How Backscatter X-ray Machines Function

The shoe bomber came first, followed by the underwear bomber, both of whom attempted to set off explosives on commercial flights. Luckily, neither of these attacks succeeded.

However, these attempts did succeed in heightening global fear among travelers about the safety of flying. Furthermore, the bungled attacks highlighted critical weaknesses in airport security protocols.

Every year, governments worldwide allocate millions of dollars to combat these threats. Since the 9/11 terrorist attacks, the U.S. Transportation Security Administration (TSA) has rolled out an extensive range of security measures aimed at reducing the risk of future attacks, from limiting shampoo bottle sizes to enforcing thorough pat-downs.

One of the latest security innovations being implemented is backscatter X-ray scanners. These machines detect potential weapons by creating a detailed body outline, often compared to a chalk sketch or an exceptionally clear X-ray image. This technology identifies materials that metal detectors and traditional X-ray machines might miss, including plastic explosives, illegal drugs, and non-metallic weapons crafted from ceramics or other substances.

The technology has sparked backlash from privacy advocates, who argue that the images infringe on personal privacy rights. Some also raise concerns about the potential health risks associated with the technology.

Despite these concerns, backscatter X-ray scanners merge fascinating physics with state-of-the-art technology. To grasp how this new innovation works, let's first explore a more traditional technology — the X-ray machines that doctors have relied on for years — to highlight the differences.

This Ray is Rated X

Traditional X-ray machines use X-ray tubes to direct high-energy X-rays onto your body, which are a form of electromagnetic radiation. Similar to visible light, X-rays consist of photons, which are the basic particles forming all electromagnetic radiation. For a more in-depth exploration of radiation, check out our article How Radiation Works.

The X-rays pass through your body and are then captured on either film or a digital sensor located on the opposite side. Denser body parts, such as bones, obstruct the X-rays to varying degrees, creating lighter and darker areas on the image based on the density of your body parts. For instance, your skin and muscles appear darker, while bones are shown in white.

In essence, the X-ray energy is captured on the transparent, radiographic film that your doctor examines. The result is a monochrome image that can help the doctor identify issues such as a broken bone.

Such X-ray machines aren't suitable for airports, where speed is crucial, and there's no time to develop film for each passenger. Instead, airports utilize a similar technology called dual-energy transmission X-ray systems to screen carry-on luggage as it moves along a conveyor belt.

These dual-energy systems emit powerful rays, which are filtered in a way that allows security personnel to detect differences in material density. Experienced operators use visual cues to distinguish between metallic, organic, and non-organic items. They are also trained to spot objects that might indicate a security threat. For more details about dual-energy X-ray systems, visit How Airport Security Works.

Backscatter X-ray scanners (also known as soft X-ray scanners) offer more advanced capabilities compared to traditional medical X-rays and dual-energy X-ray machines. Even their configuration is distinct. While conventional X-ray machines position the X-ray tube and imaging sensor on opposite sides of the subject, backscatter scanners place the imaging sensor on the same side of the body, next to the X-ray tube.

The X-rays used in backscatter systems are far weaker than those used by your doctor. These rays don't pass through your body. Instead, they penetrate your clothing and about an inch into your body, where the rays scatter off your tissues and bounce back to the sensor.

The sensor captures the scattered rays, creating an image that closely resembles a naked human body. If someone is secretly carrying a suspicious or unusual object, the authorities will detect it. However, they will also notice if the 'irregular' objects happen to be love handles, sparking privacy debates that we'll discuss later.

Backscatter Tech Backgrounder

A scanner capable of producing a nude image of your body while you're still fully clothed may seem like something from a science fiction movie. However, the technology is very much a reality.

Traditional X-rays create images by sending powerful rays through the human body. These rays can be harmful if too much exposure occurs, which is why doctors use them carefully. It's also the reason you wear a lead apron during X-ray procedures at the dentist’s office. The apron absorbs the rays before they reach your body.

Backscatter machines, on the other hand, use weaker X-rays. The radiation dose amounts to roughly 0.02 to 0.03 microsieverts, which is similar to one hour of exposure to natural background radiation [source: Health Protection Agency]. These X-rays don’t pass through the body forcefully; instead, they collide with your body and scatter.

Backscatter X-rays react differently depending on the material they interact with. Each material, whether organic or non-organic, causes the X-rays to scatter at varying intensities, which results in a high-contrast, two-dimensional image. Security personnel use these images to spot suspicious objects hidden under clothing or possibly within body cavities.

For those with a deeper interest in physics, there's a more technical way to understand how backscatter X-rays function. When the X-ray hits atoms in your body, the photons in the X-ray beam scatter. In the process, some of the photons knock electrons out of certain atoms, creating ions, and occasionally slower-moving photons as well. This is why X-rays are classified as ionizing radiation.

The scanner relies on the energy signatures of ions and slower photons to generate images that distinguish between organic materials, like your arms and legs, and non-organic items, such as weapons or the belt buckle you forgot to remove before screening. These distinctions are often quite noticeable, even to someone without training.

