GB971495A - Improvements in and relating to electron-optical image converter tubes - Google Patents

Abstract

971,495. Cathode-ray tubes. MULLARD Ltd. Oct. 28, 1960, No. 37158/60. Heading H1D. An image converter comprises an evacuated envelope provided with a photo-emissive layer 3 and phosphor and second anode layers 5, 6, a focusing electrode 7 and a first anode 8. The arrangement is such that with a steady positive voltage applied to the second anode and a smaller voltage applied to the first anode, there is a spherical field distribution between the photo-cathode, focusing electrode and first anode providing an intermediate virtual image, while the first and second anodes form a second converging lens system, variable magnification being provided by variation of the first anode potential with consequent adjustment of the focusing electrode potential. As shown, the image intensifier tube comprises a concave photo-cathode 3, sensitive to visible, infra-red, ultraviolet light or X-rays, e.g. a caesiated-antimony layer, and a focusing electrode 7 in the form of an aluminium wall coating. The first anode 8 is supported from a glass envelope projection 11, and comprises an aluminium part 9 held in position with the aid of a crimped stainless-steel spring 12 and an aluminium part 10 spun over the annular flange 13, the two parts 9, 10 being connected by two lengths of platinum tape 14. The longitudinal axial distance between first anode 8 and layer 3 is preferably between 0.5 and 1.5 times the diameter of electrode 7 and the diameter of the larger end of part 10 at least twice or three times the shortest distance between first anode 8 and the second anode 6 provided on the green light-emitting sulphide phosphor layer 5. Suitable dimensions for a particular tube are given. In operation, layer 3 is at earth potential and second anode 6 is at 10 kv. while variable magnification is obtained by variation of the potential of first anode 8 between 0 and 10 kv. with consequent adjustment of the potential on electrode 7 between 0 and 600 v. The first electron lens produces an inverted virtual image and the second electron lens constituted by the first and second anodes a real image which may still be inverted. The spacing between the first and second anodes is chosen so that the barrel distortion produced by the second electron lens compensates for the pin-cushion distortion produced by the first electron lens. The radiation image may be focused on to the photo-cathode by means of a wide-aperture lens system of the Schmidt type having a light-gathering power of at least 100 times that of the dark-adapted eyes, and the intensified image may be viewed through a Ramsden type eyepiece having an exit pupil greater than 7.7 mm., Fig. 3 (not shown).