US2813989A - Color pickup tubes - Google Patents

Description

Nov. 19, 1957 P. K. wElMl-:R

COLOR PICKUP TUBES Filed May 2, 1955 2 Sheets-Sheet 1 www IN1/Tok.'

Nov. 19, 1957 P. K. WEIMER 2,813,989

COLOR PICKUP TUBES Filed May 2, 1955 2 Sheets-Sheet 2 @oooo'oooooe 2,813,089 Patented Nov. 19, 1957 tice COLOR PICKUP TUBES Paul K. Weimer, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application May 2, 1955, Serial No. 505,266

The terminal fifteen years of the term of the patent to be granted has been disclaimed 10 Claims. (Cl. 313-67) This invention relates to television pickup, or camera tubes. In particular this invention relates to pickup tubes suitable for use in color television transmission.

Particularly, this invention deals with pickup tubes for color television and utilizing a photoemissive surface. The use of a photoemissive surface eliminates certain time limitations which may be found in the photoconductive type of pickup tube as a result of photoconductive lag. Also, the invention relates to color television pickup, or camera, tubes which may utilize opaque signal electrodes. Since the number and size of the signal electrodes partially determines the resolution obtainable from the device, a large number of relatively ne signal electrodes is preferred. Due to the fact that the signal electrodes may be opaque in tubes constructed in accordance with this invention, certain manufacturing diiculties associated With transparent conductors are eliminated.

It is therefore an object of this invention to provide a new and improved pickup, or camera, tube for use in color television.

It is another object -of this invention to provide a new and novel camera tube in which photoconductive lag has been eliminated and one having improved sensitivities.

It is still another object of this invention to provide a new and improved pickup tube for use in color television wherein the signal output electrodes need not be transparent.

These and other objects are accomplished in accordance with this invention by providing a pickup tube including, within an evacuated envelope, a plurality of color filter strips through which light passes to a photoemissive cathode. The emission from the phot-oemissive cathode, which is determined by the scene being viewed, is imaged on a target Within an opposite end of the envelope to establish a charge pattern corresponding to the light image picked up. The target includes a plurality of signal strips, which are covered by an insulating layer. The photoelectrons leaving the photocathode move to the target and establish a change pattern on the insulating layer. Means are provided for scanning the target with an electron beam, and thus discharging the charge pattern, to provide output signals from the tube.

Other objects and features of this invention will become apparent from reading the accompanying detailed description thereof in conjunction with the accompanying two sheets of drawings in which:

Figure 1 is a sectional View of a pickup tube made in accordance with this invention;

Figure 2 is an enlarged fragmentary sectional view of the target shown in Figure 1;

Figure 3 is an enlarged fragmentary sectional View of the color filters and photocathode shown in Figure 1;

Figure 4 is a sectional View of a modification of a pickup tube shown in Figure 1 and made in accordance with this invention;

Figure 5 is an enlarged fragmentary sectional view of the color filters and photocathode shown in Figure 4; and,

Figure 6 is an enlarged fragmentary sectional View of the target shown in Figure 4.

Referring now to Figure 1 there is shown a pickup tube 10 comprising an elongated evacuated envelope 11 having a neck portion 13 extending from one side of the envelope 11, i. e. at an angle with respect to the axis of envelope 1l. The angle formed by the neck 13 and envelope 11 is not critical and may be Within the approximate range of twenty to ninety degrees. Within the neck portion 13 there is provided an electron gun 14 which includes a cathode 16, a control grid 18, and one or more accelerating electrodes 20 and 21. A nal accelerating electrode 22 is in the form of a wall coating as shown. Electrical connection to the accelerating electrode 22 is made by means of spring lingers 23 mounted on the accelerating electr-ode 21. The electrodes of gun 14 provide an electron beam 24 which is deflected over a target 26 by means including a deliection yoke 28. Any conventional scanning, such as line and frame scansion, may be utilized. The electrodes of gun 14, as well as the deflection yoke 28, may be conventional.

