A method of determining the concentration of a substance in a subject's blood, the method comprising: noninvasively irradiating an eye of the subject with a measurement light beam having a first wavelength and a first power, wherein at least a portion of the measurement light is retro-reflected from the retina of the eye, thereby producing a measurement retro-reflected light beam having the first wavelength; noninvasively irradiating the eye of the subject with a reference light beam having a second wavelength and a second power, the second wavelength different from the first wavelength, wherein at least a portion of the reference light beam is retro-reflected from the retina of the eye, thereby producing a reference retro-reflected light beam having the second wavelength; detecting a power of the measurement retro-reflected light beam; and detecting a power of the reference retro-reflected light beam; said first and second wavelengths being infrared wavelengths differing by less than 0.2 micrometers; calculating a measurement ratio of the detected power of the measurement retro-reflected light beam and the first power; calculating a reference ratio of the detected power of the reference retro-reflected light beam and the second power; calculating a parameter ( T ƒ) which is the difference between the measurement ratio and the reference ratio or a difference between a logarithm of the measurement ratio and a logarithm of the reference ratio; measuring at least one of a body temperature of the subject and an ambient temperature; and determining the concentration of the substance in the subject's blood in response to the calculated parameter and the at least one of the body temperature and the ambient temperature. noninvasively irradiating an eye of the subject with a measurement light beam having a first wavelength and a first power, wherein at least a portion of the measurement light is retro-reflected from the retina of the eye, thereby producing a measurement retro-reflected light beam having the first wavelength; noninvasively irradiating the eye of the subject with a reference light beam having a second wavelength and a second power, the second wavelength different from the first wavelength, wherein at least a portion of the reference light beam is retro-reflected from the retina of the eye, thereby producing a reference retro-reflected light beam having the second wavelength; detecting a power of the measurement retro-reflected light beam; and detecting a power of the reference retro-reflected light beam; said first and second wavelengths being infrared wavelengths differing by less than 0.2 micrometers; calculating a measurement ratio of the detected power of the measurement retro-reflected light beam and the first power; calculating a reference ratio of the detected power of the reference retro-reflected light beam and the second power; calculating a parameter ( T ƒ) which is the difference between the measurement ratio and the reference ratio or a difference between a logarithm of the measurement ratio and a logarithm of the reference ratio; measuring at least one of a body temperature of the subject and an ambient temperature; and determining the concentration of the substance in the subject's blood in response to the calculated parameter and the at least one of the body temperature and the ambient temperature.