"Vascular Pattern Recognition, also commonly referred to as Vein Pattern Authentication, is a fairly new biometric in terms of installed systems. Using near-infrared light, reflected or transmitted images of blood vessels of a hand or finger are derived and used for personal recognition. Different vendors use different parts of the hand, palms, or fingers, but rely on a similar methodology. Researchers have determined that the vascular pattern of the human body is unique to a specific individual and does not change as people age."
Biometrics Catalog: http://www.biometricscatalog.org