In accordance with this invention target 26, which is disposed within one end of envelope 11, see Figures 1 and 2,A comprises an insulating support plate 30 which may be a separate support plate as shown, or in the alternative, the end of envelope 11 may function as a support plate. The target 26 comprises a plurality of conductive signal strips 32, 33 and 34 spaced apart on the inner surface of support plate 30. The conductive strips 32, 33 and 34 function as signal strips for information of three primary colors, e. g. red 32, green 33, and blue 34. Covering all of the conductive strips 32, 33 and 34, and extending onto the support plate 30 between the signal strips, is an insulating surface 35 which preferably has a high secondary electron emission ratio. The support plate may be made of an insulator such as glass or mica. Signal strips 32, 33 and 34 may comprise evaporated strips of gold, or silver, or other highly conductive material and may be of any desired thickness since they need not be transparent. Each set of signal strips for a given color, e. g. signal strips 32 for red information, is connected to a separate lead-in wire 36. The lead-in wires are shown as extending through support member 30 for simplicity of illustration. An example of the Width and spacing of signal strips 32, 33 and 34 is a strip 0.001 inch wire and a spacing between adjacent strips of 0.00025 inch. The thickness of the signal strips may range from a few hundred to many thousand angstrom units. It should be understood that the signal strips 32, 33 and 34 may be transparent, but it has been found that it is dimcult to manufacture a large group of signal strips which have good electrical continuity and which are transparent. In actual practice the insulator 35 may also provide the support for signal strips 32, 33, and 34 and thus, support member 30 may then be omitted.

In accordance with this invention, in the other end of envelope 11 there is provided a photocathode 37, which is shown in greater detail and enlarged in the fragmentary sectional view in Figure 3. Photocathode 37 includes a plurality of color filter strips 39, 40 and 41, each of which is designed to selectively pass a prdeetermined primary color, e. g. red 39, green 40 and blue 41. The order of color filter strips 39, 4@ and 41 is repeated throughout the surface of the photocathode 37. Covering all of the color filter strips 39, 42 which is preferably highly transparent. Covering the protective layer 42 is a layer of conducting photoemissive material 44. The protective layer 42 may be omitted if the photoemissive material does not react chemically with the filter materials.

The color lter strips may be any conventional type of 40 and 41 is a protective layer color filter strips such as multilayer interference filters, Fabry-Perot filters, absorption filters, or other well known types of color filters, and may pass any of the primary colors with the red, green and blue colors being selected merely for illustration. The color filters may be electrically insulating or conductive. For example, the interference filters, which are electrically insulating, may be formed by evaporating successive layers of material through a wire masking grill. Interference type color filters having the desired pass band may thus be formed. To form red color filters 39, successive layers of zinc selenide and magnesium fluoride are evaporated. Each of the layers has a thickness corresponding to an optical thickness of a quarter wave length of light of approximately 4300 angstrom units in wave length. The blue filters 40 are formed of the same materials with layers of an optical thickness of a quarter wave length of light of about 5730 angstrom units in wave length. Sufficient layers are formed so as to obtain a filter of the desired color efiiciency. For example, eleven layers have proved satisfactory in practice. The green color filters are formed by evaporating successively a three quarter wave length optical thickness of zinc selenide and one quarter wave length thickness of magnesium fluoride of light of approximately 6500 angstrom units wave length. Again sufficient layers are formed, such as nine, so as to establish an efficient green filter.

The protective coating 42 may be a coating of silica dioxide which may be a few thousand angstrom units thick and which may be deposited by evaporating silicon monoxide and subsequently baking at a few hundred degrees temperature in air. Also, the protective layer may be a material such as tin oxide. The conducting photoemissive layer 44 may be any conventional layer having a panchromatic response such as bismuth-silvercesium. The thickness of photoemissive layer 44 may be approximately three hundred angstrom units. The purpose of the protective layer 42 is to prevent harmful reaction between the signal strips 39, 40 and 41 and the photoemissive layer 49.

During operation, potentials are applied to the tube 1 0 such as those shown in Figures l, 2 and 3. It should be understood that these potentials are merely an example of one method of operating the device and other potentials may be utilized. Also, the output signals may be obtained from other circuit elements than those shown.

Assuming that no image is directed upon photocathode 3 7, the electron beam 24 establishes a charge on the surface of target 26 that is approximately three volts positive with respect to the collector electrode 22. During the :V30 of a second interval between scans by the electron beam 24 this three volt charge leaks off and may actually go a few volts negative before it is replaced the next time the particular elemental area is struck by the beam 24. This leakage of the charge may occur partially by leakage through the insulator but is primarily the collection of secondary electrons from other portions of the target 26 when these other portions are bombarded by beam 24.

