GB1104138A - Optical filtering devices and methods of making same - Google Patents

Abstract

1,104,138. Infrared filters. BECKMAN INSTRUMENTS Inc. Feb.23, 1965 [March 2, 1964(2)], No.7695/65. Heading G2J. A filter having a high transmission for infrared radiation and being substantially opaque to all wavelengths shorter than a predetermined cuton wavelength comprises a thin transmitting wafer having uniformly distributed high and low refractive index material to produce selective scattering of all wavelengths shorter than a certain wavelength and one or more absorbing materials having an absorption band in the shorter wavelength transmission region so as to sharpen the cut-on front. The thin transmitting wafer may be a polyethylene matrix with a pore size of 1<SP>.</SP>5 microns or alternatively 10 microns so that air of refractive index 1<SP>.</SP>0 is distributed in polyethylene of index 1<SP>.</SP>5. In a modification T11, T1C1, TlBr, HgI 2 or Hg 2 C1 2 is evenly distributed in a polyethylene matrix. The absorbing material may be one or more selected from BaCO 3 , BaF 2 , BeO, KBr, PbCO 3 , CsBr, CaCO 3 , Li 2 CO 3 , LiF, SrF2, TiO 2 and freshly prepared MgO, finely divided and substantially evenly dispersed throughout a polyethylene, polystyrene or polytetrafluoroethylene matrix. When using hygroscopic absorbing materials, such as KBr or CsBr which tend to coalesce when finely divided, they are mixed with the polyethylene or other matrix material and also with small amounts of a highly refined mineral oil placed in a die, heated, pressed to form a thin wafer, leached with organic solvent such as hexane or heptane, reheated, and repressed.