How Backscatter X-ray Systems Work

Backscatter Tech Backgrounder

Knives made of ceramic are no match for backscatter X-rays, which pick up on all sorts of nonmetallic objects.
Knives made of ceramic are no match for backscatter X-rays, which pick up on all sorts of nonmetallic objects.

A scanner that can render a nude image of your body while you're still clothed might sound like an insidious invention straight out of a science fiction movie. But the technology is very real.

Traditional X-rays create images by blasting powerful rays all the way through a human body. These rays are strong enough to cause health problems if you receive too many high doses, which is why your doctor uses X-rays sparingly. It's also why you wear a lead jacket when you're exposed to X-rays at the dentist's office. The jacket deflects the X-rays before they come into contact with your body.

In contrast, backscatter machines use less powerful X-rays. The radiation dose is around 0.02 to 0.03 microsieverts, or about the same as one hour of exposure to natural environmental radiation (called background radiation) [source: Health Protection Agency]. These X-rays don't barge through you by sheer power. Instead, they smack into your body and scatter.

Backscatter X-rays interact differently depending on the type of material they come in contact with. Each different type of material, be it organic or non-organic, causes the X-rays to scatter at different intensity levels, providing a lot of contrast in the resulting two-dimensional image. Security personnel monitoring the scanner can use these images to detect suspicious objects squirreled away under a person's clothing or perhaps hidden inside body cavities.

If you're a physics buff, there's a more technical way to think about how backscatter X-rays work. As the X-ray collides with atoms in your body, the photons in the X-ray beam scatter. During this process, those photons also push electrons out of some of the atoms, resulting in ions, and sometimes slower moving photons, too - this effect is why X-rays fall into the category of ionizing radiation.

The scanner uses the energy signatures of those ions and sluggish photons to produce images that identify organic material, like your arms and legs, or nonorganic objects, such as weapons or that belt buckle you forgot to remove before screening. The differences are often easy to see, even to the untrained eye.

If this sounds like a pretty high-tech, expensive prelude to your weekend trip to grandma's house, it is. Backscatter scanners aren't cheap. They cost more than $100,000 per unit [source: EPIC].

The TSA currently operates around 500 backscatter scanners in close to 100 airports and is planning to deploy at least 1,000 total scanners. With that many scanners in place, nearly 70 percent of travelers will be screened using the new technology [source: Los Angeles Times].

In order to successfully roll out hundreds more of these scanners, security officials will have to counter privacy and health concerns. Keep reading to see how these scanners work in detail -- and why some groups are working to keep these machines out of our airports.