US2169840A - Cathode-ray signal-generating tube - Google Patents

Description

Aug. 15, 1939. H. M. LEWIS El AL CATHODE RAY SIGNAL-GENERATING TUBE Filed Nov. 28, 1936 o 02 54mm 0 INVENTOR.

ATTORNEY Patented Aug. 15, E93

. 2,169,840 car oes-car sroua -osunaa'rn c Harold M.

Great Neck, and Rudolf 0. Hergenrother, Bcechhurst, N. Y., Hazeltine Corporation, a corporation oi laasslgnors to Application November 28, 1936, Se ldo. 11.33%

12 Claims. (01.250-153l mitted. The target most widely in use comprises" a thin sheet of dielectric, such as mica, having on one side a metallic or other conductive coating and on the other a coating of. highly conductive photosensitive substance usually comprising a silver-caesium-oxygen mixture or compound, the exact molecular structure of which is not clearly understood but which probably comprises a colloidal suspension of metal in a simple or complex oxide forming an insulating binder. There results a mosaic of infinitesimal condensers having as a common electrode the backing coating of the dielectric sheet and as the other electrodes the photosensitive particles. j When an image of a scene is projected on the photosensitive mosaic, there results a surface electron emission which at each elemental area is proportional to the light intensity at that point. As a result,

of secondary electrons tending to mask the pri-' mary photoelectric emission, the ultimate result is that the cathode-ray beam periodically discharges each elemental condenser in succession, causing modulation or variation in the secondary electron current to the collecting anode due to the dischargecurrents of. the condensers, these modulation currents constituting a measure of the variation-of the light intensity of the scanned scene.

The signal-generating tube of the type described has a number of disadvantages. In the first place, because of the opacity of the target, the target must be mounted in the tube at an angle to the incident cathode ray and the scene projected thereon through the side of the tube, resulting in a so-called keystone" eifect because of the angularlty of the cathode-ray beam, requiring some form of comtion. Furthermore, because of the large secondaryelectron emission eflect, the secondary electrons emitted from any given. element of the mosaic tend. to bombard and modify the charge of the adjacent elements of the mosaic, thus reducing the em;

ciency of thedevlce. It is an object of this invention to provide an improved cathode-ray tube signal-generating system which overcomes one or more of the abovedescribed disadvantagesand is relatively efli'clent in operation.

It is another object of the invention to provide an improved cathode-ray tube signal-generating system of simplified construction and of substam tially higher photoelectric sensitivity than the arrangements of the prior art.

It is a specific object of the invention to provide an improvedcathode-raytube signal-generating system in which the scene to be scanned may be projected directly on the enlarged end of the envelope of a cathode-ray tube.

A cathode-ray signal-generating tube of -a systern in accordance with this invention comprises an evacuated envelope of conventional shape including an electron gun for generating and accelerating a stream of electrons of consta nt intensity and focusing it into a beam and u photosensitive target and a collecting anode disposed in the enlarged end of the envelope remote from the electron gun. The target consists of a metallic or other conductive plate or film coated with a homogeneous substance having, the absence of light, such a high specific resistance, that is, such a low specific conductivity, as to be :what is ordinarily termed an insulator but havin a substantial volume photoconductlve property;

that is, the property of substantially loweredyolumetric resistance or substantially higher specific conductivity when illunii. The output circult of the device includes connections between the collecting anode and the conductive layer of the target including a high impedance which is, nevertheless, a small" fraction of the impedance of the cathode-ray beam circuit at signal frequencles which, with conventional electron gun structure, is of the order of 100 megohms. With such an arrangement, the projection of an imageof a scene on the photosensitive film results in a variation of the resistance of the film over the surface, corresponding to variations in the intensity of the light throughout the scene. It now,

the cathode ray is caused to scan the scene, the" resistances of successive incremental portions of? the coating are varied in accordance with the intensity of their respective illumination. The electron current of the beam, which remains constant, divides between the conductive element of the photosensitive target and the collecting anode, the latter component comprising secondary electrons-mission from the target and the division of current varying from instant to instant in accordance with illumination of the-incrementel portions being scanned. There is thus produced, in eifeot, a signal-modulated current flowing between-the target and, the collector anode, providing a signal which may be transmitted for reproduction of a scene at the television receiver. Preferably,the conductive layer of the target is disposed directly on the inside surface of the enlarged end of the envelope as a semitransparent metallic film and the photosensitive coating is disposed on the film on the side facing the electron gun. With such an arrangement, the scene may be projected directly on the end of the envelope from the camera, or the like, having an optical axis coincident with the axis of the envelope, thereby eliminating distortion effects due to accordance with the present invention.

