Backscatter X-ray
In contrast to the traditional X-ray machine, which detects hard and soft materials by the variation in transmission through the target, backscatter X-ray is a newer imaging system which detects the radiation which comes back from the target.  It has potential applications in almost every situation in which non-destructive examination is required, but only one side is available for examination.

# The technology
The resolution of the resulting images is quite high. Some backscatter X-ray scanners are able to penetrate up to 30cm (~12") of solid steel. As such, the technology is in use to search containers and trucks much more quickly than performing a physical search, and potentially allow a larger percentage of shipping to be checked for smuggled items or weapons. According to Farren  Technology, the technology exists to scan areas as far as 50 meters away from the device, producing 3D images of people's bodies and the weapons they might be hiding. In comparison to x-rays from medical applications, the backscattered x-rays are considered high energy and usually scatter instead of penetrate materials. A "high energy x-ray beam" moves rapidly over the person's form and a high resolution image of the person's nude body is constructed when the scattered x-ray "from a known position" is detected.

# Privacy concerns
The technology has been proposed as an alternative to personal searches at airport and other security checkpoints, since it can easily penetrate clothing and reveal concealed weapons and other items. The ACLU and the Electronic Privacy Information Center are opposed to this use of the technology, currently in use at Phoenix Sky Harbor International Airport. The ACLU refers to backscatter x-rays as a "virtual strip search".
It is "possible for backscatter X-raying to produce photo-quality images of what's going on beneath our clothes," thus, according to the TSA, the images have been distorted with the private parts being blurred (Layton). According to the TSA further distortion is used in the Phoenix airport's trial system where photo-quality images are replaced by cartoon-like outlines .

# Other concerns

## Privacy
The ACLU and some flight passengers have expressed privacy concerns with the technology since it shows their unclothed body and may allow screeners to gain access to otherwise confidential medical information, such as the fact a passenger uses a colostomy bag. The TSA has responded to these concerns with the statement that new technologies allow the image to be blurred and by setting up screening procedures such as having the screener viewing the image located far away from the person being screened.
In turn, some journalists have expressed concern that this blurring may allow people to carry weapons or certain explosives aboard by attaching the object or substance to their private parts .

## Health effects
Some people are concerned with exposure to radiation emitted by Backscatter Xrays. At airports, lead vests are not used and people fear being exposed to "dangerous level of radiation if they get backscattered too often" .
The Health Physics Society (HPS) reports that a person undergoing a backscatter scan receives approximately 0.005 millirems of radiation in comparison to the 0.009 millirems reported by American Science and Engineering Inc. . According to the U.S. regulatory agencies, "1mrem per year is a negligible dose of radiation, and 25 mrem per year from a single source is the upper limit of safe radiation exposure" .
Of course airport scanning is not the only place a person receives X-Ray exposure throughout the year.