US2770746A

US2770746A - Cathode ray tube and target

Info

Publication number: US2770746A
Application number: US344616A
Authority: US
Inventors: Gray Sidney
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1953-03-25
Filing date: 1953-03-25
Publication date: 1956-11-13
Anticipated expiration: 1973-11-13
Also published as: CH329225A; NL186169B; GB753155A; BE527543A; DE1015047B; FR1097340A

Description

Nov. 13, 1956 s. GRAY CATHODE RAY TUBE AND TARGET Filed March 25, 1953 INVENTOR.

Jjdlzey Gray A TTOR NE 1 CATHODE RAY TUBE AND TARGET Sidney Gray, New Brunswick, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 25, 1953, Serial No. 344,616

17 Claims. (Cl. 313-65) My present invention relates to cathode ray electron tubes and targets therefor. More particularly my invention relates to tubes in which a varying electron charge image is formed in a single plane and which are suitable for use as color television pickup tubes.

Such tubes have been made in which an electron beam scans a target with all of the scanning occurring in a single plane. The scanned surface of ray sensitive material, in the form of spaced elements or strips or in the form of a continuous coating, is usually capacitively coupled to a signal plate. The signal plate is segmented as for example in the form of strips which are of conductive material and extremely thin. Often the signal plate strips are so thin as to be transparent. Furthermore, the strips are very narrow and arranged in close spaced relation. The transparent conductive strips are formed on color filters which extend across the target while the ray sensitive material, which may be photo-conductive, is laid down on the transparent conductive strips. The color filters are interference filters such as may be formed from successive layers of suitable material; one having a high and the other a low index of refraction. Conveniently, the color filters and transparent conductive strips forming the segmented signal plate are formed by evaporating the materials through a wire masking grill as described in the U. S. Patent 2,745,773 of P. K. Weimer, Serial No. 344,498, filed simultaneously herewith and assigned to the assignee of this application. The color filters are of insulating material and arranged in a repetitive pattern of groups of colors. The transparent conductive strips on the filters of the same color are connected to bus bars, usually at both ends thereof, so as to form interlaced sets with a set for each color.

When object light falls on such a target, the light passes through the filters and transparent conductive strips into the photo-conductive material. Assuming red, green and blue filters, the scanned surface of the ray sensitive material assumes a charge pattern in the absence of the electron beam in accordance with the color components and intensity of the object light. Areas receiving light through the red filters respond only when the object light includes red and is substantially insensitive to green or blue. The electron beam acts to return the elemental area it impinges upon to what may be termed its discharged condition and simultaneously a voltage pulse appears on the transparent conductive strip. Ideally, the voltage pulse sees an infinite impedance except along the conductive strip to the bus bars connecting that strip to the output circuit.

As pointed out above the transparent conductive strips are closely spaced and in view of the interlaced arrangement described a conductive strip for one color has closely adjacent thereto a strip of each of the other colors. Because of this close spacing, as well as for other reasons including the scattering of some conductive material into the spaces, the impedance between adjacent transparent conductive strips is objectionably low. In fact, the re- States Patent active impedance between the sets of transparent conductive strips is fairly low.

In practice it is proven difiicult to make such tubes which produce high fidelity color video signals. Furthermore, the pictures obtained at the color television receiver are often streaky, while in portions of the picture colors which should be uniform are non-uniform.

I have found that color fidelity may be improved, cross talk between the different colors as well as streaks in the picture obtained may be substantially eliminated, by improving the connection between each of the transparent conductive strips and the bus bars connected thereto. 1 have found that pickup tubes while seemingly operating satisfactorily cause the defects appearing at the receiver because of high impedance connections between the conductive strips and the bus bars. I attribute the cause of the foregoing ditficulties to the fact that at least some of the transparent conductive strips are connected to their bus bars through a relatively high impedance. The impedance is sufiiciently high that the voltage pulses appearing on the strips travel to adjacent strips almost as well as they travel along 'a strip.

It is therefore a principal object of my invention to provide an improved cathode ray tube, particularly suitable as a pickup tube for color television, capable of pro ducing color video signals of high fidelity and uniformity.

