"Quantities and Units of Light and Related Electromagnetic Radiations, 2nd. Ed., International ISO 31/6, 1980(E)," International Organization for Standardization, Switzerland, 1980. Also see: "Optical-Thermal Response of Laser-Irradiated Tissue," Section 2.2.2. "Optical Parameters" p. 20-26, eds. A.J. Welch and M.J.C. van Gemert, Plenum Press, New York, 1995.