Airports in New York and Los Angeles have become the latest equipped with body scanners that allow security screeners to peer beneath a passenger's clothing to detect concealed weapons.
The machines, which are about the size of a revolving door, use low-energy electromagnetic waves to produce a computerized image of a traveler's entire body.
Passengers step in and lift their arms. The scans only take a minute, and Transportation Security Administration officials say the procedure is less invasive than a physical frisk for knives, bombs or guns.
Someday, the "millimeter wave" scans might replace metal detectors, but for now they are being used selectively.
Los Angeles International Airport and John F. Kennedy International Airport in New York saw their first scanners installed Thursday, each at a single checkpoint. Phoenix Sky-Harbor International Airport got one of the machines in October.
Modest travelers may have concerns about the images.
The black and white, three-dimensional scans aren't as vivid as a photograph, but they do reveal some of the more intimate curves of the human form, maybe with as much clarity as an impressionist sculpture by Auguste Rodin.
TSA officials say the system comes with privacy protections. Officers reviewing the images don't interact with passengers, or even see them. They sit in a separate area, look at the pictures on a monitor and push a button to either clear travelers or alert security about a suspicious item.
Images will not be recorded or stored. Passenger faces are blurred to further protect their identities.
For now, the scans will also be voluntary. Flyers selected for a secondary screening after passing through the metal detectors will have the option of stepping into the wave scanner, rather than undergoing a physical pat-down.
"We're giving people a choice," said TSA spokeswoman Lara Uselding.
Barry Steinhardt, director of the American Civil Liberties Union's program on technology and liberty, said he nevertheless had concerns.
"The images that I've seen are quite revealing," he said. "I guarantee you that as this gets more commonly used, you'll be seeing these images on the Internet."
The TSA said millimeter wave scanners, which cost as much as $120,000 apiece, are already in limited use at international airports in seven countries and at a handful of courthouses and jails in five states.
Their introduction to U.S. airports is on a trial basis while authorities evaluate their effectiveness. The TSA said the devices pose no health risk and project 10,000 times less energy than a cell phone transmission.