Patent GB-727893-A (GB)
Title: Improvements in and relating to optical apparatus
Inventor: GABOR DENNIS
IPC: G02B21/14
Filed: 1951-07-06
Abstract: 727,893. Optical apparatus. NATIONAL RESEARCH DEVELOPMENT CORPORATION. June 27, 1952 [July 6, 1951], No. 16151/51. Classes 97 (1) and 98 (2). Apparatus for recording photographically the interference pattern produced by illuminating a specimen with coherent light comprises means for splitting the light after incidence on the specimen into two separate informationcarrying beams of approximately equal intensity and for directing upon separated portions of a plane in which a photo-sensitive surface can be located, each of the said information-carrying beams, together with a different one of two background beams derived from the same source to produce two interference patterns, each background beam being of high intensity relative to that of the respective information-carrying beam and the phase displacements of the wavefronts of the two background beams relative to the wavefronts of their respective information-carrying beams differing by an odd number of greater wave. The apparatus may be modified for viewing the resulting photographs. As shown in Fig. 1, monochromatic light passing a small aperture 1 is divided, after passing a reflecting prism 3 and condenser 4, by a semi-transparent surface into a relatively weak specimenilluminating beam 7 and a relatively strong background beam 8. The beam 7 is focused at or near the specimen 10 and the light passed therethrough is directed by an objective 11 on to a prism 13 where it meets the beam 8 which has passed an objective 15, a prism 16 and a compensator 14 arranged to equalize the paths, to the prism 13, of the two beams 7 and 8. The surface of separation within the prism 13 comprises two semi-reflecting metal surfaces 12, 17, such as gold or silver, separated by a suitable thickness of transparent material 18 such as aluminium oxide or quartz. The part of the information-carrying beam 7 reflected at the surface 12 is combined with the part of the background beam 8 transmitted through the layers 12, 17 and 18, and the part of the information-carrying beam 7 transmitted is combined with the part of the background beam 8 reflected at the surface 17, the surfaces 12, 17 and the layer 18 being arranged to give the necessary phase displacements between the various partial beams. The two beams fall upon a photographic plate 21 through field lenses 23, to form two interference images. After processing the plate 21 is replaced on its support 22 and is illuminated by the beam 8. A plane mirror 25 immediately behind the plate returns the illuminating beams to the prism 13 whence they pass to a reflecting prism 26 and a viewing system comprising objective 27 and eyepiece 28 in which an enlarged image of the plate 21 may be seen. This image may, if desired, be viewed in phase contrast or in dark field illumination. A modification of the above arrangement is described in which the reflecting prisms are arranged so that the angles of incidence on their reflecting surfaces are 30 degrees, thereby reducing polarization effects. In the modification shown in Fig. 4, a single illuminating beam 31 is focused at or near the specimen 34 and an image of the source is formed by the objective 35 at a plate 37 by the direct beam passing through the specimen, the diffracted beam passing outside the plate 37. The plate 37 is a quarter-wave plate, e.g. of mica, and divides the direct beam into two components, plane polarized at right angles and differing in phase by a quarter of a wavelength. These two beams form the background beam and, together with the diffracted beam, fall upon a Wollaston prism 38. Here the background and information-carrying beams are divided to fall upon two adjacent areas of a photographic plate 39, prisms 41 ensuring correct orientation. After processing the plate is replaced on its carrier 40, a mirror 42 is placed behind it and it is illuminated by light from the monochromatic source through the optical system. The beams reflected by the mirror 42 are combined by the Wollaston prism and are reflected by a prism 43 into a viewing system comprising objective 44 and eyepiece 45, a plane polarizer 46 being provided to bring the two combined beams to the same plane of polarization to allow them to interfere. By suitable choice of a plate 36 surrounding the plate 37 one of the images may be recorded in phase contrast.