US2931931A

US2931931A - Electron discharge devices employing photo-conductive target electrodes

Info

Publication number: US2931931A
Application number: US523262A
Authority: US
Inventors: Lubszynski Hans Gerhard; Wardley John
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1954-07-27
Filing date: 1955-07-20
Publication date: 1960-04-05
Anticipated expiration: 1977-04-05
Also published as: FR1137443A; GB784863A

Description

H. G. LUBszYNsKl x-:TAL 2,931,931 ELEcTRoN DISCHARGE DEVICES EMPLOYING PHOTO-CONDUCTIVE TARGET ELECTRODES v Filed July 2o, 1955 [1o /n 12 gr 1T -1oo-ov 7@ ov `4 April 5, 1960 O -15OM nited States Patent ELECTRON DISCHARGE DEVICES EMPLOYING PHOTO-CONDUCTIVE TARGET ELECTRODES Haus Gerhard Lubszynski, Waltham St. Lawrence, and

.lohn` Wardley, Hounslow, England, assgnors to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application July 20, 1955, Serial No. 523,262

Claims priority, application Great Britain July 27, 1954 6 Claims. (Cl. S13-65) This yinvention relates to electron discharge devices employing photo-conductive `target electrodes.

Devices `of this kind have been proposed for television and similar purposes in which a layer of photoconductive material is deposited on a transparent signal electrode formed on a glass window of the envelope of the device and the target electrode is arranged to be scanned by a cathode ray electron beam so as to generate signals in accordance with the point-to-point conductivity of the target electrode when an optical image is projected thereon. In one form of device the photo-conductive layer is composed of antimony trisulphide which is deposited in the form of a spongy layer. The glass window of the envelope in such devices is usually circular and the photo-conductive layer is usually deposited over the whole area of the window. In use, however, of such devices it is usually the practice to scan a rectangular raster the orientation of which relatively to the window depends on the disposition of the scanning means, which usually comprise scanning coils surrounding the envelope of the device. Itis found when such devices are initially used or tested that if a rectangular raster is scanned on the target electrode whilst the latter is illuminated uniformly, the scanned area has a lower resistance in the dark and a greater sensitivity compared with the unscanned area. Furthermore, if a checker board image is projected on to the target electrode and scanned in a rectangular raster, it is found that the lighter areas of the scanned pattern have a lower resistance in the dark and higher sensitivity compared with the darker areas of the pattern. These elects are permanent and are undesirable since it is obvious that a uniform sensitivity and dark resistance should be present over a raster area of the target electrode.

The object of the present invention is to provide a method of treating a photo-conductive target electrode with a view to reducing the above-mentioned effects.

According to the present invention there is provided a method of treating a photo-conductive target electrode having a spongy layer with a view to increasing the sensitivity and imparting a substantially permanent and uniform sensitivity and dark resistance to the target electrode which comprises bombarding at least a raster area of the photo-conductive target electrode with a high beam current whilst setting up a high voltage across said target electrode and whilst the target electrode is illuminated with a uniform illumination. Preferably substantially the whole of the area of the target electrode is so treated particularly since after the initial use of the target elec trode re-orientation of the scanned area may occur.

A voltage of the order of 150 volts with respect to the cathode may be applied to the signal electrode in order to set up a high voltage gradient through the thickness of said target electrode although preferably during treatment of the tube in accordance with the nvention the voltage of the signal electrode is varied from a low voltage to a high voltage of the order of 150 volts and vice versa.

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 drawi ing, which illustrates a typical form of electron discharge device employing a photo-conductive target electrode.

As shown in the drawing the reference numeral 1 indicates an evacuated envelope having a glass end wall 2bn which is formed a transparent signal electrode 3 having deposited thereon a photo-conductive target electrode 4 which may consist of zinc sulphide or cadmium sulphide but preferably consists of a spongy layer of antimony r-trisulphide having a superimposed solid layer of antimony trisulphide. Within the envelope 1 there'VV is provided an anode 5, the end of which adjacent t0 the photo-conductive layer 4 is provided with a mesh 6. At the endV of the envelope 1 remote from the window 2 there is provided an electron gun comprising a cathode 7, a'cathode screen 8 and an apertured electrode 9 which limits the cross-sectional area of the electron beam pro' duced by the cathode. The cathode 7 serves to generate a beam of electrons which can be scanned over the photoconductive layer 4 by means of scanning coils 10. The electron beam is maintained infocus by means of a sole-A noid `coil 11. The device shown in the drawing is also provided with the usual alignment coils 12.

