US2820167A - Tricolor pickup tube - Google Patents

Description

Jan. 14, 1958 A. c. SCHROEDER TRICOLOR PICKUP TUBE F'ild April 30, 1954 v I NVENTOR. ElLFRED E. SEHRUEDER Unite States TRICOLOR PICKUP TUBE Alfred C. Schroeder, Upper South Hampton Township, Bucks County, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application April 30, 1954, Serial No. 426,823

4 Claims. (Cl. 313--65) This invention relates to television pickup, or camera, tubes and particularly to a target structure for television camera tubes for use in conjunction with tri-color television systems.

In the past, various target structures have been utilized for tri-color television camera tubes which have been relatively complicated in design and therefore expensive to produce on a mass production basis.

A principal object of this invention is to provide a new and improved tri-color television camera tube, and target therefore, that is simple to construct and therefore adapted for mass production techniques.

The principal object of this invention, as well as other objects and advantages thereof, is accomplished in accordance with the general aspects of this invention by providing a camera tube wherein the target structure comprises a plurality of signal output electrodes each having means including a photosensitive material thereon. The signal output electrodes are spaced in closely adjacent planes and each of the means is sensitive to different light frequencies whereby each signal output electrode provides the output signal for a different primary color. The signal output electrodes are arranged so that each is exposed to both an electron beam and radiations from a scene to be reproduced.

The novel features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself will best be understood by reference to the following single sheet of drawings wherein:

Fig. 1 is a transverse sectional view of a camera tube utilizing a target structure in accordance with this invention;

Fig. 2 is an enlarged fragmentary sectional view of the target shown in Fig. 1; and

Figs. 3 through 5 are enlarged fragmentary sectional views of embodiments of target structures in accordance with this invention.

Referring now to Fig. 1, there is shown a pickup tube comprising an evacuated envelope 11 with an electron gun 12 mounted at one end thereof. The electrodes of the electron gun include the usual cathode, control electrode, and one or more accelerating anodes which are connected to lead-in pins in the well-known manner. An electron beam from the gun 12 is directed upon target 13 at the other end portion of envelope 11. Means are provided for focusing the electron beam and scanning the beam over target 13 to form a raster and may include a focus coil 14, a deflection yoke 15, as well as an alignment coil 17. An electrode 16, which is permeable to the electron beam, is positioned adjacent to the target 13 and during operation, together with focus coil 14, functions to insure that the electron beam in its final approach to the surface of target 13 is normal thereto. Final accelerating electrode 18 is in the form of conductive coating on the interior of envelope 11. Fingers mounted on 18 to one of the lead-in pins.

2,820,167 Patented Jan. 14, 1958 Target 13 is conventionally supported adjacent to the transparent window 20 and terminal pins 19 sealed through the envelope are connected thereto. As shown more clearly in Figs. 2 and 3, target 13 comprises a support or backing member 21 having a transparent conductive coating 22 on the side thereof toward the electron beam. The transparent conductive coating functions as a signal output electrode. On the surface of the transparent conductive coating 22, i. e., the surface of coating 22 which is exposed to the electron beam, there is provided a layer of photosensitive material such as photoconductive material 23. Closely spaced adjacent to the photoconductive material 23 is a plurality of parallel conductive members 24, each of which is shown as a wire having a coating of photosensitive material, such as photoconductive material 25, on the surfaces thereof that are exposed to both the electron beam and to the light image. The conductive members 24 function as another signal output electrode. Spaced closely adjacent to the parallel conductive members 24 is a similar set of parallel conductive members 26, each of which has a coating of photosensitive material, such as photoconductive material 27, thereon. The conductive members 26 function as still another signal output electrode. It should be noted that conductive coating 22, conductive members 24, and conductive members 26 are arranged in substantially parallel spaced planar relationship substantially normal to the electron beam path.

