An infrared ray detector assembly for detecting a plurality of components such as hydrocarbons, CO.sub.2 and CO gases includes a housing member having a hollow interior and a portion transmissible to infrared rays and a holder member mounted within the hollow interior and having a plurality of separate compartments. An infrared detector is mounted in each compartment, and an optical filter is mounted over each compartment, abutting adjacent filters to respectively limit the wavelength band to each infrared detector, each optical filter having a substrate of approximately the same material and thickness.