See FBI Appendix F specifications in
500 pixels per inch or 1000 ppi at 8 bits per pixel. Capture size 1.6" x 1.5" (600 Kpixels)
for roll finger or 1" x 2" for thumb (500 Kpixels).
But once you threshold the images, you effectively get rather less than 1 bit per pixel, as
there's a lot of correlation between pixels. Also rotations all count the same. My fingers
have more like 50 ridges per inch. But that's still a *lot* of possible values.
After extracting the minutiae, there's rather less information held. One finger reader I have
states the software extracts between 10 and 70 minutiae points, held as (x,y) vectors, in a
transform claimed to be non-reversible. If coordinates are accurate to 6 bits, that means 10 x
(6+6) bits = 120 bits minimum. Still allows for significantly more possible prints than the
See also Sir James Crosby's report,
http://www.hm-treasury.gov.uk/media/6/7/identity_assuranc..., suggesting that only
non-unique digital representations should be stored. This would allow the master copy in the
database to be replaced with another version, so would provide some limited options to
"change" a compromised fingerprint.