Backscatter X-ray works from reflected, or scattered, X-ray energy, so there's no need for a film or sensor on the opposite side of the imaged object. Objects composed of elements with low atomic numbers scatter more energy than the ones made from relatively high atomic number items. All organic compounds, including drugs, explosives, and people, contain carbon (atomic number=6), oxygen (8), hydrogen (1), and so on. Steel is mostly iron (atomic number=26).
At airport security checkpoints, backscatter X-ray devices are in use to detect things a metal detector can miss. Most anything that doesn't fit the characteristic signature of clothing or tissue will be readily apparent on the backscatter display. On a larger scale, backscatter X-ray scanners can "see" through vehicles and cargo containers to reveal concealed people, drugs, and other possible contraband. The vehicle-size scanners fit into a van.
American Science and Engineering (AS&E), the largest manufacturer of backscatter X-ray devices, has sold at least 400 Z Backscatter Vans or ZBVs, but the company won't say who it sold them to. The U.S. Customs and Border Protection (CBP) agency has some of them.
X-rays are ionizing radiation, and are potentially harmful to human tissue. Exposure to radiation is an understandable concern for people who may or may not be aware they are being subjected to this kind of surveillance. It's evident enough at the TSA checkpoint, but possibly not so obvious when a vehicle-size scanner is in use. AS&E maintains that a typical scan involves about the same amount of radiation exposure as one would receive in two minutes of flying in a commercial airliner at cruise altitude (cosmic radiation increases as the atmosphere gets thinner).
Exposure to radiation isn't the only aspect of surveillance John Q. Public is worried about. No one likes to be watched, and the perception that the police are always watching you creates an aura of oppression, not safety. New police technologies usually give rise to new case law, but the rules of the game haven't changed all that much.
The legality of any police surveillance is governed by the Fourth Amendment, and whether there has been a search, usually defined as "an intrusion into a reasonable expectation of privacy." So long as the police are in a place they're allowed to be, and seeing what anyone can see with their own eyes, there's no expectation of privacy.
Use of GPS monitoring devices, even when they have been placed on vehicles surreptitiously, is not a search, or so says the 7th Circuit in U.S. v. Garcia. In this case, Garcia was a predicate felon for dealing in methamphetamine, and an informant told the police he was distributing it and planned to start manufacturing it.
Police placed a passive GPS device on Garcia's vehicle (presumably while the car was in a public place, as there was no trespass claim) and found it had traveled to a parcel of land. The police obtained a search warrant for the land, found Garcia's meth lab, and arrested him when he returned to it. Garcia tried to suppress the information from the tracking device, but the court said the device didn't provide them any information they couldn't have gained from a conventional surveillance.
The same rationale applies to ALPR data. An ALPR is essentially a cop with a good eye and really fast note-taking skills. It doesn't capture anything a patrol officer couldn't see with his or her own eyes.
X-ray devices are another matter, of course, since only Superman can see inside a vehicle without a physical intrusion. So far, these machines are used only where suspicionless searches are lawful, like TSA checkpoints and border crossings, so there are no Fourth Amendment issues-not that people don't try to raise them. The airport backscatter machines produce an image that some people feel is too close to nudity, as it shows anatomical details concealed by clothing. The remedy is to request a hand search, which most people don't like much, either.
So, make it easy on yourself and use the surveillance tech that's available to you. Maybe the next generation of technology will address the problem of where all that coffee consumed on stakeouts is supposed to go.
Tim Dees is a retired police officer and the former editor of two major law enforcement Websites who writes and consults on technology applications in criminal justice. He can be reached via editor@PoliceMag.com.