Another object of my invention is to provide an improved color television pickup tube having sets of interlaced transparent conductive strips having uniformly low impedance connections to their respective leads or bus bars.

A more specific object is the provision of a target for cathode ray tubes particulary suitable for a color television pickup tube in which the conductive signal strips are connected to their respective bus bars by uniformly low impedance connections.

In accordance with my invention it provide a target particularly suitable for color television pickup tubes in which each transparent conductive signal strip in a set of signal strips is connected to its respective bus bar by means of a conductive material which reduces the impedance between the strip and its bus: bar. Preferably, I utilize a material which not only reduces the reactive impedance but which renders intervening nonconductive films conductive.

Other objects and features of my invention will be apparent from the accompanying detailed description thereof and the drawing in which:

Figure 1 is a sectional view of a pickup tube, being illustrative of one particular type of cathode ray tube, suitable for color television;

Figure 2 is a plan view of the target of the tube shown in Figure I, greatly enlarged and not drawn to scale for convenience;

Figure 3 is a fragmentary sectional view of the target shown in Figure 2.

For the purpose of illustrating my invention I will now describe the same in connection with photo-conductive pickup tube 10 shown in Figure 1. However, it should be clearly understood that my invention is equally ap plicable to other types of tubes and in particular to other types of pickup tubes suitable for color television. Pickup tube 10 comprises an envelope 11 having the usual electron gun 12 mounted therein. As is well known, the electron gun 12 contains a cathode and various accelerating and focusing electrodes. A final accelerating and focusing anode 13 is usually in the form of a coating as shown; electrical connection thereto being made by means of spring fingers mounted on electron gun 12 and insulated therefrom. A target 14 which is scanned by an electron beam generated by the electron gun 12, is

mounted adjacent to the opposite end of envelope 11. The target 14 may also be formed as part of the envelope itself but in any case is mounted so that object light passing through a camera lens system (not shown) may be readily directed thereupon. Terminal pins 15, sealed through the envelope, afford convenient means for making electrical connection with and supporting target 14. Means are provided for scanning the electron beam over the surface of target 14 and may include focus coil 16, deflection yoke 17, and alignment coil 18. To insure that the electron beam at its final approach to the surface of target is normal thereto a fine mesh screen electrode 19 is mounted adjacent to target 14. One of the terminal pins is provided for making electrical connection with electrode 19.

Referring now to Figures 2 and 3, target 14 includes means for supporting various films or coatings and may be a transparent supporting sheet 20, such as glass, having a plurality of spaced conductors or bus bars 21 formed thereon. A bus bar 21 is provided for each of the video signals. Preferably two sets of bus bars are formed one adjacent opposite ends of sheet 20. Color filter strips 22 extend across transparent insulating sheet and are color filters that pass the desired colors such as red, green and blue. As described in detail in the above referred to patent of P. K. Weimer, the color filter strips 22 are formed by evaporating successive layers of high and low index of refraction material. The materials utilized namely zinc selcnide and cryolite or zinc sulphide and cryolite form insulating films when laid down.

Transparent conductive strips 23am formed one on each of the filter strips 22. Transparent conductive strips 23 are each somewhat longer than filter strips 22 and extend over and make electrical contact with respective bus bars 21. Conveniently on one side red filter strips 22 extend over the blue and green bus bars and serve to insulate the red signal strips therefrom. Similarly green filter strips 22 extend over the blue bus bar 21 and afford similar insulation between each green signal strip and the blue bus bar. On the opposite side of transparent sheet 20 the order of the bus bars 21 from the inside out is seen to be reversed. Intermediate each of the conductive signal strips 23 and the bus bar 21 with which it makes contact there is a short strip or finger 25 formed by evaporating conductive material through a suitably formed and registered mask. The reactive impedance between each strip and its bus bar is substantially reduced when such materials as aluminium or gold are used to form the fingers. Preferably I utilize silver or material having similar properties in forming fingers 25. I have found that because of properties characteristic of silver, fingers '25 not only reduce the reactive impedance of the connections between signal strips 23 and bus bars 21 but also make them substantially uniform and highly conductive. As shown, fingers 25 are longer than bus bars 21 are wide.

