Backscatter X-ray is an advanced X-ray imaging technology. Traditional X-ray machines detect hard and soft materials by the variation in x-ray intensity transmitted through the target. In contrast, backscatter X-ray detects the radiation that reflects from the target. It has potential applications where less-destructive examination is required, and can operate even if only one side of the target is available for examination.
The technology is one of two types of whole-body imaging technologies that have been used to perform full-body scans of airline passengers to detect hidden weapons, tools, liquids, narcotics, currency, and other contraband. A competing technology is millimeter wave scanner. One can refer to an airport security machine of this type as a "body scanner", "whole body imager (WBI)", "security scanner" or "naked scanner".
Deployments at airports
In the United States, the FAA Modernization and Reform Act of 2012 required that all full-body scanners operated in airports by the Transportation Security Administration use "Automated Target Recognition" software, which replaces the picture of a nude body with the cartoon-like representation. As a result of this law, all backscatter X-ray machines formerly in use by the Transportation Security Administration were removed from airports by May 2013, since the agency said the vendor (Rapiscan) did not meet their contractual deadline to implement the software.
In the European Union, backscatter X-ray screening of airline passengers was banned in 2012 to protect passenger safety.
Technology
Backscatter technology is based on the Compton scattering effect of X-rays, a form of ionizing radiation. Unlike a traditional X-ray machine, which relies on the transmission of X-rays through the object, backscatter X-ray detects the radiation that reflects from the object and forms an image. The backscatter pattern is dependent on the material property and is good for imaging organic material.
In contrast to millimeter wave scanners, which create a 3D image, backscatter X-ray scanners will typically only create a 2D image. For airport screening, images are taken from both sides of the human body.
Backscatter X-ray was first applied in a commercial low-dose personnel scanning system by Dr. Steven W. Smith. Smith developed the Secure 1000 whole-body scanner in 1992 and then sold the device and associated patents to Rapiscan Systems, who now manufactures and distributes the device.
Large scale
Some backscatter X-ray scanners can scan much larger objects, such as trucks and containers. This scan is much faster than a physical search and could potentially allow a larger percentage of shipping to be checked for smuggled items, weapons, drugs, or people.
There are also gamma-ray-based systems coming to market.
In May 2011, the Electronic Privacy Information Center filed suit against the United States Department of Homeland Security (DHS) under the Freedom of Information Act, claiming that DHS had withheld nearly 1000 pages of documents related to the Z backscatter vans and other mobile backscatter devices.
Concerns
Legality
Since in addition to weapons, these machines are designed to be capable of detecting drugs, currency and contraband, which have no direct effect on airport security and passenger safety, some have argued that the use of these full body scanners is a violation of the 4th Amendment to the United States Constitution and can be construed as an illegal search and seizure.
Privacy
Backscatter x-ray technology has been proposed as an alternative to personal searches at airport and other security checkpoints easily penetrating clothing to reveal concealed weapons. It raises privacy concerns about what is seen by the person viewing the scan. Some worry that viewing the image violates confidential medical information, such as the fact a passenger uses a colostomy bag, has a missing limb or wears a prosthesis, or is transgender.
The ACLU and the Electronic Privacy Information Center are opposed to this use of the technology. The ACLU refers to backscatter x-rays as a "virtual strip search". According to the Transportation Security Administration (TSA), in one trial 79 percent of the public opted to try backscatter over the traditional pat-down in secondary screening.
