Visible and near infrared (NIR) light interact with biological tissue predominantly by absorption and elastic scattering. There are several physiologically interesting molecules which have characteristic absorption spectra at these wavelengths. In particular, the spectra of oxy-haemoglobin (HbO) and deoxy-haemoglobin (HHb) differ markedly, as do the oxygenation-dependent spectra of cytochrome oxidase. Haemoglobin provides an indicator of blood volume and oxygenation, whereas the cytochrome enzymes indicate tissue oxygenation.