To learn more, read our Privacy Policy. Some years ago, the three of us found ourselves together in a very long security line at Reagan National Airport in Washington, D. Maybe, we joked, Superman could use his X-ray vision to help out the beleaguered Transportation Security Administration TSA employees and the traveling masses. But then came an exciting realization: We did have the power to solve the technical problem here. By employing our combined expertise, we began developing an X-ray system well suited for detecting such dangerous objects hidden in a carry-on bag.
Perhaps one day it will shorten the kind of line we were standing in. The goal of aviation security is, of course, to make sure that dangerous items do not end up on planes—while also providing an acceptable passenger experience. What exactly this experience should be depends on whom you ask, but it likely includes short, quick-moving lines and the capacity to bring everything you want without having to rummage through your bag to exonerate your electric toothbrush. To make this vision a reality, the TSA would need a device that is capable of rapidly scanning the many bags passing through a security checkpoint and deciding on its own whether any of them contains a threat.
X-ray projection imaging works by directing these penetrating rays at something and measuring how much energy comes out the other side. The shape and degree of darkness of the shadows are used to distinguish between different items or, in the case of medical imaging, to reveal a fracture in a bone.
While this strategy works reasonably well, anyone who has played shadow puppets knows that shadows are not always what they appear to be. This tactic provides significant advantages, such as allowing you to leave most items laptops, iPads, keys, and so forth in your bag for a single scan.
Such advanced scanners are now being deployed at U. Their 2D images are interpreted by a combination of computer algorithms and trained operators, who then determine whether you are free to continue to your plane or need to have your carry-on more carefully inspected by an agent. In general, this system works great for shape-based threats such as guns or knives. While we can debate whether a small penknife or knitting needle could really be used as a weapon, these transmission X-ray scanners detect such objects routinely.
And more recent enhancements, such as deep-learning-based image-recognition algorithms, allow automatic identification of a variety of such dangerous items. An Airport Ritual: Airline passengers have long had to suffer excessive wait times at security checkpoints, as in this scene from Denver International Airport in After all, an explosive could be fashioned into the shape of a common, benign object.
How can the TSA deal with those threats? It has been known for more than a century that X-rays can reveal the atomic structure of a material through a process known as X-ray diffraction. For this, you have to measure the rays that bounce off the atoms or molecules in the target and ricochet in different directions. X-ray diffraction can, for example, easily distinguish among coal, graphite, and diamond, despite the fact that these substances are chemically identical—they are all made up of carbon.
It can even identify different liquids, which lack crystallinity but have different interatomic spacings as a function of molecular size and repulsion. So a scanner based on this technique could, for example, determine whether the bottle someone stuffed in his carry-on was filled with water or with nitroglycerine.
Making use of X-ray diffraction measurements, however, is a thorny technical problem. To begin with, the diffracted signal is several orders of magnitude weaker than the transmitted signal. As a result, even though such fingerprinting more properly called X-ray diffraction tomography would pretty much guarantee success in identifying threats, the technology required has in the past come at a significant cost, not just financially but also in terms of complexity, scan time, and other factors.
The focus of our efforts over the last few years has been figuring out how to combine X-ray diffraction measurements with traditional transmission imaging in a practical way. For more than 30 years, researchers have attempted to use X-ray diffraction to map out differences in atomic structure from place to place within an extended object. The much more vexing problem is that the diffracted signals originate from all points in the bag.
And each pixel of your detector records all these different signals at once. The trick to disentangling these overlapping signals—so you can tell which of the scattered X-rays came from your laptop and which from your water bottle, say—involves placing an additional element in the system, one that affects the X-ray signals in a controlled way. This element, called a coded aperture , is basically a slab of highly absorptive material with a set of holes drilled in it.
Those holes are arranged in a specific pattern. X-rays can pass through the holes but are blocked by the absorptive material. The reason for using such a coded aperture is easier to understand with the help of a thought experiment.
Imagine that the piece of luggage being scanned is composed of hundreds of tiny X-ray sources, each of which can be switched on and off on command. If you turn on just one source, the X-rays it emits will pass through the holes in the coded aperture and continue on to the image sensor positioned some distance behind it. Now switch on a different one of these tiny X-ray sources instead, one located at a different position in the bag.
The pattern of spots projected on the image plane will now be different. The spots will be bigger or smaller and located in different places. The same principle applies to shadow puppets, whose shadows change in size and position depending on where the puppet is located with respect to the light source and the wall. So the coded aperture affects the X-rays originating from each different location within the bag in a unique way, which is akin to tagging them with a bar code.
