Advertisers have spent a fortune trying to measure how we see their ads, when we see them, and who we are. Online it is easier than in television and print.

Now, some entrepreneurs have introduced technology to equip billboards with tiny cameras that gather details about passers-by — their gender, approximate age and how long they looked at the billboard. These details are transmitted to a central database.

The goal is to tailor a digital display to the person standing in front of it — to show one advertisement to a middle-aged white woman, for example, and a different one to a teenage Asian boy.

The technology has been used in Ikea stores in Europe and McDonald’s restaurants in Singapore, but it has just come to the United States. Within advertising circles, these camera systems are seen as a welcome answer to the longstanding problem of how to measure the effectiveness of billboards, and how to figure out what audience is seeing them. On television, Nielsen ratings help marketers determine where and when commercials should run, for example. As for signs on highways, marketers tend to use traffic figures from the Transportation Department; for pedestrian billboards, they might hire someone to stand nearby and count people as they walk by.

Although surveillance cameras have become commonplace in banks, stores and office buildings, their presence takes on a different meaning when they are meant to sell products rather than fight crime. So while the billboard technology may solve a problem for advertisers, it may also stumble over issues of public acceptance.