Patent GB-468893-A (GB)
Title: Improvements in or relating to the duplication of diffraction gratings
IPC: G02B5/18
Filed: 1936-01-10
Abstract: <PICT:0468893/IV/1> <PICT:0468893/IV/2> An original diffraction surface is duplicated by applying to it a material capable of taking an impression therefrom and thus forming a primary negative replica, forming one or more positive replicas by applying to the negative replica a material which does not mix or dissolve therewith, and making further replicas from the positive replica in the same manner. In a modification, a number of the primary negative replicas are formed and juxtaposed, and used to form a positive diffraction surface of larger area than the original.  In another modification, a number of the direct positive replicas are formed and juxtaposed, one or more secondary negative dies are formed from the juxtaposed positives and these are juxtaposed to form a larger diffraction surface. Cellulosic material such as collodion may be used for the primary negative replica and swollen gelatinous material or photo-engraving glue for the positive replica.  In one form of the method, a pyroxylin skin is cast on the original grating, dried and floated off in warm water.  The skin is transferred to a plate having a gelatine surface soaked in a hardening agent such as potassium dichromate or formalin.  The skin is squeegeed on to the gelatine, dried and removed.  The hardened gelatine thus forms the positive replica wherefrom a number of collodion skin negatives are obtained. The skins are laid in overlapping relation, either regularly or in a pattern, on a large gelatine plate, the junctions being painted with glue.  The large gelatine positive so formed is used for casting a large collodion film negative and the process may be repeated for larger surfaces.  The diffraction surface may be reproduced on a continuous film of regenerated cellulose or cellulose acetate or other derivative or alginic material by forming a cylinder 30 or an endless band with the diffraction surface.  The cylinder is provided with a celluloid layer 29 and covered with space strips of glue 31, 34.  A positive is formed on gelatine paper 27 by squeegeeing the collodion negative thereon.  The gelatine positives are pressed by rollers 35 on to the uncovered spaces of the roller, pyroxylin solution 36 being interposed between the gelatine and celluloid. After drying, the paper is removed and the glue washed off.  Further patterns are similarly impressed on the spaces previously occupied by glue.  The cylinder is used to impress cellulose foil 41 which is passed through a water bath 43 and pressed against the cylinder by rollers 44, 45.  The roller 30 may be internally heated.  The cylinder may be prepared by coating it with dichromate glue and applying the collodion negatives thereto.  The hardened glue surface may be used for foils or for casting cellulose acetate. Thermoplastic synthetic resins may be impressed by the replicas. Moulds for synthetic resins such as phenol formaldehyde resins may be lined with the diffraction surfaces in hardened gelatine or glue or in metal formed by deposition on a replica.  Specification 468,942, [Group V], is referred to.