The airport here became the first in the United States to test new X-ray technology that can see through people's clothes with blush-inducing clarity. Critics have said the high-resolution images created by the "backscatter" technology are too invasive. But the Transportation Security Administration adjusted the equipment to make the image look something like a line drawing, while still detecting concealed weapons. Similar machines could be at the Los Angeles airport and New York's Kennedy Airport by the end of the year. During the testing, which started Friday, the machine will be used only as a backup measure. Passengers who fail the standard screening with a metal detector will be able to choose between the new device or a pat-down search. "It's 100 percent voluntary, so if the passenger doesn't feel comfortable with it, the passenger doesn't have to go through it," TSA spokesman Nico Melendez said.