Quantitation of absolute concentration change in scattering media by the time-resolved microscopic Beer-Lambert law.

In a scattering media like living tissue, there have been two lines of critical argument concerning the effect of optical absorption on the distribution of’ the optical pathlength. One is that the optical pathlength is constant when the absorption changes, though it differs markedly from the physical pathlength, such as the thickness of the tissue because of considerable scattering. The other is that the distribution of the path-length depends on the absorption (1,2). Previously, we have shown the validity of the Beer-Lambert law in rat heads (3) and thigh muscles (4). The requirement of’ the Beer-Lambert law is the independence of the attenuation of incident light by absorption from that by scattering. This was confirmed by measuring the time of flight of picosecond length light pulses in several rat tissues (5) as well as model systems (6). Equation(1), which was derived from our time-resolved study on the Beer-Lambert law, shows that light intensity along the non-linear path taken by photons through scattering media is exponentially attenuated by absorption. Monte Carlo simulation also confirmed this (7). The present paper expands equation(1) into time-resolved multiwavelength photometry for determination of the absolute concentration of absorber coexisting in scattering media, by which the optical pathlength was directly estimated.