A digital palm print scanner works very much like a fingerprint scanner: it takes a picture of your palm (or finger) and then decides whether the pattern of ridges and valleys in the image matches those in pre-scanned images stored in a networked database. Optical scanners use the same light sensor system used in digital cameras: an array of light-sensitive points that record pixels -- tiny dots that represent light that hits that spot. The light and dark pixels form an image of the scanned palm or finger.
Another type of scanner also generates an image of the ridges and valleys that make up a print, but it does this with electrical current instead of light. This type is more compact than optical devices, and harder to trick because it requires an actual finger- or palm-print shape to produce an image, instead of relying on a pattern of light and dark.
TV programs often show detectives analyzing fingerprints by laying one image over another to find a match, but prints are prone to smudging, which can make it difficult to get a perfect image overlay. This is why most scanners focus on specific features, called minutiae: points where the ridge lines end, or where one ridge splits into two. The scanner's software uses complex algorithms to recognize these features, much like we would recognize a part of the sky by the relative positions of stars. For example, if two prints have three ridge endings and two ridge splits, forming the same shape of about the same size, they are most likely from the same palm or finger.
Fingerprinting is valuable in law enforcement, but mistakes can happen, and it is possible for a clever person to trick a scanner. Optical scanners sometimes can't tell the difference between a picture of a finger and the real thing, while electronic scanners can be fooled by a mold of a person's finger. A popular motif in movies is for criminals to cut off a finger to get past a security system. Some scanners have pulse and heat sensors that can tell if a finger is living or dismembered, but even these can be fooled by placing a gelatin print mold over a real finger.