Near-Infrared (IR) light can penetrate tissue several centimeters; bone tissue like scull is even transparent. In a NIRS device, a sending optode sends infrared light into the tissue where it is scattered around and some will be catched by one or more receiving optodes. The amount received will depend on tissue composition; most important, blood amount and its oxygen content.
The dispersion of IR in tissue is mostly modelled as a type of diffusion. Such models allow to learn which portion of the tissue has the most influence on the received signal, so selected portions can be monitored by matching optode(s) position.
The method has advantages and disadvantages. It is convenient and completely non-invasive so can be applied with seriously ill patients or pre-term children. It is very fast, in the millisecond range. But it is not precise so that imaging does not yield sharp boundaries.