the angularity of the target with respect to the cathode ray.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims. Referring to the drawing: Fig. 1 is a schematic diagram of a complete television transmitting system including a cathode- V ray signal generating system in accordance with the presentinvention; Fig. 2 is a fragmentary view in perspective of the enlarged end of a cathode-ray tube showing the cathode-ray target partly. in section, while Fig. 3 is a perspective of a modified form of a cathode-ray target for embodiment in the cathode-ray tube system according to the present invention.

Referring now more particularly to Fig. l of the drawing, there is illustrated a complete transmitting television system including a camelectron gun. The collecting anode l5 may comprise a metallic film or coating on the inner surface of the envelope II; as shown. The tube 10 is also provided with a cathode-ray photosensitive target l6 disposed adjacent the end surface of the portion llb of the envelope, and described in more detail hereinafter. Disposed about the portion Ha of the tube envelope are the scanning or deflecting coils I! for deflecting the cathode ray of the electron gun rectilinearly to scan the image projected on the cathode-ray target l6. Suitable operating potentials are supplied to the several electrodes of the tube ID from a voltage divider l8 connected to a suitable source, indicated as plus and minus and preferably grounded at its positive terminal. The circuit 'of the control grid [3 includes the series blocking resistors I9, 20, and 2|. 'Ihe connec tion to the photosensitive target I6 is preferably made to a point on the voltagedivider l8 positive with respect to that to which the second anode I5 is connected and, as shown, includes a load resistor 22 of high resistance or equivalent element having a high impedance which is, nev ertheless, a small fraction of the impedance of the cathode-ray beam circuit at signal frequencies which, with a conventional electron gun structure, is of the order of megohms.

In order to block out the cathode ray during the retrace portion of the scanning cycle, there is provided a so-called pedestal generator 23 which is coupled across the resistor 20 through cou pling condensers 24. The scanning coils I! are energized from the line-frequency generator 25 and the frame-frequency generator 26, which are controlled by, timer 2'! which also synchronizes the pedestal generator 23 with generators 25 and 26. Also coupled to the timer 21 is a mixed, lineand frame-frequency synchronizing generator 28 for supplying the proper synchronizing impulses to the signal to be transmitted in order to control the corresponding scanning circuits of the receiver. The video signals picked up across the resistor 22 in the circuit interconnecting the cathode-ray target l6 and collecting anode IS, the synchronizing impulses generated by the generator 28, and the block-out or pedestal signals generated by the device 23 are combined and amplified in a mixing or modulation amplifier 29, the output of which is connected to a modulator 30 supplied also with carrier-frequency oscillations from an oscillator 3|, The resulting modulated high-frequency carrier generated by, the modulator 30 is passed through a power amplifier 32 and thence delivered to the antenna circuit 33-33.

With the exception of the signal-generating pears in the output circuit between the scanning target [6 and collecting anode l5 and across theresistor 22 video-frequency voltages which are a measure of the intensity ofillumination of the successive incremental areas of the target l8, as they are scanned by the cathode ray generated by the electron gun structure l2, [3, H. These signals areimpressed upon the modulation amplifier 29 which also receives lineand framefrequency synchronizing impulses from the generator 28 and pedestal or block-out signals from the generator 23, the several generators being inter-related by the timer 2'! to maintain the synchronizing impulses impressed upon the transmitted carrier in synchronism with the scanning waves generated and impressed upon the scanning coils H of the transmitter tube. These combined signals are amplified in the modulation or mixing amplifier 29 and supplied to the modulator 30, wherein they are converted 7 into modulated carrier-frequency oscillations which are amplified in the power amplifier 32 and radiated by the antenna circuit 33-33.

