Patent GB-705383-A (GB)
Title: Television camera tubes
IPC: H01J31/28
Filed: 1951-02-02
Abstract: 705,383. Television pick-up tubes. STANDARD TELEPHONES & CABLES, Ltd. Feb. 2, 1951 [Feb. 4, 1950], No. 2653/51. Class 39(1), A television camera tube having a secondaryemissive storage electrode has means for charging said electrodes to a high positive potential, a photocathode projecting photo-electrons corresponding to an optical image on to the storage electrode to reduce the charge thereon in accordance with the image and scanning means which reduce said charge still further to datum level to generate signals. The positive charging means may comprise a pulsed light source also liberating high velocity photo-electrons from the photo-cathode or an electron gun generating high velocity electrons. In Fig. 1, an optical image 15 liberates electrons from the photo-cathode 8 which fall on a target 9 comprising a secondary emissive coating 10, e.g. sensitized and insulated islands of silver or a thin sheet of high resistance glass. Scanning is by low velocity electrons from a gun 2, and electrode 13 collects any secondary electrons so liberated. A high intensity light source 17 releases from the photo-cathode a spray of electrons of such velocity that the secondary emission is greater than unity. The pulse generator 19 of the tube controls the power for the light source and simultaneously applies a higher potential between plate 9 and photo-cathode 8. Illumination of the photo-cathode by the image 15 is continuous, but illumination by source 17 alternates with the operation of the scanning beam. In a modification, Fig. 3 (not shown), the gun is on the side of the target remote from the photo-cathode. The target is double sided and both sides may be insensitive to light. In Fig. 4 (not shown) an electron gun disposed axially charges a double-sided target positively during the fly-back period of a scanning beam generated by a gun disposed in a side arm of the tube. The scanning side of the target may have little or no secondary emission. The scanning and charging gun may be intercharged in position. The apparatus includes synchronizers and an unblanking pulse generator. Fig. 5 shows apparatus in which in order to eliminate shading effects charging of an area of the target 36 takes place shortly after the area has been scanned by the signal beam 47. Delay circuits 54, 55 connected between the synchronizer 46 and the scanning wave generators 43, 44 ensure the charging beam 42 following the scanning beam 47 with a small delay, e.g. a fraction of a scanning line or even several scanning lines. The addition of signal of special wave form to the video signal so as to compensate for distortion is referred to. In another modification, Fig. 6 (not shown), the single electron beam carries out the two functions of charging and scanning. By suitable circuit dispositions of blanking pulse generators, synchronizers and gates the beam charges positively at a fast rate by means of a high speed electron beam after it has been scanned at a slow rate with a relatively slower or faster electron beam such as gives secondary emission less than unity. In one form the beam scans slowly in one direction of the frame and at a high rate of line scanning in the return direction which is a form of retrace. The beam is' suitably blanked when the scanning is interlaced to avoid charging of the intermediate lines. The line position may be changed after each scan. The output of the vertical scanning source may be of stair-case form to give scanning along true horizontal lines and ensure fly-back along the same path.