Patent GB-912881-A (GB)
Title: Improvements in and relating to the copying of diffraction gratings
Inventor: BURCH JAMES MORRISS
IPC: G03C5/08
Filed: 1958-03-19
Abstract: 912,881. Diffraction gratings. NATIONAL RESEARCH DEVELOPMENT CORPORATION. March 19, 1959 [March 19, 1958], No. 8800/58. Class 97 (1). [Also in Group XL (b)] A photographic copy of a diffraction, grating is formed by illuminating the grating with collimated light and exposing behind the grating a sensitized surface, the spacing between the grating and this surface being such that the zero order beam from one ruling meets the first order diffraction beam from the two adjacent rulings at the sensitized surface. The appropriate spacing may be determined experimentally as in Fig. 1 by exposing a sensitized surface 11 to light from a grating 12 inclined to the surface 11. By examining and testing the developed copy, that section of the length which is of the desired quality can be ascertained and from its position the exact spacing can be evaluated. As shown in Fig. 2, a linear grating 12 to be copied is correctly spaced from a sensitized surface 13 which is exposed to collimated light from a source 14 and passing through the grating 12, the grating and sensitized surface being moved through the beam in the longitudinal direction of the grating by, for example, a feed screw or endless belt driven from an electric motor at a slow speed. Slight known progressive errors of the grating 12 may be corrected by varying the inclination of the beam on the grating by manual rocking movement of the illuminating system about a centre 22 relatively to a scale 24. Fan error in the grating 12 may also be corrected by use of a cylindrical lens 27 which is oscillated. These errors may first be ascertained by calibrating the grating. Calibrating for progressive error may be effected by placing a second grating of the same periodicity in the place to be occupied by the sensitized surface and measuring, by means of a photo-electric cell, the light transmitted at various angles by the grating when illuminated by a collimated beam afterwards to be used for the photographic exposure. Similarly, fan error may be determined by mounting a cylindrical lens in the beam and measuring the rotation of this lens necessary at various points to maintain the output of a photo-olectric cell placed to receive the beam coming through the two gratings. The grating 12 and sensitized surface 13 are preferably mounted on glass or silica supports 28 and 31 clamped at 33 with spacers 34 therebetween in a frame 29 movable on V and flat ways 35 and 36, Fig. 3. Radial gratings may also be copied, in which case the grating and the sensitized surface which is correctly spaced therefrom are mounted on a turntable rotated at slow speed relative to the collimated beam. From the photographic copy obtained further replicas may be produced by moulding a transparent cold-setting synthetic resin against the photographic copy by methods such as described in Specification 817,051. The collimated light incident on the grating is preferably monochromatic but in order to increase the tolerance light of two different wavelengths may be used in the photographic exposure.