WASHINGTON - 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.

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's scans the entire body at a high speed. The amount of radiation used during this 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 radiation, Golden said Wednesday during a demonstration for reporters at the agency's headquarters in Arlington, Va. Instead, it uses electromagnetic waves to create an image based on energy reflected from the body.

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