emitted by the frame areas adjacent the end of scanning because these areas are driven more negative by the emission from the wall coating. The same effect may be obtained by providing a wall coating low in photosensitivity adjacent the edges of the mosaic where scanning is initiated and a highly sensitive coating adjacent the opposte edges. Similarly, the objects of this portion of my invention may be accomplished by illuminating the areas from which high emission is desired with blue light and the other areas with red light, since the effective emission under blue light will be greater due to higher electron emission velocities obtainable by use of blue light. I have found that the dark signal area may furnish about twice the signal output of that derived from the edge areas when the dark spot is not extended to these areas. Consequently, great advantages are obtained, in accordance with my invention, Without introducing dark spot compensating signals which inherently reduce the operating sensitivity of the tube.

I claim:

l. Apparatus for developing television signals comprising a tube having an electron gun to develop a beam of high velocity electrons, a mosaic of mutually separated photosensitive particles over an extended area facing said gun. a transparent photosensitive electrode substantially coplanar with said mosaic and exposed to said gun, and a signal plate in capacitive relation with said mosaic, said signal plate being coextensive only with said mosaic and having substantially no capacitance with said photosensitive electrode.

2. Television transmitting apparatus comprising a tube having an electron gun to develop a high velocity electron beam. a photosensitive mosaic of extended area adapted to be scanned by the beam from said gun, a source of photo-electrons substantially coplanar with said mosaic in position to furnish said'photo-electrons to the edges of the mosaic, and a signal plate coextensive with said mosaic adjacent the opposite side thereof from said gun, said signal plate having an area less than the combined extended areas of said mosaic and said coplanar source to minimize the capacitance between said signal plate and said source.

3. Television transmitting apparatus comprising an evacuated envelope, an electron gun within said envelope to develop an electron beam of high velo-city, a mosaic of mutually insulated photo-emissive particles on a transparent sheet of insulation adapted to be scanned by said beam. an electron collecting electrode between said gun and said mosaic, a transparent photo-emissive electrode surrounding and substantially coplanar with said mosaic, a light source outside said envelope positioned to project light through said sheet of insulation and upon said photoemissive electrode, and a signal plate having substantially no capacitance with said photo-emissive electrode coextensive only with said mosaic and removed from the path between said photoemissive electrode and said light source.

4. Apparatus for developing television signals comprising an evacuated envelope, an electron gun to develop an electron beam of high velocity. a. mosaic of photosensitive particles on a sheet cf insulation, a signal plate in capacitive relation with said mosaic, an electron collecting electrode aeeaese y between said mosaic and said gun, a, photo-emissive -wall coating adapted to be illuminated through said envelope between said collecting electrode and said mosaic, a capacitive member substantially coextensive with said wall coating adapted to be maintained at substantially the potential of said signal plate and a photo-emis- `sive electrode substantially coplanar with and surrounding said mosaic.

5. Apparatus as claimed in claim 4 wherein'said photo-emissive electrode is divided into a number of mutually insulated portions, each adapted to acquire a floating potential by loss of photo-electrons therefrom.

6. Cathode ray apparatus comprising an evacuated envelope, means within said envelope to develop an electron beam, a target electrode comprising a mosaic of photosensitive particles on a transparent insulating foundation, means to co1- lect photo-electrons therefrom including a ringshaped electrode on the wall of said envelope exposed to said mosaic, a photo-emissive wall coating adapted to assume a positive potential by loss of photo-electrons therefrom on a portion of said envelope wall between said ring-shaped electrode and said mosaic, means coplanar with and surrounding said mosaic and on said foundation to develop photo-electrons when illuminated by light projected through said foundation, and a signal plate in capacitive relation with said mosaic, separated thereform by said foundation, and over areas thereof removed from said last-mentioned means whereby capacitance between said signal plate and said means is minimized.

7. Television transmitting apparatus compris- 35 a photo-emissive electrode coplanar with and sur- 40 rounding said mosaic but having negligible capactiy with said signal plate, a photo-emissive wall coating immediately in front of said mosaic and an electron collecting electrode between said wall coating and said first-mentioned means, a load impedance connected between said signal plate and said collecting electrode and means to increase the capacitance between said wall coating and said collecting electrode comprising an electrode conductively separated from said wall coating connected to the junction between said collecting electrode and said impedance.

8. Cathode ray apparatus comprising a mosaic electrode including a thin sheet of insulation, an electrically conductive signal plate over the central area of one side of said signal plate, a mosaic of mutually insulated particles on an area of the opposite side of said sheet directly opposite said signal plate and a thin substantially transparent film of photosensitized metal surrounding said mosaic over the area of said sheet framing said signal plate, the capacitance between said film and said signal plate being substantially limited to that obtained by an abutting relation therebetween on opposite sides of said sheet of insulation.

9. Apparatus for developing television signals comprising an evacuated envelope, an electron gun to develop an electron beam of high velocity, a mosaic of photosensitive particles to receive said beam from said gun, means to scan said beam in two mutually perpendicular directions over the surface of said mosaic, a signal plate in capacitive relation with said mosaic, an electron collecting electrode said gun, a photo-emissive wall coating surrounding said mosaic adapted to be illuminated through said envelope, a photo-emissive electrode substantially coplanar with and surrounding said mosaic, means to illuminate said wall coating and said photo-emission electrode over the area thereof adjacent the end of scanning said mosaic in both of said mutually perpendicular directions.

