Eye scans are of two types, i.e., iris and retinal. Iris scans process, record, and compare the light and dark patterns in the iris' flecks and rings. Retinal scans, which are more intrusive than iris scans, require close-up infrared scanning through the pupil. Biometrics that involves distinctive facial features includes video or photo imaging; thermography, which reads the heat pattern around the eyes and cheeks; and scanning the dimensions of an individual's head. This type of biometrics is not as accurate as a fingerprint. Hand geometry as a means of identification involves scanning the outline or the shape of a shadow of a hand; too many people have similar hand shapes and sizes for this system to be dependable in situations that require tight security. Voice recognition has been used at border crossings to verify that the person crossing the border is the person already enrolled in the voice database. Voice recognition techniques, however, are susceptible to background noise and may not be accurate if the speaker has a cold. Handwriting and signature identification involve the scanning of signatures written on an electronic pad to measure the speed, pressure, and direction of the strokes. Given the highly unlikely occurrence of two individuals having the same DNA, an individual's DNA can be matched to an existing DNA database to determine identity, but only if the database contains a previously obtained DNA sample of the individual.