Two light sources of different wavelength direct their beams through two partially transmitting mirrors so that identical beam lengths are obtained in the measuring medium. One light source is modulated in opposition to the other at a frequency so that the alternating light forms algebraic vectors for processing in photodetectors as complex alternating voltages or currents, e.g. in a two-phase bridge circuit. The attenuation difference of the beams of different wavelength in the measuring medium is measured over a phase angle resulting from the vector difference of the attenuated beams in the medium. One light source, e.g. the second is controlled so that either the primary intensities of the sources remain identical or their intensity ratio is kept constant.
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