JPS6212881B

PURPOSE:To form a Brewster window having excellent water resistance and less evolution of heat and to improve the durability thereof by providing a protective film consisting of chalcogenide glass or ThF4 which is transparent to IR light at specific film thickness on both surfaces of an alkali halide substrate. CONSTITUTION:Chalcogenide glass such as As2S3, As2Se or the like or ThF4 or the like is vapor-deposited on both surfaces of an alkali halide parallel plate of KCl or the like which is transparent to IR light and has small optical distortion at the film thickness satisfying the relation expressed by the equation (lambda is the wavelength of light, nF is the refractive index of the film when the wavelength lambda, thetaB is the Brewster angle specific to the parallel flat plate), whereby a Brewster window formed with the protective film is provided. The Brewster window which is coated with the surface of the substrate, such as alkali halide, having deliquescence, has substantial water resistance under high humidity, has zero reflectivity to P polarized light, evolves less heat even with the large output like CO2 laser light and has excellent durability is thus obtained.