Patent GB-714340-A (GB)
Title: Interference- or combined interference-schlieren apparatus with unusually large measuring field
IPC: G02B27/54
Filed: 1951-07-23
Abstract: 714,340. Interferometers. ZOBEL, T. July 23, 1951 [July 24, 1950], No. 17365/51. Class 97(1) To obtain a large measuring field in an interferometer of the Nach-Zchinder type combined, if desired with a Schlierin device, the measuring beam is enlarged, for example, by the aid of a concave parabolic mirror of high quality and after passage through the field to be measured is concentrated to its original size, for example,'by a second similar mirror; the comparison beam with which interference takes place may or may not be enlarged and is suitably compensated for any differences of path between the beams other than those due to the field. In a modification a single mirror is used, and the device may be employed to test the surface accuracy of a large spherical concave mirror. Fig. 2 shows an arrangement in which both the measuring and comparison beams are enlarged. Light from a source is focused by a lens 2 on a semi-transparent mirror 3 from which the comparison beam is reflected to a concave parabolic mirror 11 which renders the beam parallel and directs it to a second mirror 12 from which the beam is focused by mirror 13 and lens 14 on a semi-transparent mirror 8. The measuring beam, transmitted by the mirror 3 is focused by lens 4 on a mirror 5 and is similarly focused on mirror 8 by parabolic mirrors 6, 7 after passing the field 19 to be measured. The interference effects between the two beams is observed on screen 10. Part of the second beam may be intercepted by a semi-transparent reflector 15 to pass the edge 18 of a Schlierin device. A modification is described in which the axis of the mirrors 11, 12, 6 and 7 are parallel but not coincident. Fig. 3 shows a similar arrangement in which only the measuring beam is enlarged. Light from the source 1 reflected by the transparent mirror 3 is directed to the transparent mirror 11 by reflectors, 14, 15 and 16 while the transmitted measuring beam, focused by a concave mirror 4 on a small mirror 5 is reflected through the field, e.g. a wind tunnel, enclosed between glass plates 21, by the concave mirror 6 and is directed upon the mirror 11 by mirrors 7, 8, 9, and 10. As before a Schlierin edge 20 may also be incorporated in the apparatus. A compensator 22 adjusts the comparison beam for the thickness of the plates 21. Modified arrangements for producing the comparison beam using lenses of concave mirrors in addition to plane mirrors is also described. Fig. 10 shows a system employing only one large parabolic mirror. Light from the source 1 transmitted by semi-reflector 3 passes to mirrors 9, 10, is focused by lens 11 reflected back on itself by a concave mirror 12 to return to the reflector 3, there to interfere with the measuring beam. The latter is reflected by the mirror 3, focused on a mirror 5 by lens 4, rendered parallel by the mirror 6 and passes the field enclosed between glass plates 16. The beam then reaches a plane semi-transparent reflector 7 and is partially transmitted to form a shadowgraph visible on a screen 8, and partially reflected back to the mirror 6 and thence to the mirror 3. the interference pattern being visible on a screen 14. The reflector 7 may be totally reflecting, the lens 4 may be replaced by a concave mirror, and the lens and mirror 10, 11 replaced by a plane mirror. The system may be combined with a Schlierin system. By using as the plane reflector 7 a truly planar surface such as a liquid, the system may be used to determine the surface accuracy of a large spherical mirror by placing the mirror in position of the mirror 6 of Fig. 10.