If this seems like an advanced and costly precursor to your trip to grandma's house, you're right. Backscatter scanners are not inexpensive. Each unit costs over $100,000 [source: EPIC].

Currently, the TSA operates around 500 backscatter scanners across nearly 100 airports and plans to deploy at least 1,000 in total. With so many scanners in place, nearly 70 percent of travelers will be screened using this cutting-edge technology [source: Los Angeles Times].

To successfully roll out hundreds more scanners, security officials will need to address concerns about privacy and health. Keep reading to explore how these scanners function in detail and why some groups are fighting to keep them out of our airports.

Public Peep Shows and the End of Privacy

The privacy concerns are clear: most travelers would not look forward to a backscatter screening knowing it could result in a nearly naked image. This is true even though the TSA assures that the images are immediately deleted, and the screeners are stationed remotely, far from the checkpoint, ensuring they cannot see the passengers being screened. By having the screeners situated far from the passengers, the TSA hopes to reduce the sense of visual violation.

This approach also aims to address criticisms about racial profiling. Since the screeners cannot see skin color or clothing, their judgments are not influenced by any preconceived notions about the person in front of the scanner.

Nevertheless, for many travelers, the fact that these scanners create an almost naked image remains a significant concern. To lessen the visual intrusiveness of these scans, the TSA is testing scanners with updated software that generates cartoon-like images of passengers. Any suspicious items are highlighted in a similarly abstract way, allowing authorities to identify individuals who might require additional screening, such as a pat-down.

So far, the privacy issues surrounding backscatter scanners have not escalated into major problems for the TSA in the same way that enhanced pat-downs or aggressive frisking have. However, there remains considerable opposition to these so-called 'strip search' scans.

The ACLU (American Civil Liberties Union) contends that the scanners can lead to embarrassment. Not only are individuals exposed, but those with medical devices, such as colostomy bags, might face unwarranted additional searches. The organization maintains that backscatter X-ray scanners should only be used when absolutely necessary, not routinely applied to a set percentage of travelers passing through security checkpoints.

In the UK, concerns have been raised that the scanners could violate child pornography laws. As a result, British minors under the age of 18 are allowed to avoid the scanners. Additionally, some religious figures have expressed opposition, arguing that the resulting nude images contradict their beliefs.

However, privacy concerns are just one part of the debate against the scanners. Some people are also worried about the potential health risks associated with their use. Continue reading to learn more about this issue.

Radiation Nation

While security and health officials assure the public that the radiation from backscatter scanners is minimal, not everyone is completely comfortable with the technology.

Despite these reassurances, officials argue that it would take around 5,000 trips through the scanners to accumulate the same level of radiation you'd get from a single chest X-ray at your doctor's office [source: Los Angeles Times].

Furthermore, the TSA asserts that simply flying exposes you to more radiation, as cosmic rays constantly bombard the Earth's atmosphere. In fact, just 60 seconds at 35,000 feet exposes you to the same amount of radiation as one scan [source: CBS Denver].

However, this hasn't stopped people from raising concerns about the safety of the scanners. Some scientists argue that the ionizing radiation from these devices always poses a potential health risk, particularly in relation to skin cancer. Since cancer can develop many years after exposure, it may be difficult to directly link the two.

The fear of cancer and other health issues has led some companies, such as Rocky Flats Gear and Flying Pasties, to take advantage of travelers' concerns. Flying Pasties offers inserts that protect your private areas from being exposed to scanners, while Rocky Flats Gear goes further by partially shielding you from X-ray radiation.

To address concerns about radiation exposure, the TSA has been trialing millimeter wave machines. These devices serve as an alternative to backscatter scanners, producing similar images but using radio waves instead of X-rays, which could ease concerns about radiation risks.

In addition to not emitting radiation, millimeter wave scanners offer another advantage over backscatter scanners: they generate 3D images instead of basic 2-D ones, providing more detailed information and potentially enhancing security effectiveness.

Backscatter to the Future

While the government continues to defend backscatter X-ray technology as safe, it is taking additional steps to safeguard privacy. In February 2011, the Senate passed a law making the unauthorized use of scanner images a felony. This legislation aims to prevent malicious screeners from posting inappropriate images online, as occurred with a scanner at a federal courthouse in Florida.

In an effort to address privacy concerns, the TSA is testing millimeter wave machines equipped with software that omits anatomical details. Additionally, some of these machines are being set up so passengers can view their scan results in real-time alongside security personnel, ensuring transparency and preventing any unauthorized digital surveillance.

Whether you're more focused on security or modesty, privacy advocates argue that the latest scanner technology is still lagging behind sophisticated terrorists. They claim that knowledgeable terrorists will hide explosives, like plastic or sheet explosives, deep inside body cavities, making it harder for scanners to detect them when they're most needed.

Regardless of where you stand on X-ray scanners, these technologies are set to become more common in airports and other high-security areas as the demand for increased safety grows.

Just as terrorist tactics will continue to evolve, so too will the security measures used to counter them. Likewise, debates over safety and privacy will persist as travelers and government officials work to find a middle ground between security and personal freedoms.