As stated above, the target I6 is preferably formed on the inside surface of the enlarged end I of the envelope I l. The structure of this target is shown more clearly in Fig. 2 which illustrates the target, partly in section, on a greatly magnified scale inorder to render it capable of illustration, as the layers or films of the target are of microscopic thickness. As shown in Fig. 2, the target I 6 comprises a metallic film, such as nickel, deposited over the entire inside end surface of the tube ll of a thickness only suiiiclent to be opaque, as shown by the portions Ilia-46a of the section. The film is formed with a rectangular window IBb approximately the size of the image to be projected on the target and there is deposited thereon on the side facing the electron gun a metallic or other conductive film 060 much thinner than the surrounding coating Ilia. This coating is suiiiciently thin as to be semitransparent, that is, capable of transmitting of the order of or more of the received illumination and preferably transmitting of the order of 50% of such illumination. On the semitransparent film I is deposited a layer lBd of photoconductive material; that is, material in which incremental changes in illumination are accompanied by substantial incremental changes in the conductivity of the material, which relationship is hereinafter termed the coefficient of photoconductivity.

In the operation of the target 16 as shown particularly in Fig. 2, the scene to be scanned is projected from the camera or projector 9 through the semitransparent film lfib, and the light transmitted therethrough causes the photo-conductivity of the layer Hid to vary throughout, proportionally to variations of the intensity of the illumination of the image of the scene. The portion I 6d of the target l6'is scanned by the cath ode ray in response to the excitation of the scanning coils IT, with the result that successive incremental portions of the layer Hid constitute effective resistances which vary in accordance with the intensity of illumination of the corresponding incremental areas of the scene to be transmitted. The variation in resistance of successive incremental portions of the film IBd varies the division of the electron current of the cathode ray (which, as stated above, remains constant) between, the conductive film I Get, I and the collecting anode I 5, the latter component comprising secondary electron emission from the target IS. The distribution of the electron 'current of the cathode ray is such that increase in illumination of the elemental portion being scanned is accompanied by an increase in the potentialof the target I6 in a positive sense and an increase in the potential of the anode l5 in a negative sense; that is, current flowing in the circuit interconnecting these electrodes is modulated or varied in accordance with variations in the resistance of the film liidthroughout its extent so that they may be utilized as output electrodes. Preferably, the target I6 is utilized as the high potential output electrode since its capacitance to ground is considerably less than that of the collecting anode l5. By including the load resistor 22 in circuit with the target cent screens of cathode-ray tubes used in television receivers is clear in that the essential characteristic of the latter material is that it emits light upon excitation by a cathode ray, which property is in no way related to its photosensitivity. The distinctions between the photosensitive material used in the tube of this invent-ion and that heretofore used in signal-generating tubes are not so superficial, and the differences may be best appreciated by a direct comparison of the materials:

(1) The photosensitive material utilized in this invention has, when unexcited, such a high specific resistance, that is, low specific conductivity, that it falls in the class of substances usually termed insulators, while that of the prior art is a highly conductive material.

(2) The characteristic property of the photosensitive material utilized herein is that its vol- I ume-conductivity changes with the intensity. of

illumination, involving what is believed to be a displacement of the electrons within the atoms from their normal positions, setting up a strain which is released by the displacement current flowing when a voltage is applied, as by a cath ode ray, restoring the material to its normal nonconductive state. On the other hand, the material of the prior art exhibits purely surface photoelectric emission, that is, absorption of the light quanta at the surface of the material and 1 actual thickness of the photosensitive-layer of this invention is very small, of the order of one ten-thousandth of an inch, this represents a layer of a thickness of many millions of molecuies, that is, a film ofa thickness of an entirely higher order of magnitude than that of the photosensitive films of the-prior art.

(4) The photosensitive material of this invention comprises a homogeneous coating of a single compound, while that of the prior art comprises a. mosaic, for example, the usual complex silver-'caesium-oxygen mixture which is thought to be a colloidal suspension in a simple or complex metallic oxide.

