Interference Filter Spectrometer for Low-Intensity Extended Sources

Recent availability of inexpensive optical interference filters has made possible spectroscopy of some extended sources too weak for conventional techniques. Because many spectra from solid-state sources are continua where high resolution is not required, and large source areas are available, interference filters and a photomultiplier may be used as a simple, very high aperture spectrometer. Fifty filters have been mounted around the periphery of a rotating disk 1 m in diameter and enclosed in a light-tight housing having ports for source and detector. Because transmission profile depends somewhat on the angle and point of incidence of light on the filter, calibration is carried out by using a translucent diffuser source equal to source size (6 cm in diameter) and at source position, and which is illuminated by light from a monochromator over the range from 2300 to 7500 Å. For source-to-multiplier spacings from 36 to 10 cm, most filters have a transmission profile in the shape of an isosceles triangle approximately 30% high. The width at half-height varies from 100 to 150 Å, respectively.