US2324505A - Television transmitting tube and electrode structure - Google Patents

Description

Y' Q, 1943- H. A.'IAMS ET AL 2,324,505

TELEVISION TRANSMITTING TUBE AND ELECTRODE STRUCTURE Original Filed Nov. 28 1940 a VERTICAL DEFLECT/ON SUPPLY HORIZONTAL DCFLECT/ON SUPPLY ii NV NTO s; BY zlg r-r' 5522;

l' 'ITORNEY Patented July 20, 1943 TELEVISION TRANSMITTING TUBE AND ELECTRODE STRUCTURE Harley A. Iams, Summit, and Albert Rose, East Orange, N. .L, assignors to Radio Corporation of America, a corporation of Delaware Original application November 28, 1940, Serial No. 367,516. Divided and this application February 6, 1942. Serial No. 429.756

3 Claims. (01. 250-467) Our invention relates to television transmitting tubes and electrode structure and particularly to tubes of the low velocity electron beam scanning type. This application is a division of our copending application, Serial No. 367,516 filed November 28, 1940.

Tubes of the low velocity electron beam scanning type, such as disclosed in our U. S. Patents 2,213,1'74-5 and referred to as Orthicon tubes and wherein an insulated surface is scanned by a beam of low velocity electrons, give a signal output which is proportional to the brightness of an optical image focused upon the target. Thus, it may be stated that the gamma of the device is unity. Unity gamma may be desired for many purposes, but when certain scenes which contain objects having high contrast and greatly difi'erent brightness levels must be transmitted over a television system capable of handling only a limited range of signals, a non-linear output, or a gamma less than unity, from the pick-uptube and system may be preferable.

No special precautions need to be taken in tubes utilizing high velocity electron beams such as of the Iconoscope type in order to attain these objectives because the voltage difference between the mosaic and the collecting electrode is, at most, a few volts, so that the maximum voltage which the mosaic can reach by emitting photoelectrons is limited. Further, the lighted areas collect increasing numbers of the redistributed secondary electrons (caused by the electron scanning beam) as the mosaic becomes more positive.

This also reduces the voltage attained by the mosaic. 1

In the usual Orthicon type of tube, the potential of the accelerating electrode may be several hundred volts higher than that of the target, and there is no redistribution of secondary electrons over the mosaic. The strong electric field draws away from the mosaic substantially all the photo-electrons which are emitted, which contributes to high sensitivity but makes the signal output proportional to the light. Such a mode of operation imposes no early limit to the voltage which a brightly lighted area can reach. In the operation of such devices a time lag in discharge of the mosaic may occur after the tube has been subjected to intense high lights. Thus, in televising indoor sporting events, photographers flash lights often cause excessive charging of the mosaic which is only neutralized after a successive number of scannings by the electron beam, resulting in loss of the picture just at. the time when the action is most desired. While it is possible to operate the tube with the accelerating electrode coating only a few volts positive with respect to the target and thereby alter these conditions, this solution results in considerable loss in sharpness of the transmitted picture.

It is an object of our invention to provide a television transmitting tube of the low velocity electron beam scanning type wherein the signal output of the tube is non-proportional to the incident light of an optical image, It is another object to provide a tube wherein the gamma of the system utilizing the tube is less thanunity. It is a further object to provide a tube of the type described having limited output which will generate television signals representative of both high and low values of incident light, and it is as-till further object to provide a tube wherein electrostatic charges representative of an optical image having high contrast and greatly different brightness levels may be utilized with greatly reduced time lag efiects.

A better understanding of our invention will be obtained and other objects, features and advantages will appear from the following description taken in connection with the ;accompanying drawing in which:

Figure 1 is alongitudinal sectional view of a television transmitting tube embodying our invention,

Figure 2 is a view of a mosaic electrode suitable for practicing our invention, and

Figure 3 is a. view of another type mosaic electrode by which our objects may be obtained.

