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.
