The agency in charge of the nation’s air security expects later this year to begin using a controversial X-ray machine that will show screeners a clear picture of what’s under passengers’ clothes – whether weapons or just bare skin.

Screeners plan to test the “backscatter” machines at several U.S. airports, the Transportation Security Administration says. The refrigerator-sized machines are considered a breakthrough in scanning technology but have been labeled “a virtual strip search” by the American Civil Liberties Union.

Security workers using the machines can see through clothes and peer at whatever may be hidden in undergarments, shirts or pants. The images also paint a revealing picture of a person’s nude body.

The devices can potentially be used to screen hundreds of millions of air travelers each year, although TSA says more study is needed to determine how the devices may be used at U.S. airports. The agency declined to say when and where it expects to test the machines.

The machines are already used by U.S. Customs agents at 12 airports, and they’re also getting a test run at a terminal in London Heathrow Airport.

Security consultant Douglas Laird says the machines are essential to spot explosives, which aren’t detected by metal detectors.