Just in time for holiday travel, O'Hare International Airport is getting a new type of body scanner that should help speed the lines at security checkpoints and that displays a generic outline of each passenger rather than specific body images that critics have blasted as "a virtual strip-search," the Transportation Security Administration said Monday.

By January, there will be 29 of the new millimeter wave body-imaging devices at O'Hare, replacing all 23 of the more bulky backscatter advanced image technology X-ray machines that were deployed at the airport starting in 2010, TSA spokesman Luis Casanova said.

The new machines started arriving last month, and more than 14 will be operating by Thanksgiving, the TSA estimates. It means that a mix of the two technologies will be used until the switch-over is completed in January, officials said.

Midway Airport will also get the new scanners next year, although a date hasn't been set, he said.

Passengers will still have to remove their shoes, belts, jewelry and all objects from pockets before screening, and raise their arms over their head during the body scan, unless they opt to be patted down by a TSA security officer instead, officials said.

But TSA officials say the change to newer technology at O'Hare and other large airports should help ease concerns and objections over body-imaging screening, introduced after the so-called underwear bomber, Umar Farouk Abdulmutallab, tried to blow up a U.S. airliner near Detroit on Christmas Day 2009. Both the old and new methods screen passengers for metallic and nonmetallic threats, including weapons and explosives concealed under clothing, without making physical contact, officials said.

There are several advantages to the latest generation in screening technology. The older backscatter devices raised privacy concerns because the contours and folds of each individual's naked body are displayed in a heavily blurred fashion on a screen inside a locked booth where a TSA officer scans the image for possible hidden weapons or explosives. The image is deleted once the passenger is cleared, officials said.

The 29 new machines set for O'Hare are equipped with "automated target recognition" software that detects any possible threats under a passenger's clothing. The machines display a generic outline, which is identical for all passengers, on a monitor in the imaging portal that the passenger can also view, officials said.

"You see exactly what the officer sees," Casanova said. "The computer-generated image is the same for everybody."

If anything unusual is detected, the object will be highlighted by an orange square on the screen, and additional scrutiny would be required to determine what the object is, he said. If no anomalies are detected, an "OK" would appear on the screen.

The millimeter wave devices can screen a person in as little as 10 seconds, Casanova said. Some radiation experts say the devices also are safer than the backscatter machines because they rely on low-energy radio waves similar to those used in cellphones.

The backscatter screenings take sometimes a minute or more per screening, in part because the TSA officer standing next to the passenger at the checkpoint must communicate via a transmitter with the TSA officer in the closed booth viewing the full-body image, officials said.

Passengers at O'Hare and other large U.S. airports have occasionally complained to the TSA that they missed their flights because of extended waits during use of the backscatter screening.

"We will now have improved efficiency to screen passengers, and a greater number of machines to get people through security faster," Casanova said.

The backscatter devices are not being retired, TSA officials said. Rather, they are being relocated to less-busy airports with lighter passenger loads. It means, however, that O'Hare passengers traveling to the smaller airports may still experience the backscatter scanners on their return flights.

In addition to O'Hare, the backscatter scanners are being replaced with the millimeter wave technology at airports in Boston; Los Angeles; Orlando, Fla.; and Kennedy International Airport in New York, the TSA said.

jhilkevitch@tribune.com

Twitter @jhilkevitch