The transmission of light from a sinusoidally modulated source to a detector through a turbid medium such as tissue can be described in terms of the phase angle and modulation of the detected signal. Such measurements provide information about the optical properties of the tissue. This paper outlines the relationship between tissue optical properties and the phase angle and modulation of the detected signal.
S. L. Jacques, "Principles of phase-resolved optical measurements," SPIE Proceedings of Future Trends in Biomedical Applications of Lasers, edited by L. O. Svaasand, 1525, 143-153 (1991).