The present invention relates to 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 and/or subject motion. The proposed device comprises an input unit (11) for receiving detection signals reflected back or transmitted through 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 three photo plethysmography PPG signals at different wavelengths from said detection signals a processing unit (13) for normalizing said at least three PPG signals and forming a first difference signal between a first normalized PPG signal and a second normalized PPG signal and a second difference signal between a third normalized PPG signal and one other of the at least three normalized PPG signals and for forming a ratio between said first difference signal and said second difference signal and an concentration detection unit (14) for calculating the concentration of a substance in the blood of the subject based on said ratio.
