Optical Techniques for Studying Stray Light in Spectrophotometers

In spectrophotometers the term ‘stray light’ is used to refer to the minute amount of unwanted light having wavelengths outside the narrow band isolated by the optical system. The effect of stray light is to reduce the accuracy of the instrument and in some cases to restrict the wavelength range over which the instrument may be used. The paper describes an optical method by which the stray light transmission of a monochromator may be determined for various wavelengths. If the particular instrument studied is found to show stray light behaviour of a type which varies continuously with wavelength, a simple technique can be applied for assessing the errors that will occur with any given sample. The relative importance of near-stray and far-stray light is discussed.