Retinal scanning is the most accurate method, but is being rejected for all but very high security situations because it requires the user to stand within six inches or so of the scanner while it reads the blood vessels in the back of the eye. Reading fingerprints is nonintrusive, but to some it smacks of a police blotter. Most but not all systems detect whether the image being measured is actually a living person or a photo, plastic replica, or some other attempt to spoof the system. A too-high FAR means that the bar to access is set so low that the system accepts users it should have rejected. A too-high FRR rejects users it should have accepted.
Systems generate scores that in effect set the bar at whatever height the host chooses.
This could be different for different users and different applications. However, Bowman of Identix says good user training is the best way to reduce false rejections, which usually drop after two weeks of use. Voice sensors can be microphones in an ATM kiosk or an ordinary telephone. Signature-based systems generally require the user to sign her name with an electronic pen on an input tablet, both of which can also be used for other applications. These systems claim the added advantage of recording details that can be used as legal evidence of intention to make the commitment.