LAS VEGAS — Federal aviation officials are giving airport scanners another try. This time, they are not looking as closely under traveler’s clothes.

The Transportation Security Administration on Tuesday began testing a new, more modest body scanning system at three airports. They hope it will assuage critics’ concerns that the nearly 500 full-body scanners at 78 airports reveal too much.

"We believe it addresses the privacy issues that have been raised," TSA chief John Pistole said at a news conference at Reagan National Airport in Washington, one of the airports testing the technology.

The system does not involve new machines. Instead, it relies on new software.

The software discards the x-ray-style image that revealed the contours of the traveler’s body — the one that left many uncomfortable at the thought of screeners being able to see them with the rough outlines of their undergarments.

Now, there is just a generic image — like the chalk outline of a body at a crime scene.

This is how it works:

A traveler passes through the scanner. Once they step out, they can see a computer monitor. It can display a large green "OK" and the traveler can move on.

If they have something in their pockets or hidden elsewhere on their body, the outline of a body appears on the screen, and a box marks the location of the object. If someone had a wallet in a front pocket, for example, the box would appear over the hips.

The box would then trigger a human pat-down search.

"One of the things this does is give greater confidence to the traveling public, because they are seeing the image also. They are seeing exactly what the security officer is seeing, that they can say ‘Oh, yeah, I forgot to take that piece of paper out of my pocket,’" Pistole said.

"And hopefully it provides a greater deterrent to possible terrorists, who may realize ‘Ok, they’re going to see it right there. If that shows up, and I’m here, then I’m going to be caught,’" he added.

On Tuesday at one of the test sites, McCarran International Airport in Las Vegas, TSA workers moved through a scanner equipped with the new software in a demonstration for reporters. On some people, the scanner picked up objects in their pockets. Those carrying nothing moved through the scanner in moments.

The other airport where the software will also be tested is Hartsfield-Jackson International Airport in Atlanta.

If all goes well for two months, TSA can install the software to 250 of the scanners nationwide at a cost of $2.7 million. The expansion will be limited to that because the software only works on machines produced by one of the two companies that make them.

TSA officials are making the decision to expand on the ability of the software to detect objects and how efficiently it can move travelers through the security checkpoints.

Critics have complained that the full-body scans are intrusive.

Chris Calabrese, the American Civil Liberties Union’s legislative counsel in Washington, applauded the TSA for responding to passenger complaints, but said the federal government needs to do more to protect travelers’ privacy.

The ACLU has called for an airport bill of rights that would legally protect passengers and prohibit pat-down searches. Ideally, the TSA would continue to tweak its body scanning software until the process was no more invasive than a metal detector machine, Calabrese said.

"You have to balance Americans’ dignity and privacy against security concerns," he said.

The U.S. Travel Association estimates more than 70 percent of all passengers consider the TSA’s searches inefficient and frustrating, said senior vice president Geoff Freeman. Incremental changes, he said, won’t address those concerns.

"It’s not about the scanners versus the pat-downs," he said. "The biggest complaint is people don’t want to take off their shoes. What this is really about is the desire by travelers to see a long-term vision, the idea that this country that put a man on the moon can find a way that is more customer-friendly to keep us safe."

At McCarran International Airport in Las Vegas, passengers shuffled through the body-scanning machines without protest Tuesday.

Joanne Argabrite, 66, who was returning to her home in West Virginia, said she wondered if women were more likely to be embarrassed by the scans than men. But she didn’t object to the security measures.

"I don’t want to have the plane blow up because someone didn’t want to take a picture," she said.

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Associated Press writer Sagar Meghani in Washington contributed to this report.