The Transportation Security Administration started testing the new machines at three airports this month and could buy up to 500 starting this fall for major airports, TSA chief Kip Hawley said.

New equipment would replace some of the 2,000 conventional airport X-ray machines, which have a limited ability to help security screeners find bombs.

"This is a strategically significant move," Hawley said. "This will provide a sharp upgrade in security."

Advanced technology X-ray machines photograph bags from two angles instead of one and show the images on side-by-side screens.

The added view could ease the TSA requirement that laptops go through X-ray machines in bins with no other items, said Peter Kant, a vice president at Rapiscan Systems, which makes the machines.

Laptops have been a security concern because their casings and circuitry are dense and could be used to shield weapons from the current X-ray machines, which take a single photograph from underneath an object. The new machines, by taking a second photo from the side, would reveal a weapon on top of a laptop, Kant said.

Hawley said travelers shouldn't expect the laptop policy to change soon. Aviation-security consultant Rich Roth said he doubts the TSA would ease its policy because there's "too much benefit from leaving laptops outside of cases."

The machines could speed up security lines if airport screeners don't have to X-ray bags a second time to see a different angle, Hawley said.

The technology aims to counter a major threat to aviation — bombs on a plane — that was highlighted a year ago when authorities disrupted a plot to blow up planes with liquid explosives.

"It's a big step," said Clark Ervin, a former Homeland Security inspector general whose investigations found screeners repeatedly missed weapons. "These steps ought to have been taken sooner."

They are already being used at airports in Europe and Canada to screen checked luggage.

Tests at New York's Kennedy, Washington's National and Albuquerque airports will determine how many machines the TSA will buy. Each machine costs about $120,000 and $50 million could be spent on them.