System for use in blood oxygen saturation measurement of a subject , the system comprising: - a coherent light source arranged to emit coherent light at a first wavelength and at a second wavelength towards a tissue region of the subject; - a detector for acquiring first and second detection data of the tissue region of the subject, said first detection data being acquired over time by detecting radiation at the first wavelength (λ1) and at the second wavelength(λ2) received from said tissue region; said second detection data being acquired over time by detecting radiation at the first wavelength and at the second wavelength received from said tissue region in response to the coherent light at the first wavelength and coherent light at the second wavelength being emitted towards the tissue region; and - a processing device for processing said first and said second detection data of the tissue region of the subject, the processing device comprising: - an input for receiving the first and second detection data; - a PPG unit for deriving, from said first detection data, a first PPG signal indicative of an absorption of light within the tissue region at the first wavelength, and a second PPG signal indicative of an absorption of light within the tissue region at the second wavelength; - a flow unit for deriving, from said second detection data, a first flow signal indicative of a flow of light scattering particles probed at the first wavelength within the tissue region, and a second flow signal indicative of a flow of light scattering particles probed at the second wavelength within the tissue region; and - a processing unit for correcting said PPG signals based on said flow signals and/or for providing a feedback signal based on a comparison of the first and second flow signals. - a coherent light source arranged to emit coherent light at a first wavelength and at a second wavelength towards a tissue region of the subject; - a detector for acquiring first and second detection data of the tissue region of the subject, said first detection data being acquired over time by detecting radiation at the first wavelength (λ1) and at the second wavelength(λ2) received from said tissue region; said second detection data being acquired over time by detecting radiation at the first wavelength and at the second wavelength received from said tissue region in response to the coherent light at the first wavelength and coherent light at the second wavelength being emitted towards the tissue region; and - a processing device for processing said first and said second detection data of the tissue region of the subject, the processing device comprising: - an input for receiving the first and second detection data; - a PPG unit for deriving, from said first detection data, a first PPG signal indicative of an absorption of light within the tissue region at the first wavelength, and a second PPG signal indicative of an absorption of light within the tissue region at the second wavelength; - a flow unit for deriving, from said second detection data, a first flow signal indicative of a flow of light scattering particles probed at the first wavelength within the tissue region, and a second flow signal indicative of a flow of light scattering particles probed at the second wavelength within the tissue region; and - a processing unit for correcting said PPG signals based on said flow signals and/or for providing a feedback signal based on a comparison of the first and second flow signals. <EOS>