How Millimeter Wave Scanners Work

Before we climb inside a millimeter wave scanner, we need to step back and review some basic information about electromagnetic radiation, which exists in nature as waves of energy made from both electric and magnetic fields. These waves travel through space and come in a variety of sizes, or wavelengths. Gamma rays, for example, have a wavelength on the order of 0.000000000001 meters, or 0.000000001 millimeters. X-rays, which run a bit larger, have a wavelength on the order of 0.0000000001 meters, or 0.0000001 millimeters. And visible light waves measure about 0.000001 meters, or 0.001 millimeters. The entire collection of waves, across all frequencies, is known as the electromagnetic spectrum.

Now consider a wave that falls in a range exactly between 0.001 meters (1 millimeter) and 0.01 meters (10 millimeters). Scientists refer to the energy in this tiny sliver of the electromagnetic spectrum as millimeter wave radiation. Millimeter waves have a variety of uses but are especially important in radio broadcasting and cell phone transmissions. And, because the wavelengths of millimeter waves are large relative to natural and synthetic fibers, they tend to pass through most materials, such as clothing, making them an ideal candidate for scanning technologies.

Millimeter wave scanners produce their waves with a series of small, disc-like transmitters stacked on one another like vertebrae in a spine. A single machine contains two of these stacks, each surrounded by a curved protective shell known as a radome, connected by a bar that pivots around a central point. Each transmitter emits a pulse of energy, which travels as a wave to a person standing in the machine, passes through the person's clothes, reflects off the person's skin or concealed solid and liquid objects and then travels back, where the transmitter, now acting like a receiver, detects the signal. Because there are several transmitter/receiver discs stacked vertically and because these stacks rotate around the person, the device can form a complete picture, from head to toe and front to back.

It's the job of software in the scanner system to interpret the data and present an image to the TSA operator. The software creates a 3-D, black-and-white, whole-body silhouette of the subject. It also employs a feature known as automated target recognition, or ATR, which means it can detect threats and highlight them for easy identification. ATR technology is capable of detecting liquids, gels, plastics, powders, metals and ceramics, as well as standard and homemade explosives, drugs and money.

The ATR software also does something else. A scanner without this software forms images that reveal a person's unique topography, but in a way that looks like a crudely formed graphite prototype. In other words, you can see some physical features, but not with the same detail as Superman or backscatter scanners, both of which possess X-ray vision. A millimeter wave scanner with ATR software produces a generic outline of a person -- exactly the same for everyone -- highlighting any areas that may require additional screening.

Millimeter wave scanners aren't metal detectors. They actually peer through clothing to look for metallic and nonmetallic objects an individual might be trying to conceal. Getting a good view requires that passengers entering the scanner follow certain procedures. Here's what you can expect if you enter one of the approximately 600 mmw scanners in use at airports across the U.S. in 2012:

Either way, the scan takes less than 10 seconds and requires nothing painful or embarrassing. But if you feel strongly that the whole-body scan of a millimeter-wave machine violates your privacy, you can opt out of the screening process. You will, however, receive alternative screening, including a physical pat-down.

According to the TSA, most people prefer the scanning process to a physical exam. In fact, more than 99 percent of passengers choose to be screened by this technology over alternative screening procedures [sources: TSA]. And people with artificial joints or other implanted medical devices appreciate mmw scanners even more because they don't have to worry about the false positives associated with old-fashioned metal detectors.

As soon as the TSA began installing millimeter wave scanners, the public began asking questions, mostly related to privacy and safety. In the former category, people objected to the idea of strangers peering beneath their clothes to see intimate details or reveal evidence of mastectomies, colostomy appliances, penile implants and catheter tubes. A representative of the American Civil Liberties Union described whole-body imaging as "nothing more than an electronic strip search."

To quell the uproar, the TSA introduced several precautions on mmw scanners. One of those, as we've already discussed, involves installing automated target recognition software on a number of the machines. The software renders every subject as a generic outline, with suspicious areas highlighted. And if it doesn't detect anything suspicious in a scan, it displays the word "OK" with no image at all. For scanners without ATR software, the security operator viewing the resulting image sits at a remote location and communicates wirelessly with the agent operating the machine.

Of course, none of these measures protects a passenger from harmful effects of the waves themselves. Luckily, several studies have determined that millimeter wave scanners pose little risk to passengers, pilots or the TSA agents who operate the machines. The waves produced by these scanners are much larger than X-rays and are of the non-ionizing variety. Ionizing radiation has enough energy to remove electrons from atoms, but radio waves, visible light and microwaves don't have this ability. As a result, they don't alter the structure of biological molecules, such as proteins and nucleic acids.

The bigger issue with millimeter wave scanners seems to be the high number of false alarms. They can get fooled by objects that come in sizes close to the wavelength of the energy. In other words, folds in clothing, buttons and even beads of sweat can confuse the machine and cause it to detect what it thinks is a suspicious object. When Germany tested mmw scanners, security officials there reported a false positive rate of 54 percent, meaning that every other person passing through the machine required a pat-down that found no weapon or concealed object [source: Grabell and Salewski]. Because of these disappointing results, France and Germany stopped using millimeter wave scanners, leaving them no good alternative to scan fliers.

Millimeter wave scanners have caused a stir, but similar waves surround us every day and help us do things we now take for granted. For example, your cell phone relies on millimeter wave technology to send and receive data and calls. That smartphone activity occurs by way of communication satellites, which receive microwave signals from ground stations and then direct them, as downlink transmissions, to multiple destinations. Remember that electromagnetic waves come in a range of wavelengths. They also come in a range of frequencies, which is a measure of how many wave crests pass a certain point every second. Microwaves used in satellite communications are super-high frequency, or SHF, waves in the range of 3 gigahertz to 30 gigahertz (GHz).

NEXRAD, or next-generation weather radar, also uses waves in the 3 GHz range to help meteorologists make weather forecasts. NEXRAD relies on the Doppler effect to calculate the position and speed of rain, snow and weather fronts. First, a radar unit emits a pulse of energy, which travels through the air until it encounters an object, such as a raindrop. Then the unit listens for an echo -- energy reflected back to it from the object. By sending a constant stream of pulses and listening for echoes, the system is able to create a color-coded picture of the weather in a particular area.

Astronomers take advantage of extremely high frequency (EHF) waves in the range of 30 to 300 GHz to study the formation of stars and galaxies millions of light-years from Earth. Instead of traditional telescopes that sense light, these scientists use radio telescopes to "see" energy with millimeter and submillimeter wavelengths. Because structures on the ground can interfere with these waves, radio telescopes are usually placed at very high locations. For example, the Combined Array for Research in Millimeter-wave Astronomy (CARMA) encompasses 23 radio dishes in the Inyo Mountains near Big Pine, Calif.

So, millimeter waves are well-understood and quite common in a number of applications we regularly use. Even the microwave oven in your kitchen zaps food with a form of energy from this narrow band of the electromagnetic spectrum. Its adoption in airport security is a natural -- and harmless -- extension of the technology, especially when you consider the type of disaster it's trying to prevent. As of November 2012, the TSA has installed hundreds of mmw scanners at airports across the U.S. And internationally, they are being used in airports and mass-transit systems in several countries, including Canada, the Netherlands, Italy, Australia and the United Kingdom.