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Continuous wave Near Infrared Spectroscopy is a well known non invasive technique for measuring changes in tissue
oxygenation. Absorption changes (&Dgr;O2Hb and &Dgr;HHb) are calculated from the light attenuations using the modified
Lambert Beer equation. Generally, the concentration changes are calculated relative to the concentration at a starting
point in time (delta time method). It is also possible, under certain assumptions, to calculate the concentrations by
subtracting the equations at different wavelengths (delta wavelength method). We derived a new algorithm and will
show the possibilities and limitations. In the delta wavelength method, the assumption is that the oxygen independent
attenuation term will be eliminated from the formula even if its value changes in time, we verified the results with the
classical delta time method using extinction coefficients from different literature sources for the wavelengths 767nm,
850nm and 905nm. The different methods of calculating concentration changes were applied to the data collected from
animal experiments. The animals (lambs) were in a stable normoxic condition; stepwise they were made hypoxic and
thereafter they returned to normoxic condition. The two algorithms were also applied for measuring two dimensional
blood oxygen saturation changes in human skin tissue. The different oxygen saturation levels were induced by
alterations in the respiration and by temporary arm clamping. The new delta wavelength method yielded in a steady
state measurement the same changes in oxy and deoxy hemoglobin as the classical delta time method. The advantage of
the new method is the independence of eventual variation of the oxygen independent attenuations in time.
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