Abstract: An optically based apparatus for non-invasively determining the concentration of optically active substances in a specimen comprises, a source of a beam of spatially coherent light which is acted upon to produce a rotating linear polarized vector therein. A beam splitter splits the beam into a reference beam and a detector beam for passage through the specimen. The detector beam is received upon exiting the specimen and compared with the reference beam to determine the amount of phase shift produced by passage through the specimen. This amount of phase shift is converted into concentration of the optically active substance in the specimen.