US2618761A - Negative stored charge pickup tube - Google Patents

Description

Nov. 18, 1952 OS 2,618,761

NEGATIVE STORED CHARGE PICKUP TUBE Filed Feb. 23, 1949 w 0.0. VOL7276E$UPPLY J INVENTOR ALBERT ROSE 0. c. I/OLMGE5UPPU B Patented Nov. 18, 1952 NEGATIVE STORED CHARGE PICKUP TUBE Albert Rose, Princeton, N. 3., assignor to Radio Corporation of America, a corporation of Delaware Application February 23, 1949, Serial No. 77,718

7 Claims. 1

This invention relates to a television transmitting tube of the type in which there is used an insulator target electrode upon which is established a negative charge distribution corresponding to an optical image to be televised.

.A successfully operated television pickup tube is of the type disclosed in my co-pending application Serial Number 631,441, now U. S. Patent 2,506,741, issued May 9, 1950 and in U. S. Patent 2,433,941 of P. K. Weimer. In television pickup tubes of this type, an optical image is focussed upon a photocatho'de electrode to provide a photoelectron emission corresponding to the optical image. The photoemission is accelerated to relatively high potential and focussed upon a thin glass target electrode, resulting in a secondary emission from the surface of the glass film. The secondary emission from the surface of the glass film leaves on the glass surface, a distribution of positive charges corresponding to the optical image focussed upon the photocathode electrode. The opposite side of the glass target electrode is scanned with a low velocity electron beam. The glass target electrode is of such thin ness that the positive charge distribution on its surface provides a positive electrostatic charge pattern on the scanned side of the target. Electrons from the low velocity scanning'beam land on the opposite side of the glass target in proportion to the positive potential in the point of scansion. The remaining part of the beam is reflected and returned as the video signal of the tube. The electrons deposited on one side of the target will unite with the positive charges established on the other side within a frame time.

In tubes of the type described above, one great disadvantage exists in that the positive charges formed on the target electrode are in pro ortion to the intensity of the light focussed upon the photocathode. This results in the highlights, of the picture televised, forming small television signals while the dark areas, of light or no light, form strong video signals. This results in a large dark current signal from dark portions of the picture televised. That is, when the photocathode is in darkness, the output television signal of the tube is at maximum and any picture signals produced by the tube are of less intensity than that of the output signal in total darkness. However, it is desirable to provide a television pickup tube which produces a maximum signal for the highlights of the picture televised and a minimum signal for the dark or nearly dark portions of the picture. Such a tube would have a dark current and will result inlittle or nq noise in the dark portions of the picture.

If, in the tube of the patent described, a negative charge pattern were established upon the glass target electrode, such a tube then would provide low dark current in the dark portions of the picture and a maximum current in the highlights of the picture. The usual arrangement for obtaining a negative charge image on an insulator target electrode, is to project an electron emission onto the target surface with an energy well below the point at which the secondary emission ratio is unity or below what is known as the first cross-over. The electrons then charge the target surface negatively. The disadvantage of this arrangement is that the resulting charge distribution tends to become distorted due to the deflection of the electrons approaching the target surface by the charges already deposited on the surface.

It is therefore an object of my invention to provide a television pickup tube forming a signal varying in proportion to the intensity of light focussed upon the tube.

It is another object of my invention to provide a television pickup tube which produces a maximum signal for the highlights of the scene to.

be televised.

It is a further object of my invention to provide a television pickup tube having a minimum signal for dark or nearly dark portions for the scene televised.

It is also an object of my invention to provide a television pickup tube having a target electrode upon which a negative charge pattern is established in proportion to the amount of light in the scene televised.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawing, in which:

Figure 1 discloses a means for televising pictures and including a novel pickup tube shown in section and in accordance with my invention.

Figure 2 shows an alternate method for televising pictures including a pickup tube shown in section and according to my invention.

In Figure 1 there is disclosed an evacuated envelope 10 having a photoemissive film or photoelectric cathode l2 deposited on one end M of the envelope to. The envelope portion 14 is formed of optically transparent material so that light focussed thereon will pass to the film l2 undistorted. Mounted within the envelope I0 is a thin aluminum metal foil l6, mounted for support upon a fine mesh metal screen 18.

on the inner surface of the envelope l0. Electrodes 20 and 22'are spaced to form therebetween an electron lens field for focussing the photoemission from film l2 upon the thin metal foil l6. Closely spaced from the supporting metal mesh is is a thin glass film 24 having a thickness in the order of 0.1 mil and mounted on a supporting metal ring 26. The glass film 24 is formedof semi-conducting glass as disclosed in the application to Albert Rose, Serial No. 407,131, filed August 1.6, 1941 now ,U. S. Patent 2,473,220, issued June 14, 1949. On the opposite surface of the glass film 26, away from the photocathode I2 is formed a photoelectron emitting coating 28.

