The U.S. Transportation Security Administration today promised to protect air travelers' privacy as TSA personnel peer through their clothes.

The TSA has begun testing a millimeter wave scanning system at Phoenix Sky Harbor Airport as an alternative to pat-downs performed by security personnel when secondary screening is deemed appropriate. The technology can see through clothing to detect weapons, explosives, and other objects.

The TSA said that energy emitted by millimeter wave technology -- 10,000 times less than a cell phone -- is safe, that the technology is intended to keep passengers safe, and that it will keep the potentially embarrassing images safe.

"We are committed to testing technologies that improve security while protecting passenger privacy," said TSA administrator Kip Hawley in a statement. "Privacy is ensured through the anonymity of the image: It will never be stored, transmitted, or printed, and it will be deleted immediately once viewed."

Ensuring privacy, as the TSA describes it, involves having security officers view images from remote locations. Thus, the security officer cannot identify the passenger, visually or by some other means, but can send word to fellow officers if a threat is detected.

According to the TSA, the scanning system applies a security algorithm to further protect passenger privacy by obscuring the passenger's face.

Not everyone finds such assurances credible. In a statement, Barry Steinhardt, director of the ACLU's technology and liberty program, spelled out three objections to the TSA's plans.

"First, this technology produces strikingly graphic images of passengers' bodies," Steinhardt said. "Those images reveal not only our private body parts, but also intimate medical details like colostomy bags. That degree of examination amounts to a significant -- and for some people humiliating -- assault on the essential dignity of passengers that citizens in a free nation should not have to tolerate."

Steinhardt also expressed doubt that such screenings could really be considered voluntary if passengers did not understand the invasiveness of the images and that the program would remain voluntary in the future. Finally, he voiced skepticism of the TSA's privacy safeguards. "They say that they are obscuring faces, but that is just a software fix that can be undone as easily as it is applied," he said. "And obscuring faces does not hide the fact that rest of the body will be vividly displayed."

Such concerns may not be shared by the majority of the public. The TSA says that since February, when it began testing backscatter scanning -- a similar technology -- in Phoenix, some 79% of those selected for secondary screening opted to submit to a backscatter scan rather than a pat-down.

The TSA plans to perform further testing of these systems at New York's John F. Kennedy International Airport and at Los Angeles International Airport. The security agency plans to purchase eight millimeter wave units for a total of $1.7 million.

Millimeter wave scanners trials are currently being conducted at airports in Japan, the Netherlands, Thailand, and the United Kingdom.