Fig. 1. Principles of cerebral oximetry. (A ) Cerebral oximeter probe contains a light source and detector. Cerebral O2saturation is related to light intensity at the source (Io), detector (I), and light pathlength (L) through the tissue according to the Beer Law (equation 1). (B ) In frequency domain cerebral oximetry, the intensity of the light source (Io) is oscillated at high frequency. After the light passes through the tissue, the detected intensity (I) is amplitude demodulated and phase shifted. Cerebral O2saturation is related to amplitude and phase, corresponding to intensity and pathlength in the Beer Law.