The other end of the tube envelope may be closed by a second face plate 30 of optically transparent material to provide undistorted transmission thereto, of light from the fluorescent screen of a cathode ray tube 32. Light from the screen of the tube 32 is focussed by a lens system 34 upon the photoelectric coating 28.

The envelope I0 is provided with a short neck portion 36 mounted eccentrically from the axis of the tube ID, as is shown. Within neck portion 36 is mounted an electron multiplier section comprising dynode surfaces 38, 40 and 42, forming respectively the first, second and third stages of the multiplier section. An electrode 44 provides a collector for the secondary emission from the third stage 42. The multiplier section within the tube neck 36 may be of any well known design as, for example, that disclosed in U. S;

Patent 2,433,941 to P. K. Weimer. The dynode electrodes 28. 40 and 42, of the several multiplier stages, respectively include fine mesh screens 48, 50 and 52 to provide fields directing the secondaries from each dynode surface toward the next stage of the multiplier. An electrode 54, formed as a wall coating on the tube envelope I0, is

- positioned between the face plate 30 and the photoemissive surface'28. Electrode 54 is maintained, during tube operation, at ground potential. In a successfully operated tube of the type shown in Figure 1, the multiplier stages 38, 40 and 42 are operated at respectively 200, 400 and 600 volts positive with respect to ground or the potential of the electrode 54, and the collector electrade 44 is operated at approximately 800 volts positive with respect to ground.

The various electrodes disclosed above may be maintained during tube operation, at their respective potentials by being connected, as is shown, to a voltage divider 55 connected across a source of direct current voltage supply. The potentials given above for the several electrodes are by way of example only and are not meant to be limiting, since other appropriate potentials may be used to provide a similar operation of the tube.

If the tube disclosed in Figure 1 is operated to televise outdoor scenes, the photoemissive cathode l2 may be formed from a caesium activated silver-antimony film formed in a manner described in U. s. Patent 2,244,729 of et a1.

Such a photosensitive film of caesium activated silver-antimony alloy has a good response in the blue and ultra-violet portions of the spectrum and can be used for televising outdoor scenes. However, for studio pickup work, in which there are controlled lighting conditions, an alternate photosurface such as the well-known caesium activated silver oxide photosurface may be used to provide greater sensitivity. The phosphor screen of the cathode ray tube 32 is preferably one having blue and ultra-violet light-emitting properties, such as suitably prepared alumina or zinc oxide. A zinc oxide phosphor of this type, emitting light in the blue and ultra-violet regions of the spectrum is described in the co-pending application of A. L. J. Smith Serial No. 29,577, filed May 27, 1948.

The thin aluminum film [6 may be formed in any desired manner. One method of forming the aluminum film I6 is that in which the metal supporting screen 3 is immersed below the surface of a pool of water. A thin lacquer film is produced on the surface of the water by allowing a drop of material, such as a solution nitro-cellulose in appropriate solvents to spread over the surface ,of the water. The lacquer film, after spreading, dries rapidly and produces a thin nitro-cellulose film. The immersed supporting screen may then be brought up through the surface of the water so that the lacquer film will cover over one side of the metal screen. After drying, the aluminum film then is applied to the mesh supported lacquer film by sputtering aluminum metal from a hot tungsten filament. Screen [8 supporting the aluminum film 16 may then be mounted by any appropriate means within the tubeenvelope [0. During the processing ofthe tube, the nitro-cellulose film is burned out so as to leave the thin aluminum foil l5 supported on the screen Ill. The thickness of the aluminum film I6 is not critical although experience has indicated that a thickness between 500 angstrom units and 5000 angstrom units is within the most useful range. The ability of high voltage electrons to penetrate thin aluminum films is well known and is described fully in the RCA Review of March 1946, pp. 5, volume 7, Number 1.

