PHOENIX (Reuters) - U.S. authorities began testing a controversial new X-ray machine to screen air passengers for weapons in Phoenix on Friday, which critics likened to a “virtual strip search.”

An airport employee checks a passenger's boarding passes as he enters a security checkpoint in terminal 4 at Phoenix Sky Harbor International Airport in Phoenix, Arizona November 22, 2006. U.S. authorities began testing a controversial new X-ray machine to screen air passengers for weapons in Phoenix on Friday, which critics likened to a "virtual strip search." REUTERS/Jeff Topping

The U.S. Transportation Security Administration rolled out an X-ray machine that uses so-called “backscatter” technology at one checkpoint at Phoenix Sky Harbor International Airport. The machine peers beneath passengers’ clothes to search for hidden explosives and weapons.

The TSA will test the machine in Phoenix for 60-90 days before deploying machines in Los Angeles and New York’s John F. Kennedy Airport for additional testing by the end of the year.

“Everyday the bad guys are working and improving their tools. We need to continue working to improve ours and introducing this technology is part of that work,” TSA regional spokesman Nico Melendez told Reuters.

Privacy groups and the American Civil Liberties Union have labeled the new screening a “virtual strip search” that could be abused.

But TSA officials said Friday they had worked with industry specialists to blur any images of body parts generated by the scan, and likened the resulting picture to a “chalk outline” of a person.

The machine is made by Boston-based American Science and Engineering Inc., and is on loan for the trial.

Passengers selected for secondary testing at the airport, which is the nation’s eighth busiest, can choose an X-ray scan or a pat-down search. It is strictly voluntary.

Airline passengers who choose to use the new machines stand in front of it with their arms in the air. A tiny laser beam scans the passenger from head to toe.

The images come up on a computer screen in a room about 50 feet away. A green or red light -- for pass or fail -- is pressed and shows up at the screening location.

Officials said the computer does not have the ability to save or store images, a concern expressed by privacy-rights groups.

“It gives our officers the absolute best opportunity to find prohibited items,” said Michael Golden, the TSA’s chief technology officer.

Airport travelers had mixed opinions about the new device, saying they hope it doesn’t slow down the process of getting through security. Few had any privacy concerns.

“If it’s something that’s going to improve safety, then I don’t have any problem with it,” said Ashley Houston, 32, as she waited for a plane to Albuquerque. “I have nothing to hide.”