I have traced the development of the optical method from Millikan's colorimetry of cat muscle myoglobin to today's high-frequency laser diode time-resolved phase modulation system study of hemoglobin and myoglobin in muscle and brain in adult humans. The path length as well as specific absorption information is obtained in terms of the rate of photon decay or by equivalent measurements using phase modulation. Localization of inhomogeneities of deoxyhemoglobin concentrations in stroke and head injury appears possible.