The operation of the device of Figure 1, is that in which an optical image is focussed by a lens system 56 onto the photo-sensitive cathode film l2. Photoelectrons are emitted from all portions of the film l2 in proportion of the intensity to the light striking the respective portion. This photoemission is focussed onto the metal foil 16. The high accelerating potential between the electrodes 20 and 22, of which the metal foil I6 is a part, will accelerate this electron emission to .a high velocity which is willcient to cause the electrons to penetrate foil [6. However, the metal foil l6 slows down the penetrating electrons to a point where they emerge from the foil as low velocity electrons and accompanied vin most cases by low velocity secondary electrons from the metal foil [6.

When the electron beam of the cathode ray tube 32 strikes the phosphor screen of the tube, a luminescent light spot is formed, which is focussed by the lens system 34 upon a corresponding portion of the photosensitive coating 28. The electron beam of the tube 32 is caused to be scanned over the phosphor screen of the tube by any well-known means as, for example. by two pairs of magnetic coils formed into a neck yoke 51. These pairs of coils of yoke 51 produce a pair of magnetic fields perpendicular aerator 5. to each other and to the axis of the tube 32. Each pair of coils. of the yoke 56, is connected to voltage pulse generators of any well-known type. such as those producing, for example, sawtooth pulses. In this well-known manner, the cathode ray beam of tube 32 is caused to be scanned across the phosphor screen of the tube in any desired raster.

The lens system 34 will focus the moving light spot of the screen of tube 32 onto the photosensitive coating 28 as a beam of light scanning over the photoelectric surface 28. Since the photosurface 28 and the semi-conducting glass film 24 are not connected to any voltage source, the photoemission set up by the scanning light beam from coating 28 will pass to the grounded collector electrode 54. However, the photoemission from coating 28 will only continue until the potential of the surface of the coating is raised to that of the electrode 54. A relatively coarse mesh screen 59, of around 100 mesh per inch, may be mounted in contact with electrode coating 54 and adjacent the photoemissive coating 28 as shown. Screen 59 at the potential of electrode 54 provides a more uniform collecting field over the photoelectric coating 28.

Since the electrode 22 and the thin metal foil l8 are maintained at around volts negative, with respect to ground, there will be a positive accelerating field between foil I6 and the semiconducting glass surface 24. The low velocity primary and secondary electrons leaving the metal foil l6 are drawn over and deposited upon the surface of the semi-conducting glass film 24. These low velocity electrons are kept from spreading or scattering over the surface of the semi-conducting surface 24 by the cellular structure of the supporting wire screen l8. In this manner, there is deposited upon the exposed surface of the semi-conducting glass film 24 a pattern of negative charges varying from point to point in an amount proportional to the light distribution on the photocathode film l2. Due to theextreme thinness of the glass target 24, negative charges on the right hand side of the target of Figure 1 will set up a corresponding negative potential pattern on the opposite or scanned side of the target. As the light beam from tube 32 is scanned over the photosensitive coating 28, the photoemission will vary in proportion to this negative potential pattern on the target 24. The photoelectrons of this emission will be of greater energy potential than that of electrode 54 and will be drawn only to the multiplier section by the high positive potential of the first multiplier stage 38.

Thus, as the light beam from the optical system 34 is scanned over the photoelectric coating 28, a modulated beam of photoelectrons is drawn to the first stage of the multiplier, from which it passes through the several multiplier stages to the collector electrode 44 as an amplified video signal. This signal may be further amplified by being applied to the control grid of an amplifier tube '58 in the output circuit of the system.

The modulated photoemission from coating 28 leaves the surface of coating 28 positively charged. The thickness and resistivity of the glass film 24 is so chosen that the electrons deposited on the exposed surface of the glass film 24-will, in a frame time, pass through the glass of film 24 and neutralize the positive charges to restore the conditions to equilibrium. Other photoelectrons deposited upon the exposed surface of the glass 24from photocathode l2 will maintain the negative charge pattern on the glass 24 to provide the modulated photoemission as the beam scans over the photoelectric coating 28.

Figure 2 discloses another form of the television pickup system disclosed in Figure 1. In Figure 2, the parts which are identical to corresponding parts of Figure 1 are indicated by the same reference numerals. However, in the system of Figure 2, there is substituted for the light spot scanning of the target 24, a cathode ray scanning.

