PHOENIX - The Phoenix airport on Friday became the first in the United States to test new X-ray technology that can see through people's clothes and show the body's contours with blush-inducing clarity.

A TSA officer looks over the image taken from an X-ray security scanner in a demonstration of the device at Sky Harbor International Airport, Friday, Feb. 23, 2007, in Phoenix, Arize. (AP Photo)

Critics have said the high-resolution images created by the "backscatter" technology are too invasive. But the Transportation Security Administration adjusted the equipment so the pictures can be blurred in certain areas while still detecting concealed weapons.

During the testing, the machine will be used only as a back-up screening measure. Passengers who fail the standard screening with a metal detector will be able to choose between the new device or a pat-down search.

"It's 100 percent voluntary, so if the passenger doesn't feel comfortable with it, the passenger doesn't have to go through it," TSA spokesman Nico Melendez said.

Passengers selected for screening by the device are asked to stand in front of the closet-size X-ray unit with the palms of their hands facing out. Then they must turn around for a second screening from behind. The procedure takes about a minute.

Passenger Kristen Rodgers, 22, of Little Rock, Ark., who did not go through the screening, likened it to going to the doctor.

"If you tell yourself they have to look at that all day long, it makes yourself feel better," Rodgers said. "If it's just for security, just for 45 seconds, I think it would be worth catching somebody with something harmful."

The machine will be tested for up to 90 days at a single checkpoint at Sky Harbor's largest terminal, which hosts US Airways and Southwest Airlines, the two busiest airlines in Phoenix.

The technology could be left in place after the trial period, and the TSA hopes to roll out similar machines at the Los Angeles airport and New York's Kennedy Airport by the end of the year.

The security officer who works with the passenger going through the screening will never see the images the machine produces. The pictures will be viewed by another officer about 50 feet away who will not see the passenger, the TSA said.

The machine cannot store the images or transmit them.

"Once we're done screening the passenger, the image is gone forever," Melendez said.

The device at Sky Harbor costs about $100,000 but is on loan from the manufacturer, AS&E of Boston, Melendez said.