Disclosed is a method for measuring the absorbance of light of a substance in a solution in a measuring cell (23; 223'), said method comprising the steps of: transmitting (S1) a first light beam (27; 27') from a light source (25; 25') towards a beam splitter (29; 29'); dividing (S3) the first light beam (27; 27') into a signal light ray (31; 31') and a reference light ray (33; 33') by the beam splitter (29; 29'); modulating (S5) the signal light ray (31; 31'); modulating the reference light ray (33; 33'); providing (S9) the measuring cell (23; 23') such that the signal light ray (31; 31') passes through the measuring cell; detecting (S11) a signal in a detectoR (39; 39'), which signal is the combined signal intensity of the signal light ray (31; 31') and the reference light ray (33; 33') detected by the detector (39; 39'); performing synchronous detection (S15) of the detected signal in order to reconstruct the intensities of the signal light ray (31; 31') and the reference light ray (33; 33') from the combin