'In order to avoid the undesirable effects hereinbefore referred to, the following procedure is adopted. The' device is conditioned for operation by connecting the' cathode 7 .to a source of zero voltage, the cathode screen 8 to a source of voltage which may vary from -100 to zero volts, the limiter electrode 9 to a positive source of 300 volts and the anode 5 to a similar source of voltage. ,The signal electrode 3 is connected to a source of positive voltage whichcan be varied from 0 to 150l volts whereby a voltage gradient can be set up through the -thickness of the layer 4. The usual current supply. is applied to the solenoid coil 11 and to the alignment coil 12 anda suitable scanning waveform is applied to the coil 10 so as to enable the whole area of thetargetelectrode to be bombarded with electrons. The electron beam is also defocussed either by varying the normal current supply to the solenoid 11 or by varying the voltage of the electrode 8, or both and the beam current is adjusted to a high value. With the device operating in this manner the whole of the target area is illuminated with a uniform illumination from a light source indicated conventionally at 13, this illumination being of the order of 15 to 200 foot candles. The voltage applied to the signal electrode 3 is then slowly raised to 150 volts and down to zero volts several times, say 2 to 6 times, whilst the whole area of the target electrode is scanned with said defocused beam. The same process is then repeated with the target electrode in the dark. The scanning beam may employ a beam current of several microamperes, say up to 10 microamperes which is high compared with the normal operating beam current of, say, one to two microamperes. Of this latter current however only a fraction thereof reaches the target electrode. Instead of apply ing a voltage to the signal electrode 3 in order to set up the voltage gradient through the layer 4 a similar voltage gradient can be established through the thickness of the photo-conductive layer 4 by applying a suitable negative voltage to the cathode 7. Such a method is found to result in an overall sensitivity gain of between 3 and 9 decibels over an untreated target electrode and retention of images is found to be substantially avoided.

What we claim is:

l. A method of treating the target electrode of an electron discharge device which includes a spongy photoconductive layer and prior to normal electron beam scanning of an optical image of variable light intensity projected thereon which comprises bombarding at least a rasterarea .'Qf Jelaldbtarget 4electrode with 'a 'high 'current electron ,streamrwhilst .said .area .is Vvilluminated with ,a uniform intensity illumination and` whilst a voltage gradient is set up through the lthickness of said target electrede., said illumination, V,current and 'voltage gradient being chosen to' increase -the ysensitivity of .said area -and to .impart yaztuxiformsensitivity .and :dark resistance :to said 181263.

2v. A method YVtu' :treating :the target `electrode of an electron dischargedevice which :includes aspongy photoconductive layer and -prior Z,to normal electron beam Scanning of an optical zimage of variable light yintensity projected thereon :which comprises homharding at least a1.r aste r1arca oflsaidftarget electrode with a high current electron stream whilst said ,areafis illuminated with a uniform intensitydlluminatiomand whilst va voltage gradient is sc tup `varying between .-a high *,value and a low ttalue A'through the thickness of said target electrode, ysaid illumination, .current Yand-voltage gradient :being chosen -to increase the Sensitivity'of said area .and yto impart a .uniform sensitivity and dark resistance fto lsaid area. 3. A method of tre ating :the 4target -electrode of an electron dischargetdeyice which includes a spongy photo conductive layer and prior -zto normal electron beam scanning pfan Optical image Of-wariable light intensity projected thereon which comprises `kbombarding :at "least .arasterfarea of said target electrode with a high :current electron stream whilst said area -is illuminated with ,a uniform intensity yilluminationand whilst a vvoltage gradientisisetaup through thethicknessoffsaid area electrode, subsequentlymaintaining:said target-intthe dark andagain bombandingisaid 'areawith ahigh-Vcurrentelectron stream, Said illumination, current and ivoltage gradient being chosen `to `increase the sensitivity ,of `lsaid area and -to impart auniform sensitivity-and dark resistanceto s aid area.

`d. A Imethod of treating the `target electrode of an electron discharge device which -includesa spongy photoconductive layer and tprior to normal electron beam scanning of an optical image o f variable lightmintensity proiected :thereon which comprises .bombarding at least .a raster area .of said :target electrode witha l.high current electron stream whilst said area is illuminated with a uniform intensity illumination and whilst a voltage gradient is set up varying between a high value and a low value through the thickness of said target electrode, subsequently maintaining said target electrode in the dark and again bombarding said area with a high current electron stream, said illumination, current and voltage gradient being chosen Yto increase the sensitivity of said area and to impart a uniform sensitivity and dark resistance to saidarea.

"5. Arnethod of treating the target electrode of an electron discharge device which includes a spongy photoconductive layer and prior to normal electron beam scanning of an optical image of variable light intensity projected thereon .which comprises scanning at least `a raster area of said target electrode with a defocussed electron beam of several microamperes whilst said area is illuminated with a uniform intensity illumination greater than the normal operating light intensity of -said device and whilst ,a voltage gradient higherthan that employed in normal 1operation o f said device is set up through the thickness of said target electrode, said illuminatiomcurrent :and voltage gradient being chosen to increase the sensitivity ofsaidfarealand to impart a uniform sensitivity and vdark `resistance to said area.

6,. Anelectron Idischarge deviceV having a target .electrode which includes afspongy photo-conductive layer havingan increased sensitivity over a raster area thereof and a uniform sensitivity and dark resistance within said area produced, prior to normal electron beam scanning of anoptical area at variable light intensity, by bombarding at aleast said raster area of said target electrode with a'high current electron stream while said area is illuminated with a .uniform intensity illumination and while a voltage gradient is set up through the thickness of said target electrode.

References Cited in the tile of this patent UNITED STATES PATENTS 2,177,736 Miller, Oct. 31, 193,9 42,741.4837 Forgue May 8, Y1956 2,837,688 McGee June 3, 1958