Continuous near-red light spectroscopy for early warning hypoxic/ischemic stress has the great advantage of simplicity and effica a "trend indicator", on the one hand, and time-resolved spectroscopic techniques in the picosecond/nanosecond regions give the exact pathle travelled by the photons, on the other. By time sharing light appropriate choice of wavelengths, the absolute hemoglobin concentration be determined in the arteriolar-venular capillary bed. At the same time changes in deoxyhemoglobin due to hypoxic stress can be quantified. Furthermore, and most important, simultaneous use of continuous light at time-resolved spectroscopy at the some point in the tissue affords calibration of the effective optical path of the continuous light method thereby greatly increasing the utility of the latter. [A simple dual wavelength phase modulation spectrophotometer achieves these goals in a compact, economic, efficient and readily marketable device.