Because facial recognition can be used covertly to identify and track people by name at a distance, some civil liberties experts call it unequivocally intrusive. In view of intelligence documents made public by Edward J. Snowden, they also warn that once companies get access to such data, the government could, too. “This is you as an individual being monitored over time and your movements and habits being recorded,” says Christopher Calabrese, legislative counsel for privacy issues at the American Civil Liberties Union. “That is a very scary technological reality.”

For the technology to work, a company or government agency must create a database containing photos or video stills of individuals. Next, a typical system extracts complex measurements — often topological — of each face. Then it converts each person’s facial data into a mathematical code, or “faceprint.” If security cameras record someone at, say, a store or a casino, the system can compare the faceprint of that live image to those in the database, taking only a few seconds to run through millions of faceprints and find a match.

Some international airports use the technology to identify employees as well as frequent fliers who have been cleared by government security services. Facebook offers face-matching software, called “Tag Suggestions,” to automatically suggest to members the names of people in photos they’ve uploaded. Google said last year that it would not approve “at this time” apps for Google Glass that use facial recognition.

Now retailers and marketers are weighing the possible ramifications of facial recognition and the practices they may need to employ it securely and ethically.

Mr. Rosenkrantz of FaceFirst argues that its current shoplifter-recognition service is less intrusive than typical in-store video security systems. Video cameras capture everyone who walks into a store and the images are usually kept for 30 days, he says, whereas FaceFirst destroys faceprints of all consumers except those whom retailers have previously caught shoplifting.

“We purposely do not store information on people not being looked for,” he says.

Yet Joseph Atick, a pioneer in facial recognition, views the technology as much more powerful than current consumer-tracking tools. Taken in context with trends like the near ubiquity of cellphone cameras and the proliferation of people who are identified by name in online photos, he says, facial recognition may soon let companies link a person’s online persona with his or her actual offline self at a specific public location. That could seriously threaten our ability to remain anonymous in public.

“I don’t think there has ever been a capability that converged in this way to give people power over you,” Mr. Atick says.