US2555091A - Cathode-ray tube - Google Patents

Description

y 9, 1951 H. e. LUBSZYNSKI 2,555,091

CATHODE-RAY TUBE Filed Jan. 26, 1949 /n venTar;

Patented May 29, 1951 CATHODE-RAY TUBE Hans. Gerhard Lubszynski, Norwood, England, as-

signer to Electric & Musical Industries Limited, Hayes, England, a company of Great Britain Application January 25, 1949,. Serial No. 72,863 In Great Britain February 3, 1948 1 Claim. it

This invention relates to circuit arrangements embodying cathode ray tubes of the type which employ a, target electrode which is scanned by an electron beam for the purpose of generating signals which are suitable" for example for use in television transmitting systems.

Various forms of such cathode ray tube have been proposed and in United States PatentsNos. 2,211,146 and 2,177,736, it has been suggested to employ a target electrode comprising a layer of material exhibiting a photo-conductive efiect. The manufacture of such layers is described in an article in the Proceedings of the Physical Society, volume 50,1938, page 374. In these prior proposals the target electrode was arranged to be scanned with a high-velocity scanning beam so that the elements of the target electrode attained an equilibrium potential corresponding substantially to the velocity of the scanning beam. Scanstantially reduced. According to the invention, there is provided a circuit arrangement embodying a cathode ray tube having a target electrode comprising a photo-conductive layer provided on a conductive signal electrode and arranged to receive an optical image, means for scanning said layer with a low-velocity scanning beam 50 that the equilibrium potential of the scanned surface of said layer corresponds substantially to that of the cathode which generates said scanning beam and means for maintaining said signal electrode at a positive potential with respect to the potential of said cathode.

In order that the said invention may be clearly understood and readily carried into effect it will now be more fully described with reference to the accompanying drawing which illustrates a cathode ray tube according to one embodiment of the invention and circuit connections therefor.

As shown in the drawing, the cathode ray tube 1 comprises a target electrode 2 composed of a transparent insulating layer 3, such as glass or mica, on one side of which is deposited a semitransparent signal plate 4. On the top of this signal plate 4 is deposited a layer of photo-conductive material 5, such as zinc selenide. The

layer of zinc selenide is arranged to be scanned by a low-velocity scanning beam which is generated by a suitably disposed electron gun. The electron gun comprises a cathode 6, a cathode shield 7, an apertured anode -8 and a further electrode 9 consisting of a metallic wall coating. The cathode 5 may be maintained at earth potential, the'shield l at a negative potential thereto, the anode 8 at apositive potential and the electrode 9 usually at a slightly lower positive potential than the anode 3. Near to the target electrode 2 is a decelerating electrode I0 which is maintained at a less positive potential than the electrode 9. The electrodes 5IU are conventionally shown as deriving their potentials from a potentiometer l I connected across a source of potential I2. The electron beam from the gun is accelerated b the anode '8 and decelerated by the electrodes 9 and It and is scanned over the surface of the target electrode 2 by scanning coils indicated at E3. The electron beam is focussed by and is arranged to scan the surface of the target electrode in the presence Of a longitudinal magnetic field set up by a solenoid l4 and the arrangement is such that the beam is caused to impinge on the target electrode substantially normally throughout the whole scanning cycle in known manner.

When using a cathode ray tube according to the invention for the generation of signals suitable for television transmission, the signal plate 4 is connected to a signal resistance l5 and is maintained at a slightly positive potential of the order of a few volts with respect to the cathode 6 of the electron gun, the magnitude of said positive potential determining the sensitivity of the tube. An image of the object for transmission is projected through the insulating layer and through the transparent signal plate on to the layer of zinc selenide. In the absence of illumination, the surface of the layer 5 scanned by the cathode ray beam will be charged to the potential of the cathode of the electron gun and a potential difference will thus be set up through the target electrode. When light falls on an elementary area of the layer 5, this area becomes conductive to a degree depending on the intensity of illumination and the potential of the surface begins to rise towards the potential of the signal plate 4. When the scanning beam impinges on such an area of the layer 5, electrons from the scanning beam will reduce the potential Of said area to cathode potential and thus produce a signal which is set up across said signal resistance I5. Owing to the low-velocity scanning beam employed,

emission of secondary electrons from the target electrode is substantially avoided, thus obviating the well known background or tilt signals and other disadvantages which occur with the use of high-velocity scanning beams. Furthermore, stable operation of a tube according to the invention should result, since the potential rise of the target electrode due to the illumination cannot exceed the potential of the signal plate 4 and due to the fact that the first crossover point of the secondary emission characteristic of a, photo-conductive layer is considerably higher than that of photo emissive layers, being between 20 and 100 volts. A tube constructed as described above is suitable for use in television transmission and is also suitquires to be treated with caesium or other materials of low work function the secondary emission-ratio of the target electrode can be maintained-low and hence a better modulation of the electron beam can be obtained rendering it eminently suitable for use where the unrequired electrons are returned to a multiplier.

What I claim is:

A circuit arrangement embodying a cathode ray tube having a target electrode comprising a photo-conductive layer provided on a conductive signal electrode and arranged to receive an optical image, including means for scanning said layer with a, low-velocity scanning beam with the equilibrium potential of the scanned surface of said layer corresponding substantially to that of the cathode which generates said scanning beam, and means for maintaining said signal electrode at a positive potential with respect to the potential of said cathode.

HANS GERI-IARD LUBSZYNSKI.

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

UNITEIj STATES PATENTS Number Name Date 1,747,988 Sabbah Feb. 18, 1930 2,150,980 Lubszynski et al. Mar. 21, 1939 2,324,504 Iams et a1. July 20, 1943 2,404,046 Flory et a1. July 16, 1946 2,404,098 Schade July 16, 1946 2,407,485 Essig Sept. 10, 1946 2,415,842 Oliver Feb. 18, 1947 2,489,127 Forgue Nov. 22, 1949

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