In accordance with this invention, conductive coating 22, conductive members 24, and conductive members 26 each function as a signal output electrode for light of a substantially different wavelength. As an example, conductive coating 22 may be the output electrode for the red portion of the spectrum, while the conductive members 24 may be the output electrode for the blue portion of the spectrum, and the conductive members 26 may be the output electrode for the green portion of the spectrum.

In the embodiment of target 13 shown in Fig. 2, the transparent conductive coating 22 is shown as being supported by a support plate 21. When desired, the support plate 21 may be omitted and the transparent conductive coating supported directly on the glass window 20 shown in Fig. 1. When utilized, the support plate 21 may be made of a material such as glass, quartz, or other insulating transparent material. The transparent conductive coating 22 may be a thin coating of any transparent conductive material such as tin oxide or tin chloride. The thickness of the transparent conductive coating 22 may be approximately $5 of an inch. The thickness and the materials given above are given merely as an example of this invention and should not be interpreted so as to limit the invention to these particular materials or the approximate thickness.

The photoconductive material 23, which is supported on the transparent conductive coating 22, may be any of the well-known photoconductive materials and in accordance with one embodiment of this invention, the photoconductive material 23 is responsive to light of a wavelength of one of the primary colors. As an example, the photoconductive material 23 may be a material such as selenium which is responsive to blue light; porous antimony sulfide, which is responsive to green light; or antimony sulfide in the solid form which is rer' sponsive to red light. Assuming that the photoconductive material 23 is responsive to light of the red portion of the spectrum, then the photoconductive materials 25 and 27 would be responsive to light of either the green or blue portions of the spectrum respectively.

It should be understood that other photoconductive materials may be utilized for photoconductors 23, 25, and 27, which are responsive to the same or other colors,

Also, other types of photosensitive materials may be utilized such as photoemissive materials. Examples of suitable photoemissive materials are cesiated silver oxide for the red, cesiat'ed bismuth silver oxide for the green, and ce'siated antimony for the blue. Also, other methods of obtaining separate light sensitivities for each of the signal output electrodes 22, 24, and 26 may be utilized. One such other method is to utilize a photoconductive material which is responsive to light of all wavelengths for all of the photoconductors 23, 25, and 27, and to utilize color filters which are responsive to the separate colors in conjunction with each of the signal output electrodes.

As can be seen from Fig. 2, when a light image is focused on the target 13, the light image strikes either the photoconducto'r 23,- the photoconductor 25, or the photoconductor 27. In other words, the portions of the light image which are not absorbed by photoconductor 25 pass through to strike either photoconducto-r 25 or photoconductor 27. Also, the electron beam scans each of the various photoconductors by means of portions thereof passing through the apertures between photoconductor 25 and photoconductor 27 to land on photoconductor 23. In other words, the composite signal electrode 26 and pho-toconductor 27, as well as the composite signal electrode 24 and photoconductor 26 are permeable to the electron beam.

In practice, it is preferable that the photoconductor which has the poorest light response be arranged on the transparent conductive coating 22 since some light of all frequencies will be filtered by passing through the transparent conductive coating 22 and the photoconductor 23.

The thicknesses of photoconductive materials 23, 25, and 27 may be approximately of an inch. The parallel conductive members 24 may be in the form of a plurality of parallel wires. The number of wires should be such that the aperture size through conductive members 24- and 25, including photoconductive materials 25 and 27 respectively, is less than the size of a picture element. In order to obtain this aperture size, a specific structure will depend upon the size of the target utilized. Generally, the number of wires in conductive members 24 and 26 may be substantially 500 per inch with a spacing between adjacent wires of approximately A of an inch. As shown, the photoconductive materials 25 and 27 are supported on parallel conductive members 24- and 26 only on the surfaces thereofwhich are toward the electron beam. This location of the photoconductive materials is shown merely as an example of the construction features of target 13. Due to the fact that the active areas of: photoconductive materials 25 and 27 are the areas of photoconductor which are struck by both the electron beam and the light image, it is necessary to provide photoconductive material only in these areas. As is obvious, the areas which are exposed to both the light image and the electron beam are the areas of photoconductive material which are on the top and bottom of each of the individual wires as shown in Fig. 2.

