MESA - An X-ray machine aimed at detecting weapons and explosives hidden on passengers is scheduled to make its debut Friday at Phoenix's Sky Harbor International Airport.

The "backscatter" will be in operation at Security Checkpoint B in Terminal 4.

While any Terminal 4 ticketed passenger can pass through any checkpoint, the B concourse is primarily used by travelers on Tempe-based US Airways.

The Sky Harbor pilot program will be voluntary, airport officials said.

Only those who don't pass the regular screening process will get to try out the new technology, and they could opt for a pat-down instead.

The X-ray technology, which was developed for prisons, was expected to start screening Sky Harbor passengers in December but the launch was postponed while the Transportation Security Administration worked out glitches and installed some new software.

While the machine visually strips off clothing, the image it projects looks more like a chalk drawing than a real person, TSA spokesman Nico Melendez said.