(5) The prior art photosensitive target comprises essentially a. large number of infinitesimal condensers which are charged by the illumination of the projected image and discharged by the cathode ray, while the photosensitive material of this invention is essentially a resistance which varies throughout its extent and which, because of the thinness of the film, is not seriously ailected by edge efiects of the material surrounding the cathode-ray spot at any particular instant. I

(6) As in the arrangements of the prior art, the operation of the photosensitive target involves a certain integrating process in that illumination of each incremental portion for a finite time is required to change its conductive prop erties to a substantial extent, this time being 15 Zinc oxide other materials whose photoconductive properties have been measured. Certain of these are mentioned and their particular properties discussed in the text Photoelectric Phenomena" by Hughes 8r Du Bridge, McGraw-Hill, 1932, beginning at page 321. Secondly, it is importantthat the material have a very high specific resistance approaching that of material ordinarily classed as an insulator, in .order that the'impedance of the photosensitive film may be comparable with that of the other circuit elements in the load circuit of a signal-generating tube. Further, it is desirable, though not essential, that the photosensitive material be highly refractory; that is, stable at the' high temperatures to facilitate baking out the signal-generating tube during the exhaust to remove the occluded gases. It has been discovered that, as between crystalline photoconductive substances, the coefficient of photoconductivity varies. approximately with the index of refraction with respect to light far removed from the absorption frequency, a property of materials as to which information is. more generally available. It has been determined that only those substances having indices of refraction of the order of two or greater have coefficients of photoconductivity of suilicient magnitude to be useful in signal-generating tubes of the type to which this invention refers.

In general, it has been found that the most suitable photosensitive materials are included in the metallic compounds of the halogens or of the oxygen-sulphur sub-group of the periodic table including the metallic halides, oxides, andsulphides. There follows a partial table of those materials, all of which are crystalline, which are suitable for use as the photosensitive film in a signal-generating tube of this invention:

Index re- Substance fraction Formula Zine sulphide.

lrconlum 0 Xld6 Zirconium silicate It has also been found that fluorescent material utilized in cathode-ray receiving tubes, such as that described in German Patent No. 440,350 to Riedel and de Hal-in, commercially available as television compound #601! of Riedel-de Han, distributed by Pfaltz and Bauer, New York city, can be'used as a photosensitive material. It is believed that this compound comprises essentially zinc sulphide and cadmium sulphide activated by a trace of silver and copper. However, this substance has a lower coefficient of photoconductivity than chemically pure zinc sulphide, included in the foregoing table.

Thus, it is seen that there is provided by this invention a novel and improved cathode-ray tube signal-generating system, in which the scene may be projected on the target coaxially with the normal position of the cathode ray, thus avoiding the keystone effect due to the angularity ofthe target with respect to the cathode ray. Further, the novel cathode-ray tube signal-generating system, according to this invention is of simplified construction and highly efllcient in operation.

In Fig. 3 there'is shown a target I I of modified form in which the photosensitive film "id is deposited directly on an opaque metallic plate Ilia disposed in the envelope in 'any suitable fashion as by mounting it in a plane angular to the cathode ray, in which case the image of a scene is projected upon the target through a window in the side of the envelope in the conventional manner.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is: I 1. A television signal-generating system including a cathode-ray signal-generating tube comprising an elongated evacuated envelope, an electron gun disposed in one endof said envelope for generating and accelerating a stream of electrons of constant intensity and focusing it into a beam, a photo-sensitive target electrode in the path of the electron beam and adapted to have an image of a scene projected thereon, said target comprising a layer of conductive material and a layer of photoconductive material disposed thereon on the side facing said electron gun, an anode disposed to collect secondary electronsemitted by said target, means for deflecting said beam to scan the image projected on said target, and an output circuit interconnecting said anode and said conductive layer and having an impedance at signal frequencies which is a small fraction of the impedance of said beam. 2. A cathode-ray signal-generating tube comprising an elongated evacuated envelope, an electron gun disposed in-one end ofsaid envelope for generating and accelerating a stream of electrons and focusing it into a beam, a photosensitive target electrode in the path of the electron beam and adapted to have an image of a scene projected thereon, said target comprising a layer of conductive material and a layer of homogeneous material disposed thereon on the side facing said electron gun, said latter layer being of extremely low specific conductivity when unexcited but of substantially increased specific conductivity when illuminated, an anode disposed to collect secondary electrons emitted by said target, means for deflecting said beam to scan the image projected on said target, and an output circuit interconnected said anode and said conductive layer and having an impedance at signal frequencies which is a small fraction'ot the impedance of said beam.