When light from a scene to be reproduced is directed onto the photocathode 37, the light is divided into various colors by the color filter strips 39, and 41. Thus, emission from the photoemissive layer 44 depends upon the color of the original scene. This electron emission is imaged onto target 26 by means of a focus coil 46. The photoemission from the excited areas of photocathode 44 is focused, by following the magnetic field lines of a coil 46, to strike areas of the insulator 35 which are directly adjacent to corresponding signal strips 32, 33 and 34. For example, photoemission initiated by red light passing through red filters 39 strike the insulator 35 adjacent to red signal strips 32. The focusing action of coil 46 is such that electrons from the photocathode 44 travel a substantially straight path across the tube and strike the insulator 35 adjacent to a signal strip 32 that is in a location on the target 26 that is equivalent to the location of the emitting area on the photocathode 44. Similar action takes place when electrons from photocathode 44 originate from blue and green light in that these electrons land on the insulator 35 adjacent to blue and green signal strips respectively.

When the light from a scene to be reproduced causes photoelectrons from photocathode 44 to strike the target 26, the areas corresponding to the bright parts of the picture become most positively charged since the photoelectrons striking the target 26 have a secondary emission ratio that is greater than unity. Thus, the photoelectrons deposit a positive charge pattern on the target 26 with the amount of charge varying from point to point depending upon the brightness of the scene to be reproduced.

Now, assuming that a scene has been focused on photocathode 44 and has developed a charge pattern on target 26. When the beam 24 strikes the elemental areas of target 26 which are charged most positively, fewer electrons are released by secondary emission from these areas than from uncharged elemental areas. The release of fewer electrons by the beam 24 in the positive areas produces an output signal in a load resistor 48 for the particular color of light which originated the electron emission from the photocathode 44. The electrons which are released by the electron beam 24 scanning the charged areas of target 26 are redistributed onto other areas of target 26.

Referring now to Figures 4, 5 and 6 there is shown a sectional view of another embodiment of this invention. In this embodiment of the invention tube 10 comprises an evacuated envelope 11 having a neck 13 which is offset from the axis of envelope 11 and which encloses an electron gun 14'. These elements are substantially the same as those previously described in connection with Figure l. In the embodiment of the invention shown in section in Figure 6 the target 26 is also substantially the same as that previously described. However, the photocathode 50 is of a configuration which provides an initial direction to the photoelectrons which are emitted therefrom. The photocathode 50, which is shown in greater detail in Figure 5, comprises a support member 52 having an inner surface which is serrated. On each of the fiat portions 53 of the serrated surface, i. e. the surface of the support member 52 which is parallel to the wall of the envelope, and which may be the inner surface of the wall of the tube envelope, there is provided a green color filter strip 55. While on one side 57 of the serrations there is provided a red color filter strip S9, and on the other side 60 of the serrations there is provided a blue color filter strip 61. Covering the color filters 57, 59 and 61 is a protective insulating coating 63. Covering the insulating coating 63 is a conductive photoemissive cathode 65. As can be seen from Figure 5 the photoemissive cathode 65 has a serrated electron emissive surface that provides an initial direction to the electrons representing light of the different colors. The materials for the color filters, the protective layer, and the photoemissive layer may be similar to those previously described.

Supported closely adjacent to photocathode 50 is a fine mesh screen 67 which is for the purpose of providing a high voltage gradient during tube operation.

Surrounding the envelope 10 is a focusing coil 46' which provides focusing action for the photoelectrons emitted from photocathode 65. Spaced adjacent to the target 30 there is provided a masking grill 69 which produces a shadow effect of the electrons from photocathode 50. The masking grill 69 substantially covers two of the signal strips 32', 33 or 34 within a cluster of three strips, with an aperture between grill wires being substantially the size of a single signal strip 32', 33 or 34' within a cluster.

During operation of the embodiment shown in Figure 4 the action of the electron gun 14 is substantially the same as that previously described. When a scene is focused on the photocathode 65 the emission from photocathode 65 leaves the photocathode with an initial direction which is different for each of the three selected primary colors. This initial direction is shown by the electron paths as sketched on Figures 5 and 6. In other words, for the tube shown, electrons from adjacent a green color filter 55 travel substantially straight along the axis of envelope 11'; while electrons from adjacent a red color ilter 59 start with an initial downward direction; and electrons from adjacent a blue color lter 61 started with an initial upward direction. Each of these separate groups of electrons for each bundle of three primary colors is directed toward the target 26 and is focused through the masking grill 69 before it reaches the insulating layer 35. The electrons approach the masking grill 69 from three distinct directions and the masking grill, by its shadowing action, causes each group of electrons to land on the insulator 35 adjacent to a corresponding signal strip for that color. Thus, the masking grill 69 insures the registry between the color iilters in one end of envelope 11 is correct with the respective signal strips in the other end of envelope 11. This shadowing action is schematically represented in Figures 5 and 6.

ln the embodiment shown in Figure 4 a single potential is applied to grill 69. However, it should be understood that a varying potential may be applied to grill 69 for providing further focusing of the emission from the photocathode 50.

