GB598545A - Improvements in or relating to luminescent screens - Google Patents

Abstract

598,545. Luminescent materials for radiolocation. GENERAL ELECTRIC CO., Ltd., JESTY, L. C., and HOPKINSON, R. G. Nov. 1, 1944, No. 21408. Drawings to Specification. [Classes 39 (i) and 40 (v)] A luminescent screen which is to be scanned by light dependant upon incoming signals comprises two superposed layers of luminescent material, the one nearer to the scanning source giving short built-up and afterglow periods, whilst permitting light to pass to other layer, only sufficient to give it relatively long build-up and afterglow periods, the visible light emitted by the first layer being able to penetrate the second layer for viewing purposes. The layers are not necessarily entirely discrete, but may merge into each other, with a preponderance of one or the other at each side. In one form, the third of three superposed layers, i.e. that remote from the viewer, receives the scanning beam, e.g., in a cathode ray tube, and its emitted radiation is capable of exciting the first layer and preferably the second layer, although the latter may be partly or wholly excited by the incident beam of radiant energy. The first layer may be zinc sulphide actuated with copper having a green long afterglow, the second layer zinc cadmium sulphide 73 per cent Cds., 27 per cent ZnS., having a short orange afterglow and third layer blue luminescing zinc sulphide actuated with silver. The first layer is dusted on to a glass carrier covered with a thin binder layer of phosphoric acid in acetone so as to give an optical transmission of 55 per cent. A dilute solution of potassium silicate is sprayed on and the second luminescent material dusted on so that the total transmission thickness of the two layers is 40 per cent. The third layer is applied in the same way. This screen gives a green stationary pattern and a red moving pattern when used in a radar-scanning apparatus. An improved colour contrast is obtained by viewing through didymium glass, or by means of two filters, the pass bands of which are separated by a region covering the overlap of the red and green light emitted by the screen and which are moved across the screen at a frequency above flicker frequency. The scanner emitting a modulated beam may be either a high intensity cathode-ray tube having a blue emitting screen or one of the mechanical type. A mask movable in synchronism with the scanning beam serves to suppress the red flash of the scanning spot. Alternatively, the screen consists of two layers the second comprising a mixture of two luminescent materials, one yielding the desired short build-up and afterglow periods and the other emitting a blue or ultra violet radiation. Orange luminescing zinc fluoride actuated with manganese and zinc cadmium sulphide activated with copper (68 per cent ZnS., 32 per cent CdS.), with a red orange luminescence are also used. A table showing various alternative combinations and mixtures of the above-mentioned materials is included in the Specification.