A method determines the concentration of a substance in a subject's blood. The method includes noninvasively irradiating an eye with a measurement light beam having a first wavelength and a first power and with a reference light beam having a second wavelength and a second power. The second wavelength is different from the first wavelength. The method further includes measuring at least one of a body temperature of the subject and an ambient temperature of the subject. The method further includes detecting a power of the measurement retro-reflected light beam and detecting a power of the reference retro-reflected light beam. The method further includes calculating a measurement ratio of the detected power of the measurement retro-reflected light beam and the first power and calculating a reference ratio of the detected power of the reference retro-reflected light beam and the second power. The method further includes calculating a parameter dependent on the measurement ratio and the reference ratio and determining a concentration of the substance in the subject's blood in response to the calculated parameter and to the at least one of the body temperature and the ambient temperature.