A Canadian professor has developed technology that allows advertisers to count the number of people who look at their billboards and screens.

Roel Vertegaal's Xuuk eyebox2 is a $999 portable device with a camera that monitors eye movements and automatically detects when you are looking at it from up to about 35 feet away.

Until now, Vertegaal says, such eye-trackers have been ineffective beyond 2 feet, required people to remain stationary and cost more than $25,000.

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"It can track interest for your advertisers so you can actually have a business model where you sell the ad by the eyeball," said Vertegaal, a professor at Queens University in Kingston, Ontario.

The eyebox2 comes as ads increasingly appear on plasma display panels in shopping malls, restaurants and other public places.

Although Internet ads can be measured by the number of hits on a Web site, it is much harder to assess the ads on plasma screens.

Vertegaal, who has been working on the technology since 1994, says advertisers can now accurately measure how much attention something receives, whether on a plasma panel, a billboard, or as the result of its placement on a supermarket shelf.

Whether a viewer actually makes a purchase is another matter.

Vertegaal said the eyebox2 is being used by advertisers in Britain, but not in the United States yet.

He recently spoke at Google Inc.'s () corporate headquarters in Mountain View, Calif.

Google said in a statement that it often invites interesting people, but such talks shouldn't infer any specific product direction Google is taking.