The technology exists to reveal objects hidden under clothes at airport checkpoints, and many experts say it would have detected the explosive packet carried aboard the Detroit-bound flight last week. But it has been fought by privacy advocates who say it is too intrusive, leading to a newly intensified debate over the limits of security.

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“If they’d been deployed, this would pick up this kind of device,” Michael Chertoff, the former homeland security secretary, said in an interview, referring to the packet of chemicals hidden in the underwear of the Nigerian man who federal officials say tried to blow up the Northwest Airlines flight.
But others say that the technology is no security panacea, and that its use should be carefully controlled because of the risks to privacy, including the potential for its ghostly naked images to show up on the Internet.

“The big question to our country is how to balance the need for personal privacy with the safety and security needs of our country,” said Representative Jason Chaffetz, a Utah Republican who sponsored a successful measure in the House this year to require that the devices be used only as a secondary screening method and to set punishments for government employees who copy or share images. (The bill has not passed in the Senate.)
“I’m on an airplane every three or four days; I want that plane to be as safe and secure as possible,” Mr. Chaffetz said. However, he added, “I don’t think anybody needs to see my 8-year-old naked in order to secure that airplane.”

Full-body imaging machines are in use in 19 airports in the United States and are being used as the primary method of screening at six. Earlier this year the Transportation Security Administration announced plans to buy 150 more machines and to use the scanners as the primary screening method for air passengers.
That prompted a letter of protest from a coalition of 24 privacy organizations to Homeland Security Secretary Janet Napolitano, including the Electronic Privacy Information Center and the American Civil Liberties Union.
“Your agency will be capturing the naked photographs of millions of American air travelers suspected of no wrongdoing,” the letter said.

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Depending on the specific technology used, faces might be obscured or bodies reduced to the equivalent of a chalk outline. Also, the person reviewing the images must be in a separate room and cannot see who is entering the scanner. The machines have been modified to make it impossible to store the images, Ms. Lee said, and the procedure “is always optional to all passengers.” Anyone who refuses to be scanned “will receive an equivalent screening”: a full pat-down.

Since the Christmas Day bombing attempt, supporters of tighter security have raised their voices in criticism of privacy advocates. “I do think the privacy groups have some explaining to do,” said Stewart A. Baker, a former homeland security official in the administration of President George W. Bush.

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