Near Infrared spectroscopy (NIRS) uses specific, calibrated wavelengths of near infrared light to noninvasively illuminate the tissue below a sensor placed on the skin. These wavelengths of light scatter in the tissue and are absorbed differently dependent on the amount of oxygen attached to hemoglobin in the microcirculation.  Light that is not absorbed is returned as an optical signal and analyzed to produce a ratio of oxygenated hemoglobin to total hemoglobin, expressed as percent StO₂.

In practice, near infrared light penetrates tissues such as skin, bone, muscle and soft tissue where it is absorbed by chromophores (hemoglobin and myoglobin) that have absorption wavelengths in the near infrared region (approximately 700-1000nm). These chromophores vary in their absorbance of NIRS light, depending on changes in oxygenation. Complex algorithms built into the InSpectra™ StO₂ Tissue Oxygenation Monitor differentiate the absorbance contribution of the individual chromophores.

Hutchinson Technology's InSpectra™ StO₂ Tissue Oxygenation Monitor uniquely measures StO₂; a new parameter that measures hemoglobin oxygen saturation in the microcirculation, where oxygen diffuses to tissue cells.

The depth of tissue measured by the InSpectra™ StO₂ Tissue Oxygenation Monitor is directly related to the distance between the illumination fibers and detection fibers: studies have shown that with 15mm sensor spacing, approximately 95% of the detected optical signal is from a depth of 0-14mm.