The invention relates to a measurement apparatus for the determination of gas concentrations. The apparatus comprises two spectral channels, wherein the channels are separated by a single chopper wheel. The chopper wheel has several functions. On the transmitting side, it brings the light of the two light sources on the same measuring path, on the receiving side it associates the light to the associated receiver; it has its chopper function to use the lock-in technique; and it opens the possibility to implement an easy zero point correction.