A method and device for optical measurements are presented for determining the concentration of a substance in patient's blood. Optical measurement sessions are applied to a measurement location in a blood containing medium during certain time period. The optical measurements include illumination of the measurement location with incident light of at least one selected wavelength, detection, at each measurement session, of at least two light responses of the medium characterized by at least two different polarization states of detected light, respectively, and generation of data representative thereof. Measured data so obtained is in the form of at least two time variations of the light responses of the medium characterized by different polarization states of detected light, respectively, a relation between the time variations being indicative of the concentration of the substance in blood.