Ray sensitive material 24 in the form of a coating is applied to transparent conductive strips 23. Coating 24- is substantially thicker than any of the other strips or coatings formed on transparent sheet 20 and is so indicated in the drawing. I11 a photo-conductive pickup tube such as tube 10, photo-conductive porous antimony sulphide may be utilized; such material being described in the U. S. Patent 2,744,837 of S. V. Forgue, filed June 1, 1951, and assigned to the assignee of this application.

As pointed out above, the various coatings and strips may be formed by evaporating suitable materials onto transparent sheet 20. In practice bus bars 21 are formed preferably by evaporating such material as gold through a suitable mask to form relatively thick conductive lines which are spaced apart as shown. While bus bars 21 are described as being relatively thick they are nevertheless non-self-supporting coatings and generally of such a thickness as not to be transparent. Fingers 25 are then laid down located so as to extend intermediate each signal strip and its bus bar. Silver fingers 25 are formed by evaporating silver through a mask about 500 angstrom units thick. The various color filter strips are formed by evaporating successive layers through a wire masking grill. Color filters having the desired pass band may thus be formed. To form the red filters successive layers of zinc sulphide and cryolite 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 are formed with the same materials with layers of and optical. thickness of a quarter wave length of light of about 5730 angstrom units in wave length. Sulficient layers are formed so as to obtain a filter of the desired efficiency. For example, eleven layers have proved satisfactory in practice. The green color filters are formed by evaporating successively a three quarter wave englh optical thickness of zinc selenide and a quarter wave length thickness of cryolite of light of approximately 6500 angstrom units in Wave length. Again sufficient layers are formed, such as nine, so as to establish an efficient green filter.

Transparent conductive strips 23 are preferably formed from gold as described in my application Serial No. 344,615, filed simultaneously herewith and assigned to the assignee of this application. The gold signal strips are extremely thin and are estimated to range in thickness from approximately 50 to angstrom units.

From the foregoing it is apparent that color filter strips 22 are also extremely thin though relatively thicker than signal strips 23. I have found that during the evaporating of color filters 22, material is scattered by the wires of the mask grill into the spaces on transparent sheet 20 lying behind the wires of the grill. This scattering of the filter material does not interfere with the formation of good filters. However, I have found that a film of filter materials of approximately a half wave length in thickness is formed over at least some of the bus bars where it is necessary for signal strips 23 to make contact. Thus, at least some of the signal strips are separated from bus bars 21 by a thin film of insulating material of a thickness of about /2 wave length. Thus, resistance measurements made after processing of targets without silver fingers indicated those signal strips were completely insulated from their bus bars as to direct current potentials. While such a connection affords capacitive coupling I have found the impedance as compared to the reactive impedance between the different sets of signal strips to be too high in the absence of fingers 25. Since the conductive fingers 25 are longer than the bus bars 21 are wide, the signal strip to bus bar capacitance is increased with a consequent reduction in reactive impedance. When the short strips or fingers 25 are of silver they not only cause a reduction in the reactive impedance, but also serve to render the nonconductive insulating films conductive. While the specific nature of this phenomenon is not completely understood at this time, it is believed that the silver fingers 25 act in some way to activate at least one of the filter materials, probably the zinc selenide, to render the otherwise insulating film conductive.

Targets 14 have uniform low impedance connections between signal strips 23 and bus bars 21. Substantially uniform resistances as low as approximately 100 ohms, the resistance of the strips themselves, have been obtained measuring from bus bar to bus bar along the signal strips as compared to 10,000 to 1,000,000 ohms or more in the past. The low unit impedance along the signal strips to their bus bars and the output circuit substantially eliminates cross talk between adjacent signal strips and the resulting mixing of colors. Furthermore, signal strips having a break along their length may yet produce a uniform color signal in view of the uniform connection at opposite ends thereof to the bus bars 21. Another advantage results from the fact that breaks along the bus bars themselves are no longer troublesome as heretofore inasmuch as such breaks are shunted by the many parallel low impedance paths of the respective signal strip sets. The possibility of streaks appearing in the color picture at the receiver is also reduced since all of the strips of each set are thus uniformly connected by a low resistance connection to their bus bars.

