Stray light in monochromators

A method of measuring stray light in single monochromators by means of a double monochromator is described. Various types of monochromator, employing ( a ) Littrow prism, ( b ) diffraction grating, ( c ) replica of diffraction grating and ( d ) composite prism with only two glass-air surfaces, have been examined by this method and comparative results obtained. An experimental type of monochromator where the light traverses the same prism twice is shown to have the same order of stray light as the conventional double monochromator. With the experimental instrument set at wavelength 500 mμ, the intensity of the stray light of wavelength 480 mμ or 520 mμ was found to be less than 10 -5 of the intensity of the directly transmitted light.