"All explosive detection done by X-ray units is measured by algorithms that judge density against particular atomic weights of known explosives," says Keith James, director for CBRNE Programs for SoBran, a technical and professional services company. X-ray technology is proficient at finding new threats such as liquid explosives, ceramic knives and guns made of plastic. As useful as the X-ray technology may be, it also has its limits, particularly when lethal objects and common objects have similar chemical densities.
"Unfortunately, backscatter X-ray can't distinguish between a bar of explosive nitrate and a bar of chocolate," James says. Frequent false positives are the result.
Another problem with the technology is the issue of personal privacy.
While backscatter X-ray systems, when used as an alternative to personal pat-down secondary searches at airports and other security checkpoints, can easily penetrate clothing to reveal concealed contraband, they also reveal intimate body contours. Essentially, backscatter technology undresses human subjects.
Some consider this a violation of privacy, the appearance of security achieved by further erosion of personal liberty.
The American Civil Liberties Union and the Electronic Privacy Information Center call backscatter scans "virtual strip searches." The issue is, the image that appears on the monitor used by airport screeners is a nude picture of the subject. It not only reveals hidden explosives or weapons, by also otherwise private, confidential medical information, such as the fact a passenger may be fitted with a colostomy bag.
Nevertheless, backscatter technology is being used in several U.S. airports and courthouses, although a possible replacement technology is in the wings. Millimeter wave technology, another way to obtain full-body imaging, uses non-ionizing electromagnetic waves to generate images based on the energy reflected from the body.
The images generated through millimeter waves are lower resolution than those of X-ray backscatter and supposedly less intrusive. The government is still looking under everyone's clothing, but with millimeter wave technology, "facial images are blurred," according to a TSA press release.
Additionally, the officer that does view the image will be remotely located and unable to associate the image with the passenger being screened. Once viewed remotely, the image will not be stored, transmitted or printed.
As of December 2009, 40 passenger imaging systems had been installed at some of the nation's largest airports, used for primary or secondary (random) screening. TSA reports it has plans to purchase 150 more.
Backscatter X-ray is also not as useful when scanning objects such as vehicles or freight containers. Since the X-rays penetrate only a fraction of an inch, only a surface scan is performed. Many blind spots will be evident in the image. Items buried in an object will not be seen at all, such as the inside of a vehicle gas tank, one popular location to smuggle contraband.
Instead, first single-energy, then dual-energy, transmission X-rays were developed to screen vehicles and shipping containers. Since these X-rays pass completely through, blind spots are minimized. However, clutter can become an issue. Single-energy systems produce black and white images, and operators often have difficulty distinguishing between metal and non-metal objects.
Dual-energy systems create color-coded images, allowing the operator to easily distinguish metal from organic material and eliminate the clutter issue. A faster and more thorough examination of the vehicle is accomplished compared to visual inspection and under-car cameras or mirrors.
Ultra-low dose, dual energy transmission X-ray systems have also evolved to screen occupied vehicles. Throughput is therefore not sacrificed, since passengers are not required to exit the vehicle.
Douglas Page writes about science, technology and medicine from Pine Mountain, Calif. He can be reached at email@example.com.