In operation, target 13 may be oriented as desired in tube it}. For example, photoconductive members 25 may be oriented parallel or perpendicular to the direction of scan. Pickup tube it: may be operated in several Ways and either low or high velocity operation is suitable. In the so-called low velocity mode of operation, the electron beam which scans target 13 arrives with such low velocity that the ratio of the secondary electrons dislodged out of a photoconductor to the number of primary electrons deposited by the electron beam is less than unity. In the absence of light from an image to be reproduced, the electron beam keeps the scanned surface of each of the photoconductive materials at substantially cathode potential. The signal electrodes, i. e., conductive coating 22, conductive members 24, and conductive members 26, 'for each of the various colors may be biased to some moderate positive potential with respect to the cathode as for sample, 50 volts.

When an image is focused on target 133, electron flow occurs through the photoconductors 23, 25, or 27, depending upon which photoconductor is actuated by the particular image. When the image contains components of the three primary colors each of the photoconductors 23, 25, and 27 is actuated by the respective colors, and output signals proportional to the light and shade of the image will be formed on the signal electrodes, i. e., conductive coating 22, conductive members 24, and conductive members 26.

The frame time, or time for completing one scansion of the target 13, may be 1 of a second While the element time, or the time the beam is on a given elemental surface portion, may be lflgooovooo of a second for a four megacycle per second signal. Considering now an elemental portion of the target 13 with light falling thereon just after the electron beam has passed, the effect of light on the photoconductive material is to charge the elemental portion positively during the next ,4 0 Of a second. Thus, during the frame scansion time, an elemental area loses negative charges as a consequence of the movement of electrons from the surface of the photoconductive material. When the beam scans the elemental area the beam deposits suflicient electrons to neutralize the accumulated charge on the surface of the elemental area and the surface thereot is returned to its dark potential, i. e., cathode potential. When the beam neutralizes an accumulated charge, it generates a video signal in the signal output electrode by means of the capacity coupling across the photoconductor.

As is obvious, when photo conductive material 23 is selected to be responsive to light of a particular Wavelength, as an example red, output signals are obtained from transparent conductive coating 22 only when light of the red wavelengths is directed onto the photoconductor 23. Similar action occurs with respect to the photoconductors 25 and 27, which may be selected to be responsive to the blue and green light in the above example. Only the sections of photoconductive materials 25 and 27 which are exposed to both the electron beam and the light image are operable for producing output signals since the nature of the photoconductive material requires both light and electron energizing to produce current flow therein.

As was set forth above various means, other than separate photoconductive materials responsive to different Wavelengths, may be utilized so that each of the signal electrodes produces a signal in response to light images of different colors. Furthermore, when desired, the transparent conductive coating 22 may be in the form of a third mesh electrode having a photoconductive material supported thereon. Still further, the photoconductive material 23 may be a discontinuous photoconductor to permit a larger amount of light to fall on the photoconductors 25 and 27.

Referring now to Fig. 3, there is shown an embodiment of a target structure for tri-color television camera tubes in accordance with this invention. The target 36 diliers from the target 13 shown in Fig. 2 in that a photoconductor 31 is supported on a signal electrode 32 comprising a fine mesh electrode adjacent to the photoconductor 23. Also, photocond'uctive material 33 is supported on a signal electrode 34 comprising a fine mesh electrode which is closely adjacent to signal electrode 32. Support sheet 21', transparent conductive coating 22, and photoconductor 23' are substantially the same as described in connection with target 13. The materials and thicknesses disclosed in connection with Fig. 2 may also be utilized in connection with Fig. 3. Furthermore, the operation of target 36 may be substantially the same as that previously described.