This works even though the X-rays originating from different regions in the bag impinge on the detector at the same time. It takes some clever calculations, but the signals generated by X-rays passing through the coded aperture and arriving at the detector can be untangled, allowing the scanner to distinguish the X-rays coming from different parts of the bag.
This process is much easier if the scanner also captures the traditional X-ray transmission images, which give you a pretty good idea of where the relevant objects are positioned in the bag. With it, we were able to identify different plastics, liquids, and solids at a resolution of better than 1 centimeter. Taking this to the next level involves designing a scanner that fully combines transmission and X-ray diffraction measurements and employs state-of-the-art detectors. This scanner should be suitable for commercialization and use in airports around the world.
To understand the possibilities of this and related systems, we required detailed numerical simulations, which helped us to compare possible configurations and to identify the best ones. As a first step in making such evaluations, we developed software that let us create virtual bags in a fully automated manner.
Running it with enough computing horsepower—either locally on high-performance clusters or using suitable cloud resources—allows us to create hundreds, thousands, or even millions of virtual bags, ones that are representative of the kinds of things travelers carry to the airport every day. We can now run those virtual bags through our simulation software, which models the relevant X-ray physics for both transmission and scattering and spits out high-fidelity estimates of the measurements you would get with a scanner of a particular design.
Coded apertures are also used in various optical devices. The pattern of light or X-rays that a source projects through the coded aperture onto the sensor plane depends on the position of that source. Here, the more distant red light projects a pattern that is smaller than the blue one and offset from it.
The light falling on the sensor plane can be analyzed to reveal the location and intensity of each source. That offset allows this two-dimensional sensor to register the faint X-rays scattered off of various objects within the bag without being overwhelmed by radiation from the main, fan-shaped beam.
Measurements of X-ray attenuation allow the scanner to produce a traditional X-ray image of the bag. By analyzing the scattered X-rays, the scanner can also determine the material composition of objects within the bag and label them as either benign or threatening.
The ability to generate many controllable, ultrarealistic virtual scans for arbitrary system configurations has allowed us to quantify how much better one scanner design is compared with another and what the fundamental detection performance of a given type of measurement would be.
As a side benefit, this same software lets us generate data sets for training and validating the machine-learning algorithms the scanners will run to recognize images and detect threats. So even as new computed-tomography baggage scanners continue to roll out at airports across the United States, we will be working to pave the way for an entirely new kind of scanner, one that can identify a dangerous item based on its material-specific fingerprint, not just the type of X-ray shadows it casts.
The magnetic field will be reflected back to the machine if there are any metal objects present, such as a watch or a belt buckle. The return signal is detected by the machine and a beeping noise is produced to alert the TSA agent.
The metal detectors ignore very small amounts of metal, like the button on your jeans or small earrings. Some equipment uses non-ionizing radiation. Non-ionizing radiation has enough energy to move atoms in a molecule around or cause them to vibrate, but not enough to remove electrons from atoms. In airports, metal detectors and millimeter wave machines use low energy, non-ionizing radiation to send energy across scanned surfaces.
The energy that bounces back from the scanned surface will show the objects that are present, or it can generate an image that TSA agents can use to show items that may need more investigation. Some screening equipment uses ionizing radiation.
Ionizing radiation has so much energy it can knock electrons out of atoms, a process known as ionization. Airports use ionizing radiation to scan passengers and luggage. Depending on the type of machine, ionizing radiation is used to identify objects that may be hidden by passengers and to create images of what is in luggage. Know the risks. Airport screening helps keep travelers safe by identifying hidden weapons and other hazards that are not approved for safe airline travel.
The risk of health effects from backscatter x-ray systems and millimeter wave machines is very, very low. However, if you are worried about x-ray or millimeter wave screening, you are not required to walk through these machines.
You can ask for a pat-down search instead. The TSA uses x-ray machines to screen carry-on items and checked luggage. In contrast, the UK Department for Transport has decided that any passenger refusing to pass through the body scanner will not fly. By ashley with 4 min read. By chelsea with 3 min read. By Tiago with 2 min read. Your email address will not be published. Search anything you need: Search for:. What the Body Scanner Sees? Millimeter wave scanner image example Backscatter x-ray scanner image example The Controversy about Body Scanners Scanners are being used as TSA screening routine instead of when there is probable cause to search a passenger.
Where Are The Body Scanners? Opting Out of the Airport Scanners In the United States, the law is that you can opt out of the scan and be subject to the touching search instead.
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