Don't look now, but the advertisements are watching you.

No, it's not a scene from the movie "Minority Report" where digital signboards served up personalized ads depending on who was passing by, but a real-life example at a Tokyo railway station. Above a flat-panel display hawking DVDs and books sits a small camera hooked up to some image processing software.

When trials begin in January the camera will scan travellers to see how many of them are taking note of the panel. It's part of a technology test being run by NTT Communications.

"On many street corners and railway stations there are many digital signs," said Tetsuya Kinebuchi, a senior research engineer at NTT's Cyber Space Laboratories and developer of the system. "To automatically measure the effectiveness of the advertisements we can put a camera and PC nearby, and by using the image from the camera we can estimate how many people are looking at the monitor."

Japanese cities are plastered with advertisements. From building-topping billboards to smaller ads around town, it seems like you're never far away from a commercial message, and increasingly these are digital signboards. The effectiveness of delivering a message digitally is still not well understood but that could change with this technology.

The system has its limits. It doesn't seek to identify individuals -- NTT is worried about the negative implications of such a system -- but it will attempt to figure out how many of the people standing in front of an advertisement are actually looking at it.

"It uses image detection software," said Kinebuchi. "We gathered together many faces and came up with an average Japanese face, and by using pattern matching the system recognizes faces from the image."

A second camera, which wasn't fitted at the station but will be when tests begin in January, will take care of estimating how many people are in front of the ad, whether they are looking at it or not.

NTT is Japan's largest telecommunications company and its interest in the system goes beyond the technology. The company has a content distribution system for digital signs and the work will help gather data that it could use to sell such a service and promote digital advertising in general.