From the foregoing it is apparent that I have provided an improved tube and target particularly suitable as a color television pickup tube. While I have described my invention in detail in connection with the photo-conductive pickup tube shown for purposes of illustration, my invention is applicable to other types of tubes. For example, my invention is applicable also to cathode ray tubes in which ray sensitive material of the electron emissive or light emissive type is used on the target. In some tubes where it is not necessary that light pass through the signal strips then the signal strips need not be transparent.

While I have described color filters which are insulating, other filters which are not insulating or even constructions in which some other arrangement is utilized for separating the primary colors may be used in accordance with this invention. In such case, insulation is required where signal strips of one set cross over bus bars of another set. Such insulation is formed by evaporating insulating fingers. Where an insulating film is formed by scattering of material on bus bars at points where contact is to be made then fingers 25 of conductive material serve to reduce the impedance in spite of the presence of the non-conductive film. Therefore, while various changes and deviations may be made from the specific construction described herein without departing from my invention, it is intended that all such changes and deviations that come within the scope of the appended claims be included in my invention.

What is claimed is:

1. A cathode ray tube, comprising a ray sensitive target, means for projecting toward said target an electron beam which may be scanned over the target, a plurality of spaced conductive signal strips extending across said target, a plurality of bus bars extending across said target, sets of said signal strips each being connected to one of said bus bars and insulated from the other sets and from the bus bars connected to the other sets, each of said signal strips having a portion extending over the bus bar connected thereto, and a plurality of finger strips of conductive material each extending intermediate at least one of said portions and the bus bar connected thereto.

2. A cathode ray tube, comprising a ray sensitive target, means for projecting toward said target an electron beam which may be scanned over the target, a plurality of spaced conductive signal strips extending across said target, a plurality of bus bars extending across said target, sets of said signal strips each being connected to one of said bus bars and insulated from the other sets and from the bus bars connected to the other sets, each of said signal strips having a portion extending over the bus bar connected thereto, and a plurality of finger strips of conductive material each extending intermediate at least one of said portions and the bus bar connected thereto, said finger strips being substantially longer than the width of said bus bars.

3. A cathode ray tube, comprising a ray sensitive target, means for projecting toward said target an electron beam which may be scanned over the target, a plurality of spaced conductive gold signal strips extending across said target, a plurality of gold bus bars extending across said target, sets of said signal strips each being connected to one of said bus bars and insulated from the other sets and from the bus bars connected to the other sets, each of said signal strips having a portion extending over the bus bar connected thereto, and a plurality of silver finger strips each extending intermediate at least one of said portions and the bus bar connected thereto.

4. A pickup tube, comprising a ray sensitive target,

6 means for projecting toward said target an electron beam for scanning said target, a plurality of spaced conductive signal strips extending across said target, a plurality of spaced bus bars extending across said target, a plurality of spaced bus bars extending across said target in a direction transversely of said signal strips, sets of said signal strips each being connected to one of said bus bars and insulated from the other sets and from the bus bars connected to other sets, each of said signal strips having a portion thereof extending over the bus bar connected thereto, and a plurality of conductive finger strips each extending intermediate at least one of said portions and the bus bar connected thereto.

5. A pickup tube, comprising a ray sensitive target, means for projecting toward said target an electron beam for scanning the target, a plurality of parallel color filter strips extending across said target, a plurality of spaced parallel conductive signal strips extending across said target one in registration with each of said color filter strips, a plurality of bus bars extending across said target in a direction transversely of said strips, sets of said signal strips associated with filters of the same color each being connected to one of said bus bars and insulated from the other sets and from. the bus bars connected to the other sets, and a plurality of conductive finger strips each extending intermediate at least one of said signal strips and the bus bar connected thereto.

6. A pickup tube as described in claim 5 wherein said finger strips are silver.

7. A pickup tube as described in claim 5 wherein said bus bars and said signal strips are gold and said finger strips are silver.

