Abstract

An effective means of removing the short wave-length light from a beam containing infra-red radiation of all wave-lengths, uses an echelette grating as a simple mirror. Light of wave-lengths short compared to the grating space, is diffracted out of the beam into the dispersed spectra. Light of wave-lengths long compared to the grating space, can only go into the undispersed zero-order spectrum, which falls in the direction of normal reflection if the grating were a mirror. By the use of echelette gratings with high intensities in the dispersed spectra, large reductions in the amount of short wave-length light in the central image can be effected. The wave-length at which most of the light changes from normal reflection into the beam to diffraction out of it, may be quite sharp, so that the effective filtering action may vary by a factor as large as four in a two-micron interval. When such a grating is substituted for a plane mirror in the illuminating system of an infra-red spectrometer, the scattered light in the monochromator is reduced by a factor of ten.

© 1947 Optical Society of America

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