GB699467A - Interference filter arrangement - Google Patents

Abstract

699,467. Colour filters. ZEISS-STIFTUNG, C.,[trading as JENAER GLASWERK SCHOTT & GEN]. Feb. 21, 1951 [Feb. 23, 1950], No. 4233/51. Class 97 (i). A filter arrangement for use at oblique incidence consists of two interference filters, each comprising superposed non-metallic layers of alternately high and low refractive index, separated by a bi-refringent layer having the direction of its bi-refringent action inclined at an acute angle to the plane of incidence of the light and which produces, for the angle of incidence chosen, a rotation of the plane of polarization of light passing through it. By such a system differences in the spectral composition of the reflected and transmitted beams due to differences in reflections of the components of the incident beam polarized in and perpendicular to the plane of incidence, may be avoided. Fig. 2 shows such a filter comprising a bi-refringent layer d of, e.g. gypsum, mica or stretched cellulose having on each side an interference filter comprising layers b of zinc sulphide separated by layers c of lithium fluoride, the whole being enclosed between prisms a and a<SP>1</SP>. A beam incident on one face of the prism a passes to the first filter system. Of the beam transmitted by this system the plane of polarization of the component polarized parallel to the plane of incidence is rotated through 90 degrees by the layer d so as to be polarized perpendicular to the plane of incidence when falling on the second filter. This component is reflected by the second filter, is again rotated through 90 degrees by the layer d to form the other component of the incident beam reflected by the first filter. The resulting reflected beam is therefore unpolarized, as is the transmitted beam.