10. Apparatus for developing television signals comprising a tube having an electron gun to develop a high velocity electron beam, a photosensitive mosaic of extended area exposed to said beam, means to scan said beam over said mosaic area in mutually perpendicular directions, a photo-emissive electrode substantially coplanar with and surrounding said mosaic, a signal plate coextensive with said mosaic adjacent the opposite side thereof from said gun, said signal plate having an area less than the combined extended areas of said mosaic and said photo-emissive electrode to minimize the capacitance between said signal plate and said electrode, means to illuminate said photo-emissive electrode along predetermined areas thereof adjacent. the ends of scanning in said directions with a light intensity greater than along areas thereof opposite sai-:l predetermined areas.

1l. Television transmitting apparatus comprisingan evacuated envelope, an electron gun within said envelope to develop an electron beam of high velocity, a substantially rectangular mosaic of mutually insulated photo-emissive particles on a transparent sheet of insulation, means to scan said beam over said mosaic in line scansion from one corner thereof to the diagonally opposite corner, an electron collecting electrode between said gun and said mosaic, a transparent photoemissive frame electrode surrounding and substantially coplanar with said mosaic, a light source outside said envelope positioned to project light through said sheet of insulation and upon said photo-emissive frame electrode, a signal plate having substantially no capacitance with said frame electrode coextensive only with said mosaic and removed from the path between said photo-emissive electrode and said light source, and a lter between said light source and said photo-emissive electrode having different light transmission factors over the surface thereof, the factor being smaller over that portion through which light is incident upon the frame edges adjoining said one corner at which scanning is initiated.

12. Apparatus as claimed in claim 1l wherein said photo-emissive electrode frame is divided into a number of mutually insulated portions, each adapted to acquire a floating potential by loss of photo-electrons therefrom.

13. Cathode ray apparatus comprising an evacuated envelope, means to develop an electron beam, a target electrode comprising a mosaic of photosensitive particles distributed over a rectangular area, means to collect photo-electrons therefrom including a ring-shaped electrode on the wall of said envelope exposed to said mosaic, a photo-emissive wall coating adapted to assume a positive potential by loss of photoelectrons therefrom on' a portion of said envelope wall between said ring-shaped electrode and said between said mosaic and plate and said frame is minimized, a photoemissive wall coating on said envelope surround ing and exposed to said mosaic and said frame, and means to illuminate areas of said frame and adjacent portions of said Wall coating along diagonally opposite portions of said frame with light of diercnt intensity.

14. Television transmitting apparatus comprising an evacuated envelope, an electron gun within said envelope to develop an electron beam of high velocity, a mosaic of mutually insulated photo-emissive particles on a transparent sheet of insulation adapted to be scanned by said beam, an electron collecting electrode between said gun and said mosaic, a photo-emissive frame adjacent said mosaic, a light source positioned to project light upon said photo-emissive frame, and a signal plate capacitatively associated with said mosaic and having substantially no capcitance with said photo-emissive frame.

l5. Apparatus for developing television signals comprising an evacuated envelope, a mosaic of photo-sensitive particles, a photo-emissive frame around said mosaic, an electron gun for developing an electron beam of high velocity for scanning said mosaic and at least partially scanning said frame said last-mentioned seanning being adapted to project high velocity electrons onto intermediate portions of said mosaic and means for lighting said photo-sensitive frame to project low velocity electrons onto the portions of said mosaic adjacent said frame. f

16. Apparatus for developing television signals comprising an evacuated envelope, a mosaic of photo-sensitive particles, an electron gun to develop` an electron beam of high velocity for scanning said mosaic, a signal plate in capacitive relation with said mosaic, an electron collecting electrode between said mosaic and said gun, a photo-emissive wall coating adjacent the sides of said mosaic electrically connected to said electron collecting electrode and means for illuminating said photo-sensitive wall coating.

17. Apparatus for developing television signals comprising an evacuated envelope, a mosaic o1 photo-sensitive particles, an electron gun to develop an electron beam of high velocity for scanningsaid mosaic, a signal plate in capacitive relation with said mosaic, an electron collecting electrode between said mosaic and said gun, a photo-emissive wall coating adjacent the sides of said mosaic electrically connected to said electron collecting electrode, means for projecting light on said coating and means whereby said light produces more photo-electrons from one portion thereof than from another.

18. Apparatus for developing television signals comprising an evacuated envelope, a mosaic of photo-sensitive particles, an electron gun to develop an electron beam of high velocity for scanning said mosaic, a signal plate in capacitive relation with said mosaic, an electron collecting electrode between said mosaic and said gun, a photo-emissive Wall coating adjacent the sides of said mosaic electrically connected to said electron collecting electrode, means for projecting light on said coating and means whereby the greatest production of photo-electrons is produced by the light incident on a side of said coating adjacent an ending side of said scanning.

19. Apparatus for developing television signals comprising an evacuated envelope, a mosaic of photo-sensitive particles, an electron gun to develop an electron beam of high velocity for scanning said mosaic, a signal plate in capacitive relation with said mosaic, an electron collecting electrode between said mosaic and said gun, a photo-emissive wall coating adjacent the sides of said mosaic electrically connected to said electron collecting electrode, means for projecting light on said coating and means whereby the greatest production of photo-electrons is produced by the light incident on the sides of said coating adjacent the ending sides of the line and frame scanning.

O'ITO H. SCHADE.

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