Hand geometry measures the shape and proportions of a hand, not a fingerprint. A reader images the hand, converts the measurements into a small numeric template, and compares it at the door. It is quick and tolerant of dirty hands, but less distinctive than fingerprint or iris recognition.
Hand readers suit sites where people wear gloves or work with their hands, because the template does not depend on a clean fingertip. The trade is a coarser identity check.
What the reader captures
A guide positions the hand and a camera records its silhouette and the length and width of the fingers. The result is a template of numbers, not an image of the hand.
Where it fits and where it does not
It works well for a known workforce at a staff entrance. It is a poor fit for a large population with no enrollment time, or for doors that need very high assurance.