Abstract: Oxygenation of the retina is determined by measuring the fluorescence of flavoprotein in the retina. A spot of excitation light of a frequency of about 450 nanometers is scanned across the retina. Fluorescent light emitted from the retina at a frequency of about 520 nanometers is detected. The emission light may be detected at two frequencies of about 520 nm and 540 nm to allow for compensation for absorption and transmission variables in the eye. To compensate for fluoresence of the lens of the eye, the center of the pupil is imaged onto scanning mirrors so that the scanning beam of excitation light pivots at the center of the eye lens. Further, the center of the lens is imaged onto an optical stop in the emission optical path so that emitted light which passes back through the lens periphery, and not fluorescent light from the center of the lens, is detected. The emission light detector is a photon counting photomultiplier tube to permit safe limits on the intensity of the excitation light.