' The envelope in of the tube of Figure 2 is provided with an additional tubular neck portion 68. in which is mounted a conventional electron gun. This electron gun is provided with an indirectly heated cathode cylinder 82 having at one end thereof an electron-emitting coating (not shown) formed of barium and strontium oxides, as is well known in the art. Surrounding the cathode cylinder 62 is a control grid 54 which is maintained during tube Operation negatively to tentials.

the cathode 82 to provide a cut-off voltage of the electron emission from the cathode 62. Spaced axially along the envelope neck portion .68 is a positive screen-grid electrode 86 and a positive tubular anode electrode 68. Electrode 66 is usually maintained, during tube operation, at several hundred volts positive relative to the cathode 62, to provide an accelerating arid focussing field of the electron emission from the cathode. The first anode electrode 68 is normally maintained at several hundred volts more positive, with respect to the cathode 52, than the accelerating grid 66. A second anode electrode 10, formed as a wall coating on the neck portion 60 of the tube, is extended into envelope I!) to a point adjacent the thin glass target electrode 24. Also, the second anode electrode 10 is maintained during tube operation at approximately 1000 volts positive with respect to the potential of the oathode 62.

In a manner similar to that disclosed in Figure 1, the potentials upon the various tube electrodes may be respectively maintained during tube operation by connecting each electrode to an appropriate point in a voltage divider l5, as is shown in Figure 2. The voltage divider may be connected between terminals of a direct current voltage supply source to provide the appropriate po- Again the voltages given are for example only, and are not meant to be limiting as other appropriate voltages may be used.

These potentials applied during tube operation to the several electrodes of the electron, provide accelerating and focussing fields between .electrodes 86, 68 and I0 so as to form the electron emission from the cathode 62 into an electron beam which will strike the surface of target 24 in a well-defined spot. The potential of the second anode electrode 10 may be set at ground potential and may be considered as the reference potential by which the other potentials of the tube are determined.

The electron beam formed by the gun structure, of the tube of Figure 2, is caused to scan over the surface of the glass target 24 by any well known means. For example, a neck yoke 12, mounted on the tubular portion 60, comprises two pairs of coils (not shown), which provide a pair of scanning or deflecting fields perpendicular to each other and to the axis of the tubularneck portion 68. As described above with respect to the neck yoke 55, these pairs of coils of yoke 12 .is well known in the art.

mately 5000 volts negative are connected into appropriate circuits (not shown) to provide saw-tooth pulses for furnishe ing the scanning magnetic fields. In this manner the electron beam of the gun is scanned over the surface of target 24 in any desired raster, as The electron beam will strike the glass target surface at a potential above the first cross-over point, so as to provide a secondary emission from the glass surface of greater than unity. This secondary emission will be drawn over to the anode electrode 10 until the scanned surface of target 24 is raised to the reference potential of electrode I0. At this point, the electrons leaving the target surface it will equal the number of electrons passing to the target 2d to maintain the scanned surface of the target at this reference potential.

The operation of the tube of Figure 2 is similar to that for Figure 1. The optical image to be televised is focussed by a lens system It upon the photoelectric cathode film l2. The photoemission from every point of the film l2 will be in proportion to the amount of light striking it. This photoemission as described above, is accelerated to an electron velocity of approximately 5000 volts, so as to strike the thin metal foil It with suflicient energy that the photoelectrons will penetrate through the foil. As in Figure l, electrode 22 is maintained at approximately 5 volts negative to ground or reference potential, while the potential of electrode 20 is approxito reference potential. Thus, the electrostatic lens action between electrodes 20 and 22 will provide acceleration and focussing of the photoemission from photocathode i2 upon the metal foil IS.

The photoelectrons penetrating through foil I6 wil1 emerge as low velocity electrons and will be in most cases, accompanied by secondary electrons. These low velocity electrons will pass to the glass target electrode 26, which, due to the scanning action of the electron beam, will be maintained during tube operation at reference potential or approximately 5 volts positive relative to the metal film l6. These low velocity primary and secondary electrons, from the aluminum film IE, will be deposited upon the adjacent surface of the glass film 24 to form a negative charge pattern corresponding very closely to the optical pattern focussed upon the photoelectric cathode 12. These negative charges on the right hand side of the thin glass target 24 will set up a corresponding negative potential pattern on the opposite or scanned side of the target 24. As the electron beam of the gun is scanned across the left hand surface of the target electrode 24, a

secondary emission will be produced, from each.

elemental portion of the target surface, in proportion to the potential of the target at that point. This secondary emission will /leave the surface of target 24 with a potential greater than that of the electrode and will be drawn toward the positively charged first stage 38 of the multiplier section.

