Discovery of the near-infrared window into the body and the early development of near-infrared spectroscopy.

Extension of optical monitoring of intact tissues from the visible and ultraviolet to the near-infrared (NIR) range (700-1300 nm) was first undertaken in 1977 for the purpose of monitoring the redox behavior of Cytochrome c oxidase (cyt c ox) in vivo. Soon it became evident that the much greater NIR translucency of skin and bone made it possible to reach brain and muscle tissue without surgical intervention. The presence of hemoglobin absorption led to complications forcing the construction of algorithms to separate the signals of the two molecular entities. It was also realized, however, that the hemoglobin signals provide information regarding the source of oxygen in the tissue, while the cyt c ox signals indicate the intracellular availability of oxygen for oxidative phosphorylation. This ability of recognizing the source/sink relationship greatly enhances the value of NIR spectrophotometry (NIRS) for research and clinical purposes. © 1999 Society of Photo-Optical Instrumentation Engineers.