Considered broadly, the apparatus of our invention comprises an evacuated envelope having a target preferably of the mosaic type at one end and an electron gun surrounded by an electron collecting electrode or electrodes at the opposite end of the tube. The target, if of the mosaic type, is provided on its front surface with an extremely large number of mutually insulated photo-electrically sensitized particles, and it is so positioned that it may be scanned by an electron beam from the gun and that it may have focused thereon an optical image of the object of which a picture is to be transmitted. In operation the potential between the electron gun and target is so adjusted that the electron beam is projected at relatively low velocity and directed upon the target at extremely low or substantially zero velocity, that is, with a velocity approaching zero at the point of impact therewith. In operation, elemental areas of themosaic electrode acquire electrostatic potentials of a value dependent upon the intensity of the incident light; thus,

particles of the 'mosaic which are more highly illuminated acquire the more positive electrostatic charges with respect to the lower illuminated particles, and these positive charges which represent an electrostatic image of a picture to be transmitted are neutralized by the low velocity electron scanning beam. Intermediate and extending wholly between the electron gun and target electrode we provide a uniform axial magnetic field and means within the axial field to generate an electrostatic field to scan the beam over the target without producing undesirable deflection or distortion effects.

Referring specifically to Figure 1, our tube and electrode structure comprises an evacuated envelope I enclosing at one end a target or mosaic electrode 2 made in accordance with our invention and at the opposite end an electron gun assembly adapted to project electrons toward the mosaic electrode.

The electron gun assembly is of the conventional type and comprises a thermionic cathode 3 from which electrons may be drawn, an apertured cold electrode, such as a control electrode 4 connected to the usual biasing battery and a first anode 5 maintained positive with respect to the cathode 3. The electron stream leaving the first anode 5 is accelerated at relatively low velocity and further accelerated and directed upon the front surface of the target or mosaic electrode by a second anode 6 which is preferably an apertured tubular member partially surrounding the first anode 5. The first anode 5 and the second anode 6 are maintained at the desired positive potentials with respect to the cathode by a battery I. The anodes 5 and 6 are not for the purpose of focusin the electron beam, in fact, any electrostatic focusing of the beam is detrimental in .the presence of the magnetic field described later which maintains the beam in a focused condition because such electrostatic focusing imparts transverse velocity to the electrons, thereby introducing defocusing effects. To avoid this difficulty the anodes 5 and 6 may be operated at the same potential or only one of these anodes may be incorporated in the structure. Closely adjacent the electron gun and between the anode B and the target we provide a centrally apertured electron collecting electrode 8 to which the electrons of the beam not reaching the target are directed through the slotted shield 8a.

The mosaic electrode 2 which faces the electron gun in accordance with one teaching of our invention comprises asubstantially transparent sheet of insulation such as the mica sheet 9 having on its rear surface a translucent or semitransparent electrically conducting signal plate or film II], the opposite surface of the mica sheet being provided with an exceedingly great number of mutually separated photosensitive particles l I. In making the mosaic electrode we make the mica sheet 9 of the desired area having a uniform thickness of approximately .002 inch and as a preferred first step coat one side of the sheet with a film of metal of sufficient thinness as to be substantially transparent so that an optical image may be focused on the photosensitive particles II. The mosaic of mutually separated particles may be formed by vaporizing and condensing a film of silver on the front surface of the mica sheet 9. The condensed silver film is then broken up into the mutually separated particles II by suitable heat treatment followed by oxidation and sensitization such as by caesiating, Such a process is described by Leonard Klatzo'w in U. S. Patent 2,178,233 and alternative processes by S. F. Essig in U. S. Patents 2,020,305 and 2,065,570. The conducting film I0 is connected to the input electrode of a translating device l4 and to the battery I through the impedance l5 so that the potential of the conducting film I 0 may be maintained substantially at cathode potential, If, however, signals of an opposite polarity are desired, the translating device 14 and output impedance I5 may be similarly connected in the circuit of the collecting electrode 8 to ground.

