The present invention relates to quantitative analyses of absorptive constituents in biological tissues by employing a phase modulation spectroscopy. 
Continuous wave (CW) tissue oximeters have been widely used to determine in vivo concentration of an optically absorbing pigment (e.g., hemoglobin, oxyhemoglobin) in biological tissue.  The CW oximeters measure attenuation of continuous light in the tissue and evaluate the concentration based on the Beer Lambert equation or a modified Beer Lambert absorbance equation.  The Beer Lambert equation (1) describes the relationship between the concentration of an absorbent constituent (C), the extinction coefficient (.epsilon.),  the photon migration pathlength &lt;L&gt;, and the attenuated light intensity (I/I.sub.o). ## EQU1##
The CW spectrophotometric techniques can not determine .epsilon.,  C, and &lt;L&gt; at the same time.  If one could assume that the photon pathlength were constant and uniform throughout all subjects, direct quantitation of the constituent concentration (C) using CW oximeters would be possible. 
In tissue, the optical migration pathlength varies with the size, structure, and physiology of the internal tissue examined by the CW oximeters.  For example, in the brain, the gray and white matter and the structures thereof are different in various individuals.  In addition, the photon migration pathlength itself is a function of the relative concentration of absorbing constituents.  As a result, the pathlength through an organ with a high blood hemoglobin concentration, for example, will be different from the same with a low blood hemoglobin concentration.  Furthermore, the pathlength is frequently dependent upon the wavelength of the light since the absorption coefficient of many tissue constituents is wavelength dependent.  Thus, where possible, it is advantageous to measure the pathlength directly when quantifying the hemoglobin concentration in tissue.