Two coherent narrow bandwidth infrared beams, the Signal and the Reference, are incident at an angle and at high frequency sequentially and alternately at the same spot of a whole blood/body tissues sample. The Signal beam has a center wavelength which falls within an absorption line of glucose in whole blood (e.g. 1.409μ). The Reference beam has a center wavelength which does not coincide with any known absorption lines of glucose in whole blood (e.g. 1,278μ). Radiation emitted from the spot at which the beams penetrate into the sample and subsequently emanate from it after multiple scattering and spurious absorption effects is collected by a lens onto an infrared detector. The ratio of the voltage detected from the emerging Signal beam over that of the Reference beam is processed to yield the value of glucose concentration in the whole blood/body tissue sample.
