US2645740A - Circuit arrangement embodying electron discharge devices employing target electrodes - Google Patents

Description

y 4, 1953 H, G. LUBSZYNSKI 2,645,

CIRCUIT ARRANGEMENT EMBODYING ELECTRON DISCHARGE DEVICES EMPLOYING TARGET ELECTRODES Filed June 21. 1949 In var; Tor

A, 5 40 5mm fforney Patented July 14, 1953 UITED S ES CIRCUIT ARRANGEMENT EMBODYING ELECTRON 'DISCHARG'E DEVICES EM- PLOYING TARGET ELECTRODES Application June 21, 1949, Serial No. 100,444 In Great Britain June 22, 1948 Claims.

This invention relates to circuit arrangements embodying electron discharge devices employing target electrodes such as are used for generating picture signals for television transmissions.

One form of such a device comprises a mosaic target electrode in the form of a sheet ,of dielectric material having on one side or surface an electrode, commonly called the signal plate, and on the other side or surface a mosaic of photo-sensitive elements formed by depositing antimony or an antimony compound or silver or other suitable substances, said mosaic elements being then rendered photo-sensitive as by treating them with caesium or similar materials. An optical image is projected onto the mosaic electrode and the latter is scanned by an electron beam generated by the cathode of the device, the electron beam being decelerated as it approaches the target area so that the target is efiectively scanned by a low velocity beam whereby the elements of the target electrode are stabilised to an equilibrium potential corresponding substantially to the of the cathode of said device. Devices are also known in which an electron image is projected onto one side of a non-photosensitive target electrode the opposite side of which is scanned by a low velocity beam. In devices which are operated in this manner, it is found that insulated areas on the surface of the target electrode outside the scanned patch impair the operation of the device. Such areas produce instability and what appear to be white whiskers at the edge of a picture reproduced from the generated signals and in some cases gives rise to geometrical distortion of the scanned patch. The mosaic elements in a device employing a photo-sensitive mosaic are sometimes formed by evaporating a metal through a stencil which is held in contact with said insulating surface. The area of the mosaic formed in this manner is in most cases slightly larger than the area of the scanned patch and it is found that the border of the mosaic outside the scanned patch is the area on the target electrode from which the above-mentioned difiiculties mainly arise.

The object of the present invention is to avoid or reduce these difliculties.

According to the present invention, there is provided a circuit arrangement embodying an electron discharge device having a target electrode and means for scanning said target electrode with a low velocity scanning beam so that elemental areas of said target electrode are stabilised at an equilibrium potential corresponding substantially to that of the cathode from which said scanning beam is derived wherein the area of said target electrode which is scanned by said beam is surrounded by a conducting border and means are provided for maintaining said border at such a potential that electrons from said beam are prevented from impinging on said conducting border, whereby said border defines the area from which signals can be obtained for said target electrode.

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 drawings in which:

Figure 1 is a diagram of an electron discharge device and a circuit arrangement in accordance with the invention, and.

Figure 2 is an enlarged view of the target electrode of the device shown in Figure 1.

As shown in the drawings, the reference numeral I indicates the envelope of an electron discharge device suitable for use in the generation of picture signals for television purposes. The device includes a, target electrode in the form of a mosaic 2 of the photo-electric type formed on an insulating surface 3 such as that of a sheet of mica. The mosaic may be formed by depositing antimony or an antimony compound through. a stencil and then photo-sensitising the elements. On the opposite surface of the insulating sheet on which the mosaic elements are formed, a transparent signal electrode 4 is arranged through which an optical image for transmission can be projected by means of a lens system 5' onto the photo-sensitive mosaic elements. The mosaic elements are arranged to be scanned by an electron beam generated by a thermionic cathode 6 surrounded by a cathode screen 1, the electrons in the beam being accelerated by an accelerating electrode 8 and then decelerated by a wall anode 9 and a further decelerating electrode Iii. The electron beam from the cathode 6 is scanned over the surface of the target electrode by scanning coils indicated at II, the device being immersed in a longitudinal magnetic field set up by a solenoid coil I2 which is arranged to cause the electrons from the cathode 6 to impinge on the target electrode substantially normally throughout the scanning cycle in known manner. In operation of the device shown in Figure 1, when an optical image is projected onto the mosaic target electrode photoelectrons are liberated causing the elements of the target electrode to acquire positive charges and these charges are periodically restored to an equilibrium potential by the action of the scanning beam, restoration of the charges to said equilibrium potential settin up picture signals in a signal resistance l3 which is connected to the signal electrode 4. The signal electrode in the example shown in the drawing is connected through the envelope I to a conductor I4 provided on the outside of the envelope l. The elements of the target electrode are arranged to b stabilised at a potential corresponding substantially to that of the cathode 6 and for this purpose the cathode 6 may be maintained at earth potential, the cathode screen I at zero potential or a few volts negative with respect thereto, the accelerating electrode 8 at a positive potential of about 300 volts and the wall anode 9 at a positive potential of about 200 volts. The potentials are shown as being derived from a potentiometer l connected across a source of potential l6.