8. A pickup tube for color television, comprising a photosensitive target, means for projecting toward said target an electron beam for scanning the photosensitive surface of the target, a plurality of insulating color filter strips extending across said target, a plurality of spaced conductive signal strips one on each of said filter strips, a plurality of bus bars extending across said target in a direction transversely of said strips, signal strips on filter strips of a given color extending across and connected in parallel by one of said bus bars and insulated from the remaining signal strips and the bus bars connected thereto, and a plurality of silver finger strips each extending intermediate at least one of said signal strips and the bus bar connected thereto.

9. A pickup tube for color television, comprising a photosensitive target, means for projecting toward said target an electron beam for scanning a surface of said target, a plurality of insulating color filter strips extending across said surface of the target, a plurality of spaced conductive signal strips one on each of said filter strips, a plurality of bus bars extending across said target in a direction transversely of said strips, signal strips on filter strips of a given color extending across and connected in parallel by one of said bus bars and insulated from the remaining signal strips and the bus bars connected thereto, a plurality of silver finger strips each extending intermediate at least one of said signal strips and the bus bar connected thereto, and a coating of photoconductive material on said signal strips.

10. A ray sensitive target for a cathode ray tube, comprising an insulating supporting member, a plurality of spaced conductive signal strips extending across said member, a plurality of bus bars on said member, sets of said signal strips each being connected to one of said bus bars and insulated from the other sets and from the bus bars connected to the other sets, and a plurality of conductive finger strips each extending intermediate at least one of said signal strips and the bus bar connected thereto.

11. A ray sensitive target as described in claim 10 wherein said fingers are of highly conductive material providing connections between the signal strips and bus bars that are highly conductive as to direct current potentials.

12. A ray sensitive target for a cathode ray tube, comprising an insulating support member, a plurality of spaced conductive signal strips extending across said member, a plurality of bus bars on said member, sets of said signal strips each being connected to one of said bus bars and insulated from the other sets and from the bus bars connected to the other sets, and a plurality of silver finger strips each extending intermediate one of said signal strips and the bus bar connected thereto.

13. A photosensitive target for a color television pic'kup tube, comprising an insulating supporting sheet, a plurality of color filter strips extending across said sheet in parallel relation, a plurality of substantially transparent conductive spaced signal strips one on each of said color filter strips, a plurality of bus bars extending across said sheet in a direction transversely of said strips, the signal strips on color filter strips of the same color extending across one of said bus bars and electrically connected in parallel by said bus bar and being insulated from the remaining signal strips and the bus bars connected thereto, and a plurality of conductive finger strips each extending intermediate at least one of said signal strips and the bus bar connected thereto.

14. A photosensitive target as described in claim 13 wherein said finger strips are of highly conductive material providing connections between the signal strips and bus bars that are highly conductive as to direct current potentials.

15. A photosensitive target for a color television pickup tube, comprising an insulating support sheet, a plurality of color filter strips extending across said sheet in parallel relation, a plurality of substantially transparent conductive spaced signal strips one on each of said color filter strips, a plurality of bus bars extending across said sheet in a direction transversely to said strips, the signal strips on color filter strips of the same color extending across one of said'bus bars and electrically connected in parallel by said bus bar and being insulated from the remaining signal strips and the bus bars connected thereto, and a plurality of silver conductive finger strips each extending intermediate one of said signal strips and the bus bar connected thereto.

16. A photosensitive target as described in claim 15 .comprising photoconductive material on said signal strips.

17. A ray sensitive target for a cathode ray tube, comprising an insulating support member, a plurality of spaced conductive signal strips extending across said member, a plurality of bus bars on said member, sets of said signal strips each being connected to one of said bus bars, each of said sets of signal strips being insulated from the other sets and the bus bars connected to said other sets, and a plurality of conductive finger strips each extending intermediate one of said signal strips and the bus bar connected thereto.

References Cited in the file of this patent UNITED STATES PATENTS 2,446,791 Schroeder Aug. 10, 1948 2,577,368 Schultz et al Dec. 4, 1951 2,589,386 Huffman Mar. 18, 1952