The federal government will begin testing a body-scanning machine that could eventually be used instead of the metal detectors passengers walk through at airports.

Tests were scheduled to begin Thursday at Phoenix Sky Harbor International Airport with passengers pulled out of the security line for secondary screening.

Passengers may request the full-body scan — which blurs faces so the person being screened cannot be recognized — instead of the traditional pat-down used across the country.

The new machine uses radio waves to detect foreign objects.

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Since February, the Phoenix airport has been testing a similar machine that uses so-called backscatter radiation to scan the entire body. The backscatter uses a narrow, low-intensity x-ray beam that scans the entire body at a high speed.

The amount of radiation used during the backscatter scan is equal to 15 minutes of exposure to natural background radiation such as the sun's rays.

Officials are trying to determine if the body-scan machines are a more effective search tool than a pat-down.

Both types of machines check for explosives, metal, plastic and liquids — anything hidden on the body, said Mike Golden, the Transportation Security Administration's chief technology officer.

The new type of device being tested, called a "millimeter-wave" machine, doesn't use harmful radiation, Golden said Wednesday during a demonstration for reporters at the agency's headquarters in Arlington, Va.

Instead, it uses radio-frequency waves to create an image based on energy reflected from the body.

The millimeter-wave machine works like this: A person walks into a large portal — nearly 9 feet tall and 6 feet wide — pauses and lifts his arms while the machine takes two scans using radio waves.

The scans take 1.8 seconds, and it takes about a minute for the image to appear on a computer screen in a separate location.

To protect privacy, the image will be shown on screens in a completely different area than where the screening is taking place. The TSA officer doing the screening will never see the computer image, and images will not be saved, TSA spokeswoman Ellen Howe said.

Reporters were only shown an example of a female body image, which was a three-dimensional image of a very fit woman in her brassiere and underwear. TSA describes this as similar to a "fuzzy photo negative."

Privacy advocates say the images are more graphic than that.

"If you want to see a naked body, this is a naked body," said Barry Steinhardt, director of the American Civil Liberties Union's program on technology and liberty.

Steinhardt also received a demonstration of the new machine, which he says shows the same graphic image as the backscatters.

"I continue to believe that these are virtual strip searches," Steinhardt said. "If Playboy published them, there would be politicians out there saying they're pornographic."

Amsterdam Schiphol Airport and the Alexandria, Va., federal courthouse use the millimeter-wave machines, TSA said.

TSA purchased eight of the millimeter-wave machines, which cost between $100,000 and $120,000, and is considering deploying them at John F. Kennedy and Los Angeles international airports during the testing period.

The results of the testing will determine whether to use these machines for primary screening, Howe said.