In operation of the device shown in Figure 1, it is found that undesired signals are generated when the target electrode is scanned as hereinbefore referred to. In order to avoid this difficulty, the area of the target electrode which is scanned by the scanning beam is surrounded by a conducting border I! and in operation this border l! is maintained at such a potential that electrons from the scanning beam are prevented from impinging on said conducting border so that the border defines the area of the target electrode from which picture signals can be obtained. The potential of the border ll may be zero volts for which purpose it can conveniently be connected to the signal electrode 4 and to the conductor [4. Alternatively the conducting border ll may be maintained at a slightly negative potential with respect to the cathode 6. Said conducting border H may be applied to the mosaic target in any suitable manner as by evaporating a filmpf metal after formation of the mosaic elements, the target electrode being of course suitably masked during the evaporation process or, alternatively, the conducting border may be deposited onto the insulating surface 3 before formation of the mosaic elements. the size of the space within said conducting border should coincide exactly with the scanned area but since in some cases this may be difficult to arrange, it may be desirable in such cases to make the size of the space within the border slightly less than the area occupied by the mosaic elements.

Although the invention has been described above as applied to a device employing a photosensitive mosaic target, it will be appreciated that the invention can also be employed in conjunction with devices in which an electron image is arranged to be projected onto one side of a non-photo-sensitive target electrode, the other side of which is arranged to be scanned by a low velocity electron beam.

What I claim is:

1. A circuit arrangement embodying an elec- Preferably v tron discharge device having a target electrode and means for scanning said target electrode with a low velocity scanning beam to stabilize elemental areas of said target electrode at an equilibriumpotential corresponding substantially to that of the cathode from which said scanning beam is derived, a conductive border disposed in the path of said scanning beam and surrounding the surface of .said target electrode scanned by said beam and means for maintaining said conductive border at a potential to prevent electrons from said beam from impinging on said conducting border, whereby said border defines the area from which signals can be obtained from said target electrode.

2. A circuit arrangement according to claim 1, including a signal electrode, and a target electrode in capacitive relation with said signal electrode, said conductingborder bein connected to said signal electrode.

3. A circuit arrangement according to claim 2, said target electrode comprising a photo-sensitive mosaic in capacitive relation with said signal electrode, said signal electrode being transparent for projection of an optical image therethrough onto said mosaic.

4. A circuit arrangement accordin to claim 1, said target electrode comprising a signal electrode, and a photo-sensitive mosaic in capacitive relation therewith, said signal electrode being transparent for projection therethrough of an optical image onto said mosaic.

5. A' circuit arrangement embodying an electron discharge device having a target electrode provided with mosaic elements, means for scanning said target electrode with a low velocity scanning beam to stabilize elemental areas of said target electrode at an equilibrium potential corresponding substantially to that of the oathode from which said scanning beam is derived, a conductive border disposed in the path of said scanning beam and surrounding the surface of said target electrode scanned by said beam, said conductive border having an aperture area at least as small as the surface of said target electrode covered by mosaic elements, and means for maintaining said conductive border at a potential to prevent electrons from said beam from impinging on said conducting border, whereby said border defines the area from which signals can be obtained from said target electrode.

HANS GERHARD LUBSZYNSKI.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,131,892 Iams- Oct. 4, 1938 2,149,455 McGee et al. Mar. 7, 1939 2,150,980 Lubszynski et al. Mar. 21, 1939 2,225,063 Lubszynski Dec. 17, 1940 2,325,676 Hepp et a1. Aug. 3, 1943

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