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Abstract

A method and apparatus for measuring the concentration of a substance capable of absorbing infrared, visible or ultraviolet radiation energy, the substance being in a mixture. The method and apparatus involve passing a beam of radiant energy having predetermined spectral response characteristics through the mixture, modulating the beam at a predetermined frequency .omega. to form two sequential separate intervals of radiation, the two intervals being separated from each other by dark intervals and being different from each other in spectral response, controlling the first one of the radiation intervals with a filter which selectively absorbs or transmits energy in the spectral region of the absorption of the substance being measured, controlling the second one of the radiation intervals with a filter which does not selectively absorb or transmit energy in the spectral region of the absorption of the substance being measured or in the spectral region of interference components, measuring the intensities of the radiation intervals after the beam has passed through the mixture using a wide-band detector to produce two output signals, phase rectifying the detector output signals with the frequencies .omega. and 2.omega. to form two signals S.sub..omega. and S.sub.2.omega. and forming the ratio of the two signals as a measure of the concentration of the components of interest.

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