Patent GB-721943-A (GB)
Title: Improvements in or relating to television pick-up tube arrangements
Inventor: THEILE RICHARD
IPC: H01J29/02
Filed: 1951-05-09
Abstract: 721,943. Television pick-up tubes. PYE, Ltd. May 2, 1952 [May 9, 1951; July 5, 1951]. Nos. 10971/51 and 15980/51. Class 39 (1). [Also in Group XL (b)]. A pick up tube employing high velocity scanning, such as an iconoscope or image iconoscope, has an electron multiplier 11, Fig. 1, in the form of a closed or almost closed frame of annular, rectangular or the like configuration and comprising at least' one dynode 12 and a secondary electron collector 16, the multiplier is located between the charge storage target 6 and the scanning gun 7 and out of the path of the scanning beam, and an electron permeable screening electrode 17, preferably in the form of a screening box, is provided between the multiplier and the charge storage target and is operated at such a negative potential with respect to the dynode 12 as to produce a field penetration ratio of the dynode 12 through the screening electrode 17 to the charge storage target which is less than 10 per cent. This construction ensures the uniforrr and efficient collection of secondaries from the whole surface area of the charge storage target The photocathode 2, scanning gun 7 and charge storage target 5, 6, focussing coils 4, 8 and scanning coils 9 and collector wall coating 10 may be conventional. The multiplier 11 may be slightly offset from the axis to minimize masking of electrons from gun 7, or may be coaxial, and comprises four dynodes 12-15 and collector 16 enclosed in a screening box 17 having mesh 18 through which secondaries from the charge storage target can enter the multiplier. Members 10, 17 18 are at about the same potential which is negative to first dynode 12 and to the stabilized target potential and mesh 18 gives less than 10 per cent, for example 3 per cent, penetration of the dynode field towards the storage target. The ratic of the diameter of the annular multiplier to the distance between mesh 18 and the charge storage target is such as to produce as uniform a field as possible over the target surface, a final favourable adjustment being made for example by making the screening box 17 slightly positive or negative with respect to the wall coating 10. Alternatively adjustments may be such as to produce a desired non-uniform field to compensate for shading or other spurious signals. To further reduce non- uniformity of the field over the storage target it is constructed to have a small surface leakage, as by glass layer 25 separated by insulating layer 26 from charge storage surface 5. Peripheral electrodes 27 connected to adjustable source 28 are stated to improve the efficiency of operation. To decrease shot noise in the electron stream from photocathode 2, especially at high levels of illumination, the photo cathode 2 is energized only in the frame and/or line blanking intervals, by negative pulses, as from pulse generator 30 or by voltage amplification of the blanking pulses. A switch 31 may be employed to cut out the pulses and give a fixed bias for steady photo emission at low light levels, and the switch may operate automatically in dependence on the level of illumination. In other forms the mesh 18 is replaced by a sheet with one or more slots or apertures. In one such form, Fig. 2 (not shown) the sheet has a single annular slot with the slot edges bent back to shield the final collector from the secondaries emitted by the charge storage target. In this case there is only a single dynode within the screening box. The multiplier structure may be rectangular instead of annular, and may be made in a single unit or in separate sections spaced apart, e.g. four straight sections spaced apart by small distances, to form a rectangular structure. The sections may be connected together or operated separately. The Provisional Specification refers generally to the provision of a screen or screens between the multiplier electrode and storage target.