3. A cathode-ray signal-generating tube comprising an elongated evacuated envelope, an electron gun disposed inone end of said envelope for generating and accelerating a stream of electrons and focusing it into a beam, a photosensitive target electrode in the path of the electron beam and adapted to have an image of a scene .projected thereon, said target comprising a layer of conductive material and a thin layer of homogeneous crystalline material disposed thereon on the side facing said electron gun, said latter layer having a high coefiicient of photoconductivlty, an anode disposed to collect secondary electrons emitted by said target, means for deflecting saidbeam to scan the image projected on said target, and an output circuit interconnecting said anode and said conductive layer and having an impedance at signal frequencies which is a small fraction of the impedance of said beam.

4. A cathode-ray signal-generating tube comprising an elongated envelope, an electron gun disposed in one end of said envelope for generating and accelerating a stream of electrons and focusing it into a beam, a photosensitive target electrode in the path of the electron beam and adapted to have an image of a scene projected thereon,-said target comprising alayer of conductive material and a thin layer of homogeneous refractory material disposed thereon on the side facing said electron gun, said latter layer having a normally low specific conductivity and a high coefiicient of photoconductivity, an anode disposed to collect secondary electrons emitted by said target, means for deflecting said beam to scan the image projected on said target, and an output circuit interconnecting said anode and said conductive layer and having an impedance at signal frequencies which is a small fraction of the impedance of said beam.

5. A cathode-ray signal-generating tube comprising an elongated evacuated envelope, anelectron gun disposed in one end of said envelope for'generating and accelerating a stream of electrons and focusing it into a beam, a photo sensitive target electrode in the path of the electron beam and adapted to have an image of a scene projected thereon, said target comprising a layer of conductive material and a thin coating thereon consisting essentially of a refractory metallic halidehaving a high coefficient of photoconductivity, an anode disposed to collect secondary electrons emitted by said target, means for deflecting said beam to scan the image projected on said target, and an output circuit interconnecting said anode and said conductive layer and having an impedance at signal frequencies which is a small fraction of the impedance of said beam.

6. A cathode-ray signal-generating tube comprising an elongated evacuated envelope, an electron gun disposed in one end of said envelope for generating and accelerating a stream of electrons and focusing it into a beam, a photosensitive target electrode in the path of the electron beam and adapted to have an image of a scene projected thereon, said target comprising a layer of conductive material and a thin coating thereon on the side facing said electron gun, said coating consisting essentially of a refractory metallic compound of an element of the oxygen-sulphur subgroup having a high coeflicient of photoconductivity, an anode disposed to collect secondary electrons emitted by said target, means for defleeting said beam to scan the image projected on said target, and an output circuit interconnecting sald' anode-and said conductive layer and having an impedance at signal frequencies which is a small fraction of the impedance of said beam.

7. A cathode-ray signal-generating tube comprising an elongated evacuated envelope, an electron gun disposed in one end of said envelope for generating and accelerating a stream of electrons.

and focusing it into a beam, a photosensitive target electrode in the path of theelectron beam and adapted to have an image of a scene projected thereon, said target comprising a layer of conductive material and a thin coating thereon consisting essentially of a refractory metallic oxide having a high coefficient of photoconductivity, an anode disposed to collect secondary electrons emitted by said target, means for deflecting said beam to scan the image projected on said target, and an output circuitinterconnecting said anode and said conductive layer and having an impedance at signal frequencies which is a small fraction of the impedance of said beam.