The electron beam deflected by the electron gun is scanned in one direction over the mosaic electrode by a pair of deflection plates 16, preferably formed as described in our above-mentioned patent, the plates being connected to a source of deflection potential and to ground through a center-tapped resistor of from 1 to 10 megohms. The electrostatic field produced between the plates IS in combination with a coaxial magnetic field deflects the electron beam over the mosaic electrode 2 in a direction perpendicular to the plane of the drawing of Figure 1. The coaxial magnetic field is preferably generated by a magnetic coil I! which is of slightly larger diameter than the envelope I and extends over and beyond the space between the electrode 8 and the mosaic electrode 2. Deflection of the electron beam in a direction normal to that proposed by the plates [6 may be accomplished by a pair of deflection coils [8, this latter deflection preferably being the frame or vertical deflection since in standard television systems the frame deflection is at lower frequency and the horizontal line deflection produced by the plates I6 should preferably be at the higher of the two frequencies. Electrons from the electron gun are thus scanned in two mutually perpendicular directions and are accelerated by the anodes and the accelerating electrode or wall coating l9. However, since the signal plate or film II] is operated at or'near cathode potential, the electrons of the beam are decelerated and approach the mosaic with substantially zero velocity. It will be understood that our invention is not limited to this particular type of low velocity electron beam scanning tube but that other types such as those utilizing fullmagnetic deflection of the electron beam may be utilized with equal advantage. Referring again to Figure 1, the mosaic electrode 2, is subjected to light in the form of an optical image such as represented by the arrow 20, focused such as by a lens system upon the photosensitive particles ll. However, in accordance with our invention the light passing to the photosensitive particles will be representative of the original image but will consist of small areas of light and shadow, the areas of light still being representative of the original optical image. 7

More particularly in accordance with our invention the output vs. light input characteristic or gamma of television transmitting tubes having a strong electrostatic field at the photosensitive target is controlled by a structure which establishes over the surface of the target a regular or random pattern of localized areas which remain at or near a fixed potential when otherlocalized areas of the target are exposed to light such as light representative of an optical image. Such a pattern has negligible effect upon the photo-emission from the other areas of the target until the said other areas have attained a detectable positive voltage under the influence of the light. As the positive, voltage tends to build updue to high lights of the optical image the regular or random pattern of areas provided by our structure begins to serve-the function of agrid and begins to suppress the photo-emission. Finally, when the emitting areas have charged far enough positive, such as due to very bright high lights, the retarding'potential due to the regular or random pattern is as great as the emission velocity of the photo-electrons, and further photo-emission is suppressed.

More particularly, and in accordance with one modification of our invention, the'regular or random pattern of tiny areas which remain at or near a fixed potential when the target is exposed to light may be formed photo-optically. Thus, i

in accordance with our invention, minute shadows may be cast on the photosensitive portion of the mosaic electrode 2, the shadows being formed .by deletion of light from the optical image over tiny areas thereof taken either at random or over a regular pattern. The objects and features of our invention may be obtained utilizing a mosaic electrode as shown in Figure 2 wherein the mica foundation 9 supports the photosensitive particles I I on the side facing the electron gun. The opposite side of thesheet of mica 9 is provided with the semi-transparent signal plate Ill and,

.in addition, a network of opaque material 26 through which various areas of the semi-transparent signal plate are exposed. The signal plate in this modification of our invention may be omitted provided this material is electricallyconducting, so that a signal may be applied to the output device I4. The signal plate I0, however, may be used in the modification of Figure 2 when the network 26 is of non-conducting material. Thus, the electrode may be provided with a semiconducting metallic film on which is deposited the opaque material either in the form of continuous lines or material deposited at random, such as by spraying aquadag or other finely divided opaque material'thereon. In accordance with a further teaching of our invention and referring to Figure 3, the mosaic electrode may comprise the sheet of mica 9 or other substantially transparent insulator having on one side thereof only the semi-transparent electrode or signal plate ID. The opposite side, however, that is, the side facing and scanned by the electron beam, is provided in addition to the photosensitive particles II with an opaque structure in the form of intersecting metal strips having apertures therein exposing areas of the insulating foundation of mica sheet 9 carrying the mutually separated and insulated photosensitive particles II. The metallic structure 25 may be connected to a source of potential or may float at the potential acquired by collection of low velocity electrons from the scanning beam. The metallic structure 25 operates in the same manner as the modification of our invention described above, limiting the emission from the photosensitive particles deposited on the mica foundation. While we have shown the particles I I on the front surface of the mica sheet 9, the particles are preferably over the entire exposed surface as shown in Figure 3 at Ma. Thus, the metallic mesh-like structure may be applied to the sheet of mica first and the particles II formed both on the exposed areas of the mica and also on the metallic mesh. Not only is this structure more easily made, but it offers definite advantages in that the control of emission from the illuminated areas is more uniform. 7

While we have described in connection with areas.