I claim:

l. A television camera tube for color television transmission comprising an envelope, a target supported Within said envelope and including a plurality of conductive signal output electrodes and an insulating layer thereover, a photocathode in said envelope and spaced from said target and including a plurality of color filters and a photocmissive layer adjacent said filters, said target receiving photoelectrons from said photocathode during operation of said tube, and means for directing an electron beam toward said target to be scanned thereover.

2. A camera tube for color television transmission cornprising an envelope, a target in said envelope including a plurality of conductive signal strips and a layer of insulating material covering said strips, a photocathode in said envelope including a plurality of color lter strips within said envelope and spaced from said target, and a photoemissive layer supported adjacent said -color lter strips, said target receiving photoelectrons from said photoemissive layer during operation of said tube, said signal strips each being in registry with a different one of said color filter strips, and means within said envelope for directing an electron beam toward said target to be scanned thereover.

3. A camera tube for color television transmission comprising an envelope, a target electrode in one end of said envelope including a plurality of conductive signal strips and a layer of insulating material, said signal strips being electrically insulated one from the other, a photocathode in the other end of said envelope including a plurality of color iilter strips and a photocmissive layer, each of said signal strips being positioned opposite one of said color filter strips whereby photoelectrons from adjacent each of said color lter strips land on said insulating material adjacent to a different one of said signal strips, and means within said envelope for directing an electron beam onto said insulating material.

4. A pickup tube for color television transmission comprising an envelope, a plurality of conductive signal strips in one end of said envelope, an insulating layer covering said signal strips, a plurality of color filter strips in the other end of said envelope, a photocmissive layer covering said color iilter strips, each of said signal strips being arranged opposite one of said color iilter strips, an electron gun for producing a beam of electrons, and means for scanning said beam over said target.

5. A pickup tube comprising an envelope, a plurality of color iilter strips in one end of said tube, a photoernissive layer covering said color filter strips and providing an electron image in response to light from a scene to be reproduced, a target electrode in the path of said electron image, said target electrode including an insulating layer and a plurality of conductive signal strips, an electron gun for producing an electron beam, said target being arranged in the path of said beam.

6. A pickup tube comprising an elongated envelope, a target electrode in one end of said envelope and including an insulating layer and a plurality of signal strips, a photocathode in the other end of said envelope and including a plurality of color filters and a photocmissive layer, said photocmissive layer having a serrated surface facing said target, and an electron gun for producing an electron beam, said target being supported in the path of said beam.

7. A pickup tube comprising an evacuated envelope, a photocathode in one end of said envelope and including a plurality of color filters and a photocmissive layer, a target electrode in the other end of said envelope and including a plurality of Signal strips, a masking electrode between said target and said photocathode, and electron beam producing means within said envelope.

8. A pickup tube for color television transmission comprising an elongated envelope, a photocathode in one end of said envelope and including a photocmissive layer and a plurality of color iilter strips, a target in the other end of said envelope and including an insulating layer and a plurality of signal strips, a tine mesh screen spaced closely adjacent to said photocathode, a masking grill between said mesh screen and said target, and said photocathode having a serrated surface facing said mesh screen with one of said color lilters on each of the serrations thereof.

9. A camera tube comprising an evacuated envelope, means for producing an electron beam, a plurality of signal output electrodes in one end of said envelope, an insulating layer covering said output electrodes, a serrated member in the other end of said envelope, each of the serrations on said member having a different color lilter strip thereon, and a photocmissive layer supported by said member.

l0. A pickup tube comprising an evacuated envelope, electron beam producing means within said envelope, a target in one end of said envelope and comprising a plurality of spaced apart conductive signal strips and an insulating layer on said strips, a photocathode in the other end of said envelope and including a plurality of color lter strips and a photoemissive layer on said color filter strips, and focusing means surrounding said envelope for focusing the emission from said photocathode onto said target.

Lubszynski Dec. 2, 1941 Schroeder Aug. *10, 1948

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