A method to estimate the ratio of absorption coefficients of two wavelengths using phase-modulated near infrared light spectroscopy.

Near infrared spectroscopy is a very useful tool for monitoring the oxygen saturation of living tissue noninvasively. We can calculate the hemoglobin oxygen saturation within tissue, using the ratio of the absorption coefficients (mua) at two different wave lengths of light. Biological tissue has a very high effective scattering factor (mu's), which elongates an optical path length and makes it difficult to compute the mua by the conventional method using continuous light. Phase-modulated spectroscopy (PMS) measures the path length which is a complex function of the mua and mu's. To obtain the ratio for mua, we have to eliminate the effects of the mu's from the obtained value by the PMS method. In this report, we present a theory and an experimental result which show that the inverse of the squared ratio of two phase angle differences at two different separations obtained by two different light wavelengths provides a good estimate of the ratio at these wavelengths.