ARLINGTON , Virginia ( CNN ) -- A new type of walk-through security machine will debut at several U.S. airports in the coming days as the Transportation Security Administration tries out the latest in body scanning technology . Privacy experts with the ACLU are concerned the walk-through scanning machine is too invasive . It 's called `` millimeter-wave passenger imaging technology , '' and it produces a more detailed picture than the metal detectors in use now at airports to screen for weapons and explosives.. Because it produces such a detailed image , however , technology and privacy experts at the American Civil Liberties Union are not satisfied that the new technology meets privacy standards . In a written statement issued Thursday , Barry Steinhardt of the ACLU said the technology can pick up graphic body images and even medical details like whether a passenger has a colostomy bag . Steinhardt called the screening an `` assault on the essential dignity of passengers that citizens in a free nation should not have to tolerate . '' TSA spokeswoman Elle Howe said privacy will be respected with the new millimeter-wave technique . `` We want to preserve passengers ' privacy and make them feel comfortable with trying a technology like this , '' she said . A TSA officer will escort a passenger to the machine for the test , but the person looking at the actual body scans will be at a different location and will not see the passenger , the TSA said . In addition , the scans will have a `` modesty filter '' to blur out faces , and no images will be saved . But the ACLU expressed concern that TSA officers would not be able to resist the temptation to save images of certain people , such as celebrities , and that the plan to blur faces might later be changed . This is how the new scanners work . The passenger steps into a machine where he or she is quickly scanned with radio waves . Those waves reflect off the body to transmit a three-dimensional image of the passenger that looks like a fuzzy photo negative . A TSA officer studies the image on a screen and looks for unusual shapes that might mean a passenger is carrying something suspicious . Passengers who are asked to undergo a second screening can choose a pat-down search or the millimeter-wave test . The TSA says the machines scan a passenger twice , each scan taking less than two seconds . But it takes another minute or two for a screener to review the scans before signaling a passenger to move on . The TSA demonstrated the screening technology at a news conference Wednesday near Washington . Howe said the millimeter wave is harmless and `` can see more than a magnetometer , '' which is the first screening machine airport passengers encounter . `` A magnetometer only picks up metal or weapons , so this could see other materials that might be hidden on the body and it also produces an image '' rather than just a beep , she said . Asked if the millimeter wave could detect an object hidden in a body cavity , she said only that the TSA will learn more about the technology as it 's tested at U.S. airports . The TSA has been testing another type of imaging technology called backscatter . This technology also came under some fire because it shows very detailed body images -- which led some critics to call it an electronic strip search . So , the backscatter was altered and blurred to show more of an outline of the body . The TSA will continue to test the backscatter scanners in some airports . TSA officials said they are a long way from deciding whether they 'll settle on just one of these imaging technologies . Phoenix Sky Harbor Airport in Phoenix , Arizona , begins using the new machines Thursday -- to be offered as an option for people who are asked to be screened a second time . Los Angeles International Airport in California and John Fitzgerald Kennedy International Airport in New York are also slated to try the machines . Britain , Spain , Japan , Australia , Mexico , Thailand and the Netherlands are using the millimeter wave screening . In the United States , some courthouses and jails are trying it . E-mail to a friend .
NEW : ACLU says machines ' detailed images violate privacy , assault dignity . New machines use radio waves to scan passengers for hidden objects . They 'll be used on passengers selected for secondary screening . TSA says blurred image , remote location of scanner protect privacy .