Grant Evans beams a smile as he ponders the potential of his company's 3-D face-recognition technology.

The upbeat chief executive of A4Vision sees huge sales of cameras, hardware and software on the horizon as the Sunnyvale startup joins the global push to boost security since the Sept. 11 terror attacks.

Face scanning has emerged as one of the hottest "biometric" methods for identifying people. The goal is to use unique body parts -- such as fingerprints, irises or face or hand patterns -- to help identity someone as friend or suspected foe.

"What I say is, 'It's tough to leave home without your face,' " Evans said.

Evans and others selling face-scanning gear say their systems are merely computers that mimic the natural way humans recognize one another -- by faces.

Reality, though, is more complex and controversial.

Privacy advocates fear a day when police could monitor someone almost constantly through a network of security cameras.

The potential applications are many: A soldier gazes into a camera mounted to a door to a restricted building on a base. The computer searches through a database of those allowed to enter, and clicks open the door once the familiar face is found. Or, travelers at a border crossing line up to look into the camera to prove they are the person listed on their passport.

Organizations from the U.S. Defense Department to the airport in Lyon, France, are testing or using new 3-D face-recognition technology. But older 2-D face-recognition technology, sold by biometric leaders like Identix of Minnesota, is still the norm. Current international technology standards -- key to lucrative deals for passports or other government programs -- support 2-D systems.