Patent GB-800226-A (GB)
Title: Improvements in or relating to image intensifiers
IPC: G03B21/62
Filed: 1956-09-05
Abstract: 800,226. Semi-conductor devices. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Sept. 5, 1956 [Sept. 8; 1955], No. 27173/56. Class 37. [Also in Group XL (a)] The frequencies of the operating voltages of successive image intensifiers in an image reproducing device comprising at least two optically cascade-connected solid-state intensifiers are so chosen that the input level of each intensifier lies within the operating range of this intensifier. Each intensifier contains an electro-luminescent material and a photo-sensitive material controlling the emission of this electro-luminescent material, arranged between electrodes to which the operating voltage is applied. The " input level " is defined as the intensity of the irradiation of the photo-sensitive layer in the halftones of an image produced by an external source of radiation, e.g. a preceding image intensifier, and the " operating range " is defined as the range of intensities of irradiation of the photo-sensitive layer within which a variation of irradiation intensity produces a variation in the brightness of the electroluminescent layer. In the arrangement shown in Fig. 3, three intensifiers 31, 32 and 33 are connected to sources 35, 36 and 37 which supply voltages of substantially equal amplitudes but of different frequencies #1, #2 and #3 respectively. An image of the screen 39 of a television projection tube is produced on the photo-sensitive layer of intensifier 31 by an optical system 38. The frequency #1, which may be 50 c/s., is chosen in accordance with the input level of intensifier 31, which is determined by the half-tones of the image on the screen 39 together with the optical system 38. If the gains of the intensifiers 31 and 32 are F1 and F2 respectively, the frequencies f2 and f2 f3 f3 are chosen so that - = F1 and - = F2. f1 f2 Thus if F1 and F2 are both 10, suitable values of f 2 and f3 are 500 c/s. and 5000 c/s. respectively. In the arrangement shown in Fig. 4 a source of radiation 43 produces an image of a photographic positive 44 on the photo-sensitive layer of an intensifier 41 connected to a voltage supply 42 having a frequency f4 of about 100 c/s. An image of the resulting pattern on the electro-luminescent layer of the intensifier 41 is projected by an optical system 45 on to the photo-sensitive layer of a larger intensifier 46 connected to a supply 47 having a frequency 15 so chosen that if F4 is the gain of intensifier 41 and # is the ratio between the intensities of the light emitted by the intensifier 41 and that ref5 F4 ceived by the intensifier 46, then - = -. If f4 # F4 is about 100 and # about 110, then f5 is about 45 c/s. The-linearity of the total intensification of a series of cascade-connected intensifiers can be improved by also choosing the frequencies of the operating voltages so that successive intensifiers operate on oppositely curved portions of their characteristics. Each intensifier may comprise a layer of photosensitive cadmium sulphide and a layer of electro-luminescent zinc sulphide activated with copper and gallium, separated by an intermediate opaque layer of black lacquer or a mosaic of minute metal particles, these layers being mounted between transparent electrodes each in the form of a thin layer of gold or conductive stannic oxide or a metal grid, the whole arrangement being supported by a glass plate or a fabric of thin glass filaments. Specification 783,812, [Group XL (a)], is referred to.