Near-infrared range and has been used in newborn infants since 1985. Fibre-optic bundles or optodes are placed either on opposite sides of the tissue being interrogated (usually a limb or the head of a young baby) to measure transmitted light, or close together to measure reflected light. Light enters through one optode and a fraction of the photons is captured by a second optode and conveyed to the measuring device. NIRS uses a frequency band between 650 nm and 1000 nm and relies on three important phenomena: (1) human tissue is relatively transparent to light in the near-infrared region of the spectrum; (2) pigmented compounds known as chromophores absorb light as it passes through biological tissue; and (3) human tissues contain substances whose absorption spectra at near-infrared wavelengths are well defined and depend on their oxygenation status.