"Let's say a digital camera is taking a picture of someone's face. So now it gets represented in computers as a bunch of numbers," he said. "Humans have no problem [saying] that's someone I know. The computer has to look at those numbers and say, 'Are these the same set of numbers corresponding to a person I took a photo of a year ago?'"
It's a complex process, and it is not flawless. For computers, those numbers representing human features can change based on the person's expression, lighting and overall quality of the image, according to Bhagavatula.
To combat this, researchers are constantly looking for new algorithms to analyze facial features. Currently, many researchers are looking at features that don't change, such as the distance between the eyes, the angle made by the tip of the nose or the length of an eyebrow, he said.
"Many methods try to capture these kinds of things that are unique to people's faces," he said. "You hope that these numbers stay the same when a person smiles or frowns."
The kind of monitoring that would enable facial recognition to work well has not caught on in the United States, at least not yet, according to Paul Saffo, a technology forecaster in Silicon Valley.
"The English have always had a slightly different attitude toward privacy," Saffo said. "They've never had a strong a privacy culture as America has had. The English do not have a constitution. Their protections are in common law. It is easier for the government to overstep notions of privacy than it would be here, because you have people invoking the Bill of Rights."
But Saffo believes that given the right crisis, the United States would eventually accept the technology.
"Do not underestimate the psychic shock of the London subway bombings," he said. "We bleat and cry about privacy, but we happily surrender our privacy for the cheapest of coin."
So far, most legislative pushes for video monitoring by city governments have been thwarted.
This week in Washington, D.C., a bill pushed by the city's mayor calling for nearly $1 million in funding for citywide public cameras was voted down by the city council.
"People sometimes talk about video surveillance systems as moving forward inexorably in the United States, but we've seen quite a few successful protests," said Mark Rotenberg, the director of the Electronic Privacy Information Center. "I think there are a lot of questions that need to be asked about video surveillance. The most obvious one is: what is the purpose?"
"[Britains] have embraced a really extraordinary amount of monitoring by the government that I don't think the U.S. would accept," he said.
Some critics also take issue with the accuracy (or lack thereof) of facial recognition technology.
In perfect conditions, facial recognition can be fairly effective, according to experts, but in less than perfect conditions it can be wildly inaccurate. For example, it is difficult for a computer to identify a person who is walking on a city street or in an airport where his face might be blurred, obscured or shadowed.
"We have gotten a long way from where we were 10 years ago," says Carnegie Mellon's Bhagavatula. "But good algorithms have an 80 percent accept rate. It's pretty good, but not perfect."