Abstract: A process and apparatus for the photo-optical measurement of the absorption behavior of solid, liquid and gaseous media. The analysis medium and a comparison medium are exposed to two light beams. The light beams are broken up by a chopper means into a periodical sequence of light pulses and the measured difference of the beam attenuations in the analysis medium and comparison medium is evaluated in a circuit which is adjustable with regard to the intensity of the radiation from the optical light source and the sensitivity of the light receiver. The light pulses are divided by defined variation of their intensity into two different pulse series of which one series comprises measuring light pulses and the other series comprises control pulses.