Absolute SO2 measurements in layered media.

Until recently, tissue near infrared spectroscopy has been largely restricted to measuring quantified changes in tissue oxygenation. This limitation arises because the multiple light scattering properties of most biological tissues renders the usual Beer-Lambert law inapplicable and hence prevents a straightforward conversion between measured light attenuation and the intrinsic absorption coefficient of the medium μ a . Techniques are now being proposed which overcome this limitation and allow an absolute determination of blood oxygenation to be made. Broadly these methods can be categorised as:-