Fingerprint recognition compares the ridge pattern of a presented finger with a stored template and scores the similarity. It is a common biometric for door readers because enrollment is quick and readers are familiar. Wet, worn or dirty fingers reduce reads, so a card or code is usually kept as an alternative.
Fingerprint readers are the biometric most people have used. Their convenience depends on enrollment and on the condition of the finger, not only on the sensor.
Templates and matching
The reader does not store a picture of the finger. It derives a template from distinguishing features and compares that with the enrolled template for a claimed identity. Templates cannot be turned back into an image, but they are still personal data and deserve the same protection as any other credential.
Practical enrollment decisions
Enroll in the conditions people will actually use: dry hands, gloves removed, the same finger each time. Harsh soaps, chemicals and abrasion wear prints down, and outdoor readers suffer from cold and moisture. A second credential method keeps the door usable when a print will not read.