LAS VEGAS — Federal aviation officials are giving it another try. This time, they are not looking as closely under travelers’ 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 administrator 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: Travelers pass 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, 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 … then I’m going to be caught,’” he said.

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. A search later showed wallets or cell phones. Those carrying nothing moved through the scanner in moments.

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

TSA officials will be basing the decision to expand on the ability of the software to detect objects and how efficiently it can move travelers through checkpoints.