There is a device, system and method for determining the concentration of a substance, such as the oxygen saturation, in the blood of a subject, which reduce or remove the influence of specular reflection. The proposed device comprises an input unit (11) for receiving detection signals reflected back from a skin area of the subject in response to irradiation of the skin area by a radiation signal, a signal extraction unit (12) for extracting at least two photo-plethysmography, PPG, signals at two different wavelengths from said detection signals, and a processing unit (13) for computing the concentration of a desired substance in the blood of the subject based on said PPG signals, wherein said computation is adapted to the skin tone of the subject.</p
