Fundamental problems of incoherent optical heterodyne detection are analyzed. Output of the heterodyne detection depends on the spatial and temporal coherences of incoming incoherent signal light on the photodetector surface. The directivity of optical heterodyne detection is concluded to be the same as with that of direct detection in an ideal case in which the image of the object is well focused on the photodetector surface. This incoherent heterodyne detection is applied to air pollution monitoring. In the laboratory, the absorption spectra due to NH₃, Freon, and SF₆ are measured using an incoherent light source, and the concentrations of each gas were determined by using the least-squares method.

© 1978 Optical Society of America

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