B. A cathode-ray signal-generating tube comprlsing an elongated evacuated envelope, an electron gun disposed in one s end of said envelope for generating and accelerating a stream of electrons and focusing it into a beam, a photosensitive target electrode in the path of the electron beam and adapted to have an image of a scene projected thereon, said target comprising a layer of conductive material and-a thin coating thereon consisting essentially of a refractorymetallic sulphide having a high coefiicient of photoconductivity, an anode. disposed to collect secondaryelectrons emitted by said targehineans for deflecting said beam to scan the image projected on said target, and an output circuit interconnecting said anode and said conductive layer and having an impedance at signal frequencies which is a small fraction of the impedance of said beam.

9. A cathode-ray signal-generating tube com-, prising an elongated evacuated envelope, an electron gun disposed in one end of said envelope for generating and accelerating'a stream of electrons and focusing it into a beam, a photosensitive target electrode in the path of the electron beam and adapted to have animage of; a

scene projected thereon, said target comprising a layer of conductive material and a thin coating thereon consisting essentially of a refractory zinc sulphide having a high coefllcient of photoconductivity, an anode disposed-to collect secondary electrons emitted by said target, means for deflecting said beam to scan the image pro jected on said target, and an output circuit interconnecting said anode and said conductive layer and having an impedance at" signal frequencies which is a small fraction of the impedance of said beam.

10. A cathode-ray signal-generating tube comprising an elongated evacuated envelope enlarged at one end, an electron gun disposed in one end of said envelope for generating and accelerating a stream of electrons and focusing it into a beam, a conductive first coating on the inside enlarged end surface of said envelope, said coating being at least semitransparent, and a second coating of photoconductive material superposed on said first coating, said coatings constituting a cathoderay target electrode and adapted to have an image of a scene projected thereon from a direction opposite to that of said electron gun, an anode disposed to collect secondary electrons emitted by said target, means for deflecting said beam to scan the image projected on said target, and an output circuit interconnecting said anode and said. conductive layer and having an impedance at signal frequencies which is a small fraction of the impedance of said beam.

11. A cathode-ray signal-generating tube comprising an elongated evacuated envelope enlarged at one end, an electron gun disposed in one end of said envelope for generating and accelerating a stream of electrons and focusing it into a beam.

10 a thin metallic film deposited on the inside enlarged end surface of said envelope. said film being at least semitransparent, and a thin coating of homogeneous material disposed thereon having a high coemcient of photoconductivity, said film and coating constituting a cathode-ray target 25 pedance of said beam.

CERTIFICATE OF CORRECTION.

Patent No. 2,169,8h0.

12. A cathode-ray signal-generating tube comprising an elongated evacuated envelope enlarged at one end, an electron gun disposed in one end of said envelope for generating and accelerating a stream of electrons and focusing it into a beam, an opaque coating on the enlarged end surface of said envelope having a rectangularwindow, a thin metallic film deposited on said inside end surface and coextensive with said window, and a thin coating of homogeneous refractory material disposed on said metallic film and having a high coefficient of conductivity, said film and its coating constituting a cathode-ray target electrode and adapted to have an image of a scene projected thereon from" a direction opposite to that of said electron gun, an anode disposed to collect secondary electrons emitted by said target, means for deflecting said beam to scan the image projected on said target, and an output circuit interconnecting said anode and said conductive'layer and having an impedance at signal frequencies which is a small fraction of the impedance of said beam.

' HAROLD M. LEWIS.

RUDOLF c. HERGENROTHERL August 15, 19 9.

HAROLD M. LEWIS, ET AL. It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Page 5, sec-- 0nd column, line 52, for the word "photoelectric" read photoelectronic;

page h, second column, line 55, page 5, 59, same page, second column, lines LL, 25, bland first column, lines 1, 20, )l0 and page 6, first column,

line 5, and second column, line 1, claims 2 to 12 inclusive, after "A" sert television signal-generating system including Pag LL, second column,

line 58-59, page 5, first column, lines i 25, 0nd column, lines 7, 26-27, til-t5 and 6h,

second column,

connec ted" read interconnecting;

line 5, claims 21:0 12 inclusive, after "electrons" of constant intensity; page 1 second column,

i i-L b, and 62, same page, seepage 6, first column, line 9, and

insert line 72 ,-claim 2 for "interand that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 5th day of" December, A. D. 1959.

(Seal) Henry Van Arsdale, Acting Commissioner of Patents.

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