Wiedefeld said the change to the advanced-imaging program is a welcome development. "Last Thanksgiving, there was almost a passenger revolt over this technology," he said.

The new procedures provide "a better experience for the customer," the airport chief said. "They see the same imaging the screener sees, so that takes some of the mystery out of this technology."

The new technology has been deployed at four of BWI's five piers — A, B, D and E. The checkpoint on Pier C, at the oldest part of the airport, has not been equipped with the advanced-imaging machines because the devices won't fit until an expansion project is completed in several years.

The software upgrade is being introduced only at airports that use the "millimeter wave" version of advanced-imaging software. Passengers who pass through airports equipped with the other version of the technology, known as "backscatter," will still encounter the more intrusive scan — though they can choose a pat-down instead.

Speaking for one of the chief critics of the TSA's screening program, American Civil Liberties Union legislative counsel Chris Calabrese said the new technology is an "important step forward."

"At least the employee is not looking at the naked person every time," he said. "We appreciate their listening. Obviously, we wish they had done it sooner."

Calabrese noted that the software upgrade does not apply to backscatter machines at airports around the country. "We're only halfway there," he said.

The ACLU spokesman said the software fix doesn't alleviate all of the group's concerns about the airport security program. "We still have very intrusive pat-down practices and groping procedures," he said.

Passengers who object to the advanced imaging are permitted to choose pat-downs instead, but the TSA has warned that those searches can be uncomfortably thorough.

The Electronic Privacy Information Center says the main issue is what is done with the information gathered by the screening device.

Lily Coney, EPIC's associate director, said the software change is a "step in the right direction" but concerns remain.

"They haven't taken out the image capture capability," she said. "They're just showing you a different presentation of the same data."

EPIC, which has taken the TSA to court over the issue, still wants a formal process to write rules governing how the technology will be used, Coney said.

Riley said some of EPIC's concerns about the technology are misplaced.

"It cannot store, it can't transmit images, period," he said.

The TSA spokesman defended the agency's use of the old technology to this point, saying it had to move forward with the "best available technology" to deal with potential threats to aviation,

Riley said the TSA intends to replace the imaging software in the backscatter machines.

"We're moving toward it right now," he said, declining to offer a timetable.