Referring now to Fig. 4, there is shown an embodiment of a target structure for use in a tri-color camera tube in accordance with this invention wherein the apertured signal electrodes comprise: a plurality of parallel vertical conductive strips 37 having photoconductive material 38 thereon, and a plurality of parallel horizontal conductive strips 39 having a photoconductive material 40 supported thereon. Other arrangements of the plurality of parallel metallic members having photoconductive material thereon may be utilized. However, in order to avoid moire effects, the adjacent parallel conductive members 37 and 39 should be parallel, or at a sufiicient angle with respect to each other. Support member 21", transparent conductive coating 22", and photoconductor 23" are substantially the same as those previously described.

The conductive members 37 and 39 may comprise a plurality of strips of conductive material, such as copper or nickel, and are preferably arranged so that the smaller edge thereof is exposed to boththe electron beam and the light image whereby the least amount of filtering action of both the light and the electron beam occurs and the largest possible amount of photocondnctive material is exposed to both the electron beam and the light image. The materials, sizes, and mode of operation of target 36 are substantially the same as that disclosed in connection with target 13 described above so that further description thereof is not deemed necessary.

Referring now to Fig. 5, there is shown an embodiment of a target for use in a tri-color pickup tube in accordance with this invention wherein an apertured signal electrode comprises an apertured plate 59 having a photoconductive material 51 supported on both sides of the apertured plate 50 to surround each individual aperture of the apertured plate 50. Supported closely adjacent to the apertured plate 50 is a similar signal electrode which comprises an apertured plate 52 supporting a photoconductive material 53. The apertured plates 58 and 52 may be constructed by etching or stamping a sheet of conductive material as is well-known in the art. The support plate 21, the transparent conductive coating 22", and the photoconductor 23" are similar to those of target 13. The materials and operation of target 49 are also substantially the same as that previously described.

It should be understood that in all of the embodiments of the target structures shown in Figs. 2 through 5 inclusive, other means may be provided for making each of the individual signal electrodes responsive to one primary color. One other such means is set forth above in connection with the description of Fig. 2. Also, other types of photosensitive materials may be utilized.

What is claimed is:

1. A tri-color pickup tube comprising an envelope containing means to project an electron beam along a path and a target in said path, said target comprising at least three signal output electrodes each supporting a separate photoconductive material, said signal electrodes being supported in spaced planes substantially normal to said path, and at least two of said signal output electrodes each comprising a set of substantially parallel spaced metallic members.

2. A target for a tri-color pickup tube comprising a first signal electrode comprising a transparent conductor having a first photoconductor thereon, a second signal electrode including a plurality of spaced apart parallel wires and having a second photoconductor thereon, said second photoconductor partially filling the space between adjacent wires of said second signal electrode, a third signal electrode including a plurality of spaced apart parallel wires and having a third photoconductor thereon, and said third photoconductor partially filling the space between adjacent wires of said third signal electrode.

3. A target for a tri-color pickup tube comprising a first signal electrode comprising a transparent conductive sheet, a first photoconductor on said sheet, a second signal electrode comprising a first plurality of spaced parallel metallic strips, a second photoconductor on said first plurality of strips, said second photoconductor partially filling the spaces between adjacent strips, a third signal electrode comprising a second plurality of spaced parallel metallic strips, a third photoconductor on said second plurality of strips, said third photoconductor partially filling the spaces between adjacent strips, and said metallic strips of said second electrode being at an angle with respect to said metallic strips of said third signal electrode.

4. A tri-color pickup tube comprising an envelope containing means to project an electron beam along a path, a target in said path, said target comprising at least three signal output electrodes each supporting a separate photoconductive material, said signal electrodes being supported in spaced planes substantially normal to said path, at least two of said signal output electrodes being sets of substantially parallel conductive strips, the photoconductive material supported by said two signal output electrodes partially filling the apertures therein, and the conductive strips of one of said sets being supported at an angle with respect to the conductive strips of the other of said sets.

References Cited in the file of this patent UNITED STATES PATENTS 2,100,841 Farnsworth Nov. 30, 1937 2,322,807 Iams June 29, 1943 2,614,235 Forgue Oct. 14, 1952 2,687,484 Weimer Aug. 24, 1954

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