Patent GB-1066949-A (GB)
Title: Improvements in or relating to interferometric apparatus
IPC: G01B9/02
Filed: 1963-10-02
Abstract: 1, 066, 949. Photo-electric interferometers. GENERAL PRECISION, Inc. Oct. 2, 1963 [Oct.2, 1962 No.38884/63. Heading G1A. [Also in Division G2] An interferometic optical goniometer comprises a stator 12, Fig. 1A, for producing a pair of separated beams of radiation 28, 30, 32, and 62, 64, 66, which impinge respectively on cube corner reflectors 42, 68, or other reflectors of a multiplereflecting kind, which are mounted on a gonior 14 for angular movement as a unit about an axis perpendicular to the beams, the reflectors 42, 68, reflecting back beams 44, 46, 48, and 70, 72, 74, in the same angular direction relative to the incident beams 28, 30, 32, and 62, 64, 66, regardless, within limits, of the angular position of the gonior 14, to recombining means mounted on the stator 12, for producing interference fringes. As shown, monochromatic light from source 16 is collimated at 18 to beam 20, 22, 24, which is partially transmitted at beam splitter 26 to form beam 28, 30, 32, and partially reflected as beam 34, 36, 38 to a mirror 60 to form beam 62, 64, 66. The transmitted beam 28, 30, 32, reflected back as beam 44, 46, 48, is reflected by mirror 50 as beam 54, 56, 58, to recombiner 52. The reflected beam 34, 36, 38, forming beams 62, 64, 66, is reflected back from reflector 68 as beam 70, 72, 74, which is transmitted through partial reflector 52 to recombine at 76, 78, 80, with rays 54, 56, 58. The recombined beams are reflected at 88, enter a constant deviation dispersing prism 96, to provide spectral separation of residual unwanted wavelengths provided by source 16, are collected by stigmator lenses 98, 100, pass through pinholes 102, 104, and enter photo-multiplier tubes 106, 108. The outputs of the photo-multiplier tubes are fed to a reversible electronic counter to count the interference fringes produced. To provide direction sensitivity the lower surface of beam splitter 26 has half its width coated with a one quarter wavelength coating 40 of silica or titania so that half of the beam of the transmitted light 28, 30 is retarded 90‹. To determine the zero position of gonior 14, white light is introduced from a tungst 112 by movable mirror 110, in the 110' position, the output of the photo-multiplier tubes being a maximum when equal path lengths and thus zero angle of gonior 14 is obtained. In a modification, Fig. 2 (not shown), the amount of light recombined is substantially independent of the angular rotation of gonior (142) within much wider limits by providing mirrors (150, 194), adjacent the reflectors (140, 186) of the gonior to direct the emergent beams through the cube corner reflectors a second time, the shift of each beam reflected by the cube corner reflector as the gonior rotates being cancelled out by a reverse shift on the second time through the reflector. In a further form, Fig. (not shown), the monochromatic light from source (16) collimated at (18) is partially reflected and transmitted at beam-splitter (258) positioned at the centre of an optically flat glass or quartz slab (256). The reflected beam (262, 264, 266), is reflected at (276) enters cube corner reflector (284) on gonior (286) passes to mirror (294) one half of which has a phase retarding coating (296), back to reflector (284) and by mirror (276) returns to beam splitter (258). Similarly the transmitted beam (270, 272, 274), is reflected at (316), enters cube corner reflector (318), passes to mirror (294), returns to reflector (318) and mirror (316) so as to recombine at beam splitter (258). In a further form, Fig. 4A (not shown), the cube corner reflectors are replaced by first surface dihedral mirrors (336, 344) on gonior (320) and four such devices may be combined, Fig. 5 (not shown), so that angles extending from 0 to 360‹ may be measured. In these latter embodiments the polychromatic light for determining the zero position is continuously fed into the system from tungsten lamps (372) and forms portion (378), Fig. 4B (not shown), of the total light passing through the interferometer, it being finally reflected by prism (364) into photo-multiplier (370).