MILWAUKEE — Watch an advertisement on a video screen in a mall, health club or grocery store and there's a slim — but growing — chance the ad is watching you too.

Small cameras can now be embedded in the screen or hidden around it, tracking who looks at the screen and for how long. The makers of the tracking systems say the software can determine the viewer's gender, approximate age range and, in some cases, ethnicity — and can change the ads accordingly.

That could mean razor ads for men, cosmetics ads for women and video-game ads for teens.

And even if the ads don't shift based on which people are watching, the technology's ability to determine the viewers' demographics is golden for advertisers who want to know how effectively they're reaching their target audience.

While the technology remains in limited use for now, advertising industry analysts say it is finally beginning to live up to its promise. The manufacturers say their systems can accurately determine gender 85 to 90 percent of the time, while accuracy for the other measures continues to be refined.

This technology doesn't identify people individually — it simply categorizes them by outward appearances.

So a video screen might show a motorcycle ad for a group of men, but switch to a minivan ad when women and children join them, said Vicki Rabenou, the chief measurement officer of Tampa, Fla.-based TruMedia Technologies Inc., one of the leaders in developing the technology.

"This is proactive merchandising," Rabenou said. "You're targeting people with smart ads."

Because the tracking industry is still in its infancy, there isn't yet consensus on how to refer to the technology. Some call it face reading, face counting, gaze tracking or, more generally, face-based audience measurement.

Whatever it's called, advertisers are finally ready to try it, said advertising consultant Jack Sullivan, a senior vice president of Starcom USA in Chicago. "I think you're going to see a lot of movement toward it by the end of this year in the top 10 markets," he said.

Because face tracking might feel reminiscent of Big Brother, manufacturers are racing to offer reassurances. When the systems capture an image of who's watching the screen, a computer instantly analyzes it. The systems' manufacturers insist, however, that nothing is ever stored and no identifying information is ever associated with the pictures. That makes the system less intrusive than a surveillance camera that records what it sees, the developers say.

The idea still worries Lee Tien, a senior staff attorney with the Electronic Frontier Foundation, a civil-liberties group in San Francisco. Tien said it's not enough to say some system is "not as bad as some other technology," and argues that cameras that study people help erode privacy.

In general, the tracking systems work like this: A sensor or camera in or near the screen identifies viewers' faces by picking up shapes, colors and the relative speed of movement.

When the ad system pinpoints a face, it compares shapes and patterns to faces already identified in a database as male or female. That lets the system predict gender almost immediately.

Age is trickier: The software can categorize age only in broad ranges — teens, younger to middle-aged folks and seniors. There's moderate demand for ads based on ethnic information, but the companies say determining ethnicity is harder than figuring out gender and age range.