Patent GB-747618-A (GB)
Title: Improvements in and relating to articles incorporating diffraction gratings
Inventor: SAYCE LEONARD ALFRED
IPC: G02B5/18
Filed: 1953-04-09
Abstract: 747,618. Diffraction gratings; photographic processes. NATIONAL RESEARCH DEVELOPMENT CORPORATION. April 7, 1954 [April 9, 1953], No. 9689/53. Classes 97 (1) and 98 (2). [Also in Group XV] A surface having a pattern of areas of diffraction grating replicas belonging to at least two different diffraction grating systems is prepared by coating a grating replica of the first system, made for example as described in Specifications 468,893 or 468,942, [both in Group XVI], with a layer of photo-sensitive material which fills and thus optically neutralizes the grooves of the replica, exposing the layer to light, for example, through an appropriate mask, removing the exposed or unexposed portions of the layer to reveal areas of the first grating and forming the second grating on the outer surface of the'remaining portions of the layer, for example by direct impression with a replica grating of the second system. The second grating may be formed at any stage after which it will not be damaged by subsequent steps in the process. Fig. 2 illustrates the steps in the process where the photo-sensitive layer is a silver halide emulsion prepared from three solutions the first being a solution of 5 gms. of gelatine in 70 ml. of water, the second of 4 gms. of potassium bromide in 15 ml. of water and the third of 4.8 gms. of silver nitrate in 15 ml. of water. The first solution is maintained at 40‹ C. and the other solutions are run into it drop by drop at equal rates and at positions as far apart as possible, the bromide solution being given a start of some half a dozen drops and the whole being stirred. This gives uniform grain size and high contrast in the resulting emulsion. Finally, 2.5 ml. of a 1 per cent solution of chrome alum is added while the emulsion is still being stirred. Referring to Fig. 2, a glass plate 22 is coated with a gelatine layer 23 which is allowed to set (step A). The plate is soaked in water for a few minutes and a replica grating 21 of the first system, formed of cellulose acetate is squeezed on to the gelatine layer (B) and a hardened gelatine solution used to form an edging 24 therefor (C). The grating is then coated with a layer 25 of the above-described emulsion (D), is exposed through a mask 26 to light from lamps 27 (E). The layer is then developed, fixed and washed and treated with a solution of cupric chloride, acetic acid and hydrogen peroxide which bleaches the silver and dissolves away the gelatine containing the silver leaving a broken layer 25 (F). A replica 29 of the second grating is then pressed against the layer 25 (G) and, after drying, is removed, leaving a surface formed partly of the grating 21 and partly of the second grating (H). This surface may be used as a mould to produce other surfaces. In a modification the light-sensitive layer is formed of gelatine sensitized by dichromate and developed with warm water. The layer may be sensitized before or after application to the first replica and the impression of the second grating may be formed thereon before or after exposure to light.