New software designed to make airport security scanners less intrusive debuted at the Las Vegas airport Tuesday, a response to last year's uproar from passengers who thought the blurry but revealing images were an invasion of privacy.

The machines now produce a gray, cookie-cutter outline of the human form. The silhouette appears on a screen about the size of a laptop computer that is attached to the scanning booth.

If a passenger is cleared by the scan, the screen will flash green with an "OK." Suspicious items detected by the scanner appear as little boxes outlined in red, showing their location on generic front and back silhouettes on the screen.

Passengers who trigger an alert, and anyone who refuses to go through the scanners, will receive the rigorous frisking that has drawn sharp objections.

The new software is expected to debut soon at Reagan National Airport and in Atlanta. If it does as well in the field as it has in testing, it could be installed in the 486 scanners now in use at 78 major airports, the Transportation Security Administration said.

"We believe it addresses the privacy issues that have been raised," said John Pistole, head of the TSA. "It's basically a software modification to existing equipment, so there's very little cost."

Robin Kane, who heads the TSA's technology office, said that once the less-invasive approach is proved effective, then the controversial monitors, on which a TSA officer reviews scans in a private screening room, will be removed from all airports.

The images produced by the current software led to an uproar over privacy concerns. Pistole had said in the fall that he wanted to see modifications, but the technology that was being tested yielded too many false positives. Many passengers found the alternative "enhanced" pat-downs by TSA agents even more disturbing.

In the demonstration at National on Tuesday, "passengers" filed through the scanner, some of them producing gray silhouettes with green "OK" screens, others producing silhouettes with boxes noting where the machine detected something hidden.

Kate Hanni, founder of the California-based group FlyersRights, called the new software "a great step forward."

"We're grateful to the TSA for addressing these issues that were of concern to so many people," Hanni said. "But privacy was our secondary issue. Our primary concern about the body scanners is that they are ineffective. We're also concerned about the possibility of surges in radiation."

Two types of scanning machines - backscatter and millimeter wave - have been installed at airports. Both machines produce the kind of full-body images that attracted controversy; they work by bouncing X-rays or radio waves off skin or concealed objects.