The principle used in identifying intracranial hematomas with Near-infrared spectroscopy (NIRS) is that extravascular blood absorbs Near-infrared light more than intravascular blood since there is a greater (usually 10-fold greater) concentration of hemoglobin in the acute hematoma then in the brain tissue where blood is contained within vessels. Therefore, the absorbance of Near-infrared light is greater (and therefore the reflected light less) on the side of the brain containing a hematoma, than on the uninjured side.