Both types of scanners give off energy in the form of 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 various sizes, or wavelengths. Backscatter machines produce low-energy X-rays, which have a wavelength on the order of 0.0000000001 meters, or 0.0000001 millimeters. Millimeter wave scanners produce a special type of microwaves with wavelengths that fall in a range exactly between 0.001 meters (1 millimeter) and 0.01 meters (10 millimeters). In other words, the waves emitted by mmw scanners are much larger and therefore have less impact on small structures, such as human proteins and nucleic acids.
In terms of construction, a single backscatter machine includes two radiation sources so that both the front and the back of the person can be imaged without producing any blind spots. Each radiation source is housed in a rectangular structure resembling a large industrial freezer. The two units face each other with a gap between them big enough to accommodate a person.
A millimeter wave scanner, however, looks a lot like an oversized, hexagonal telephone booth. Two of its six panels are open to serve as an entrance and an exit, while four of the panels sport transparent glass or plastic. Two stacks of disc-shaped transmitters, each surrounded by a curved protective shell known as a radome, sit inside the wall of the structure and pivot 180 degrees around a central point.
Now that we've gotten the gist of the structures, let's see how they produce images of you.
Backscatter machines use rotating collimators to generate X-rays, which pass through a slit and strike a passenger standing inside. The X-rays penetrate clothing, bounce off the person's skin and return to detectors mounted on the machine's surface. The radiation also bounces off weapons, explosives or other threats concealed in clothing or lying against the skin. By sensing and analyzing this so-called backscatter, the machine is able to create an image of a person, as well as any organic or inorganic items carried on that person.
Millimeter wave scanners use small, disc-like transmitters to make an image. 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. One disc would only scan a small portion of the test subject, so a single machine contains two stacks of discs connected by a bar that pivots around a central point. 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.