GB1061319A - Improvements in and relating to light filters - Google Patents

Abstract

1,061,319. Optical filters. INTERNATIONAL BUSINESS MACHINES CORPORATION. Dec. 10, 1965 [Dec. 18, 1964], No.52430/65. Heading G2J. A light filter 14 comprises a plurality of partially transparent spaced layers 18 of silver chloride disposed within a member of transparent material, the layers being spaced so that (i) along any line normal to one of the two outer layers the spacings between the layers are substantially equal, (ii) the size of the spacings taken along a series of lines normal to a particular line varies progressively along said particular line, and (iii) coherent reflection occurs for light of different frequencies when directed along different ones of said series of lines. The filter is formed by setting up standing waves of a beam of monochromatic light in a photographic emulsion containing silver halide, thereby depositing silver at the antinodes of the waves, the intensity of the beam varying from one edge to the other. The emulsion is developed, a fixer causing it to shrink and hence causing the spacings between the layers of the deposited silver (later treated with mercuric chloride) to vary according to the intensity of the light directed over the surface. When light directed at any point on the surface of the developed emulsion is at such a frequency that its wavelength is twice the distance between adjacent reflecting layers beneath that point, a coherent reflective scattering of light is obtained. A scale (50) Fig. 6 (not shown) may be provided along one edge of the film (14) to indicate the frequency at which light is reflected from the film, which may be used as a frequency generator Fig. 4 (not shown) or to identify an unknown frequency Fig. 5 (not shown).