The secondary emission entering the multiplier section of the tube will be an electron stream, modulated according to the charge distribution on the glass target 24. As described for Figure 1, the modulated electron stream is amplified by the several stages 38, 40 and 42 respectively of the multiplier section and is finally collected by electrode 40 connected to the grid circuit of amplifier tube 76, to provide the output video signal of the tube.

This secondary emission from the target sur= face 26 will leave the left hand surface of the target with a positive charge corresponding to the negative potential at each point. In a frame time, the electrons deposited on the right hand surface of the target 24 will, by conduction, neutralize the positive charges on the left hand side of the target to restore the targetsurface to equilibrium.

While certain specific embodiments have been illustrated and described, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What I claim is:

1. A signal generating apparatus comprising an evacuated envelope, a semi-conducting target film within said envelope, a. photoelectric cathode within said envelope for providing a photoelectron emission corresponding to an optical. image, a thin metallic foil mounted within said envelope in the path of said photoemission and spaced between said semi-conducting sheet electrode and said photoelectric cathode for retarding photoelectrons passing to said semi-conducting film to establish a negative charge distribution on one surface of-said semi-conducting film, and means for sequentially removing the negative charge distribution from the other surface of said semiconducting film.

2. A signal generating apparatus comprising an evacuated envelope, a semi-conductor target electrode mounted within said envelope, a photoemissive film within the envelope to provide a photoelectron emission along a path, a thin aluminum foil mounted within said envelope in the path of said photoemission and between said semi-conductor target electrode and said photo emissive film for retarding photoelectrons passing to the semi-conductor electrode to establish a negative charge distribution on an adjacent surface of said semi-conductor electrode, and means for sequentially removing the negative charge distribution from an opposite surface of said semiconductor electrode.

3. A signal generating apparatus comprising an evacuated envelope, a semi-conducting target electrode mounted within said envelope, 2. photoemissive cathode within said envelope for providing a photoelectron emission corresponding to an optical image, means within said envelope for directing said photoelectron emission toward said semi-conducting target electrode, a thin metallic foil mounted within said envelope in the path of said photoelectron emission and spaced between said insulator target electrode and said photoelectric film for retarding the photoelectrons directed toward said insulator electrode to establish a negative charge distribution on the adjacent surface of said semi-conducting electrode, and means for sequentially removing the negative charge distribution from an opposite surface of said semi-conducting electrode.

4. A signal generating tube comprising n envelope, a thin semi-conducting glass film mounted within said envelope, photoelectric cathode means within said envelope for providing a photoemission corresponding to an optical image, a thin metallic foil mounted within said envelope in the path of said photoemission and between said glass film and said photoelectric cathode means for retarding the photoelectrons passing to said glass'film to establish a negative charge distribution on one surface of said glass film, a photoemissive coating on the opposite surface of said glass film for removing said negative charge distributionlby photoemission.

5. A signal generating apparatus comprising an envelope, a thin film of semi-conducting glass mounted Within said envelope, a photoemissive cathode layer Within said envelope for providing a photoelectron emission corresponding to an optical image, means for directing said photoemission toward one surface of said glass film, a thin aluminum foil mounted within said envelope in the path of said photoemission and spaced between said glass film and said photoemissive layer for retarding photoelectrons passing to said glass film to establish a, negative charge distribution on said surface of said glass film, means for scanning the opposite surface of said glass film to remove said negative charge distribution.

6. A signal generating apparatus comprising an envelope, a thin film of semi-conducting glass mounted within said envelope, a photoemissive cathode within said envelope for providing a photoelectron emission corresponding to an optical image, means for directing said photoe'mission toward one surface of said glass film, a thin aluminum foil mounted within said envelope in the path of said photoemission and spaced between said glass film and said photoemissive cathode for retarding photoelectrons passing to said glass film to establish a negative charge distribution on said surface of said glass film, means within said envelope for forming a defined beam retarding electrode including an electron per-.

meable metal foil, and means for scanning the opposite face of said glass target film for removing charges therefrom.

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

UNITED STATES PATENTS Number Name Date 2,150,160 Gray Mar. 14, 1939 2,214,973 Rose Sept. 17, 1940 2,254,617 McGee Sept. 2, 1941 2,525,832

Sheldon Oct. 17, 1950

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