Abstract: A metallic block has a conical measuring-light inlet port and a conical reference-light inlet port formed therein. The ports intersect at a light sensor. The inlet ports are closed by respective multilayer interference filters. Temperature control means, which may be responsive to the light sensor output, maintains the metallic block, and thus the filters and sensor, at a fixed temperature.

Abstract: To detect the density of a measurement component of a sample gas, infrared light is directed alternately through a measuring-light optical system and a reference-light optical system to a heated light sensor. The measuring-light optical system includes a sample cell, a first filter for transmitting infrared rays of a fixed wave-length band, and a temperature compensating cell having a gas which has a part of its absorption wave-length band within the transmission wave-length band of the first filter but has a spectrum different from the spectrum of the measurement component. The reference-light optical system includes a light throttle, an interference cell having a gas containing a fixed density of the measurement component, an adjustable-length interference compensating cell filled with the sample gas, and a second filter for transmitting rays of a wave-length band that includes the major part of the transmission wave-length band of the first filter.