A device, system and method for determining the concentration of a substance, such as the oxygen saturation, in the blood of a subject are provided, which reduce or remove the influence of specular reflection. The proposed device comprises an input unit configured to receive 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 configured to extract at least two photo-plethysmography, PPG, signals at two different wavelengths from said detection signals, and a processing unit configured to compute 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.
