port: to allow officials to detect weapons – such as plastic explosives strapped to the body – that metal detectors and other security measures might miss.

A potential sticking point is that this machine, known as a backscatter, can see through clothes. Its deployment at the Phoenix airport is a test to see how well it works – and to assess how air travelers respond to its use. If passengers at Terminal 4 who were asked to undergo body scans are an indication, security trumps privacy.

"Sure, I'd be happy to do it," said Ella Adams from Atlanta, who had stopped in Phoenix on Friday to catch a connecting flight to San Diego. "Privacy to me isn't nearly as important as our security, especially if they assure me the X-rays aren't harmful."

In all, the Phoenix airport has seven security checkpoints, and the body- scanning machine has been installed for this pilot program at just one. About 8,000 travelers a day move through this checkpoint, says Paul Armes, security director at Sky Harbor for the federal Transportation Security Administration (TSA).

The backscatter will not be used for everyone who passes this way, at least not at first. A traveler would have to have set off alarms on the routine metal detector, or be randomly selected for further screening. Even then, travelers have two options: the new X-ray machine, or a pat-down, which has caused passenger complaints about invasiveness.

What the backscatter 'sees'

Privacy advocates remain wary of backscatter technology. Barry Steinhardt, director of the technology and liberty program for the American Civil Liberties Union, likens it to a "virtual strip search." He hasn't seen a demonstration of the latest version of the technology, but he saw an earlier one at a Los Angeles city jail.

"The one I saw was very graphic, almost like a nude picture," he says. The technology has not been installed at airports until now because of questions about privacy and how well it can detect possible weapons, adds Mr. Steinhardt. "Utility is ... important here. People are being asked to trade their privacy for security. But first show us there is some security [benefit]."

Backscatter technology has been around for years, says Joe Reiss, vice president of marketing for American Science and Engineering Inc. (AS&E), which makes the backscatter being tested in Phoenix. The technology has been used, for example, to check large cargo containers and passenger vehicles.

"About 15 years ago, this technology was incorporated in personal [body] scanning systems," Mr. Reiss adds. For airport use, AS&E has adjusted the technology so that what's depicted is only an outline of a passenger – with private parts blurred – and any objects on him or her, such as "a handgun, or a blade of a ceramic knife that wouldn't be discovered by a metal detector."

"This technology gives us an additional layer of capabilities to detect objects," says Michael Golden, a technology expert for Southwest Airlines, who is on loan to the TSA. "This actually will show us where prohibited items are on the body," he says, though he declines to name all objects this machine can detect. "We think it is a good balance between security and privacy."

Other officials say the backscatter will be able to detect several items that other security technology – including metal detectors – cannot. Those items include plastic or ceramic knives, plastic explosives, and some liquid explosives. They say it can at best prevent a bomb plot like the one uncovered in London last summer, in which alleged terrorists planned to attack using liquid explosives.

What the screened passenger does

Here's how the screening is working so far. A traveler, after agreeing to the X-ray, is walked through the process by a TSA employee. He or she stands on the red-painted footprints facing the scanner, with arms raised as if a police officer had yelled, "Hands up." Then the traveler turns around for a back pose.

Another TSA employee in a tiny, walled room about 50 feet away assesses the image. A man views images of male passengers, and a woman views images of women. The images are not stored, government officials say, and they aren't transmitted anywhere else. The process should take about 30 seconds.

Kenneth Johnson, a Vietnam veteran with artificial shoulder joints and an artificial knee, was among the first to be pulled out of line Friday for closer screening. He chose the backscatter X-ray over the pat-down, saying he didn't mind it a bit.

As the TSA decides whether to buy the $100,000 machine, its pilot project is expected to last up to 90 days. It may be expanded later this year, using another manufacturer, at Los Angeles International Airport and John F. Kennedy International Airport in New York.

"We want to look at the operational capabilities of this machine and have a dialogue with the public about their perceptions of its use," says Ellen Howe, assistant administrator for the Department of Homeland Security.