A device includes an element (7) for illuminating the sample (1), alternately emitting first and second radiation beams; device (9) for receiving and analyzing radiation derived from the sample (1) which converts the output radiation into control signal (S0); monitoring and controlling device (31) emitting first (S1) and second (S2) signals controlling the illuminating means (7), and a measuring signal (Sm); and device for calculating absorption characteristics of the sample (1), on the basis of the measuring signal (Sm). The monitoring and control elements (31) are adapted so that, after at least one period (Tp) of variation of the radiation beams, the amplitude of the control signal (S0), sensed on emitting phases, is equal to the amplitude of the control signal (S0), sensed on emission phases of the other radiation, and the measuring signal (Sm) represents at least one of the control signals (S1, S2).