Figure 2 the use of opaque material on the reverse side of the mica foundation 9, or on the front side as in Figure 3, it is obvious that the opaque material may be embedded in a sheet of insulation and still serve its desired purpose. Likewisaopaque particles may be deposited on the front surface of the insulating foundation, leaving areas of the foundation exposed. The photosensitive mosaic in this case is deposited both upon the opaque areas and on the exposed portions of the insulating foundation. Target electrodes made in accordance with these latter teachings operate in the same manner as the structure shown in Figures 2 and 3, the principal purpose being to subdivide the optical image into small discrete areas of light mixed with areas in shadow.

In operation the shadowed areas exert a control on the photo-emission liberated from the illuminated areas, effectively limiting the maxitensity of the optical image increases, a smaller signal Will be generated in the high lights with respect to the incident light than on less illuminated areaswith respect to the light on these The signal output of the device will still vary with respect to the light and shade areas of the optical image to be transmitted, but the signal will no longer be proportional to the incident light established in the high lights of the imagTe wherein the signal is reduced by the suppression of photo-emission from the highly lighted areas of the mosaic.

In all of the modifications referred to above the size of the shadows cast, such as by the opaque particles of Figure 2, or the metallic followed by the electron beam in scanning the target. Although the random arrangement of non-emitting areas may be used, it is usually preferable to use a predetermined pattern such as a series of crossed parallel lines. Such a pattern of metal lines may be copied from a master film by following a photo-engraving process which we believe to be novel. The mica sheet or insulating foundation may be prepared by pouring Cold top enamel as furnished by the N. Bechak Company, New York city, or other soluble light sensitive emulsion on the mica sheet followed by draining and drying. The mica sheet is then exposed to light, using the master film as a mask. The sensitive and partially exposed film is developed and the emulsion washed away from the unexposed areas. A film of metal such as platinum is then sputtered or condensed on the remaining emulsion, whereupon the mica is fired at a temperature of 800 C. to decompose the remaining emulsion coating. The coated mica is then lightly brushed with a camels hair brush or other material and the metal deposited on the bare mica remains, while the part formerly covering the emulsion material brushes off.

The shape of the signal vs. light curve, and consequently the gamma, may be controlled by varying the spacing between the non-emitting areas, their width and their tendency to emit photoelectrons. Thus, in the modification shown in Figures 2 and 3 the material comprising the elements 25 and 26 may be made highly opaque to sharply limit the signal output, or somewhat translucent to give a more nearly linear characteristic. In the structure shown in Figure 2- the opaque grid-like structure 26 should be as close to the plane of the mosaic particles as possible so that the cast thereon aresharply limited for effective control of the gamma. Similarly,'the areas of shadow or non-emitting areas from the structure of Figure 2 decrease the gamma as the size of the non-emitting areas decreases to the order of the mica or foundation thickness. In the case of non-emitting areas on an insulating surface, however, such as shown in Figure 3, the photo-electric suppressing effect increases as the size of the areas is decreased to the order of the thickness of the insulation. When the size of the opaque areas is reduced further, both capacitance to the adjacent emitting areas and the leakage of charge over the surface of the mosaic act to make the whole illuminated area rise in potential uniformly.

While we have indicated the preferred embodiments of our invention of which we are now aware and have also indicated only one specific application for which our invention may be employed, it will be apparent that our invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the scope of our invention as set forth in the appended claims.

We claim:

1. A light sensitive electrode comprising a sub stantially transparent foundation, a lightsensitiv electrically discontinuous mosaic structure onone side of said foundation and an opaque signal plate in capacitive relation with said structure said signal plate being provided with apertures distributed over the surfacethereof to allow light to be' projected through said foundation and upon said structure as discrete areas separated one from another by areas in shadow and by said signal plate.

2. A light sensitive electrode comprising a substantially transparent foundation, a signal plate closely adjacent one side of said foundation, an

, electrically discontinuous light sensitive mosaic structure of extended area closely adjacent the other side of said foundation and opaque means between said signal plate and said structure to shield discrete areas of said structure from light projected through said signal plate and said foundation and form separate areas of light and shade distributed over the extended areas of said light sensitive structure.

3. A light sensitive electrode comprising a light transparent insulating foundation, a network of opaque material on one side of said foundation, an electrically discontinuous light sensitive mosaic structure on said network and in the interstitial spaces of said foundation not covered by said network and a signal plate in compacitive relation with said light sensitive structure on the other side of said foundation.

HARLEY A. IAMS. ALBERT ROSE.

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