Abstract: For measuring the proportion or quantity of a component in a radiation-transparent mixture, a beam is passed through the mixture and subsequently split by a beam splitter into a measuring beam and a reference beam. The measuring beam is passed through a narrow-band interference filter which has a transmitting wavelength that corresponds to a discrete absorption wavelength of the component. This filter is oscillated in a range of oscillation which traverses the absorption maximum. The reference beam is passed through a narrow-band interference filter which has a transmitting wavelength that differs from the absorption wavelength of the component. Subsequently, the radiation intensities of the measuring beam and the reference beam are separately detected and their difference is formed. This difference is a function of the proportion or quantity of the component.