An instrument to make simultaneous accurate measurements of the intensity of light in each of two (or more) spectral channels using a single interference filter employs mirrors or prisms to break up the observed light beam into two (or more) beams, propagating in slightly different directions. The angle dependence of the filter spectral passband provides two distinct spectral channels which are focused onto separate detector elements. Advantages of this instrument are illustrated by its application for remote point Raman temperature measurements in hot gases.