Both types of scanners rely on software to convert reflected electromagnetic energy into images. The exact configuration of the software determines the level of detail seen in the final example. For example, a backscatter machine with the most basic version of the software produces a whole-body silhouette of the subject that resembles a chalky sketch. Some details about a person's build and shape are visible in this image. With a privacy algorithm applied, however, the software blurs these details and only highlights potential threats.
Millimeter wave scanners also can produce images that reveal a person's unique topography, but in a way that looks like a crudely formed graphite prototype. Since their introduction, the TSA has armed these machines with automated target recognition, or ATR, software, which produces a generic outline of a person -- exactly the same for everyone -- highlighting any areas that may require additional screening. And that occurs only if the scanner detects something it perceives as suspicious. If it doesn't, it will display the word "OK" with no image.
For passengers, the process of being scanned is essentially the same in either machine. They must remove everything from their pockets, as well as belts, jewelry, lanyards and cell phones. Then they step up a small ramp, stand in the center of the machine, raise their arms, bent at the elbows, and remain motionless as the device does its thing. The only difference is the time required to complete a scan. For backscatter machines, the process takes about 30 seconds. For mmw scanners, it takes about 10 seconds.
Here's another difference, perhaps more significant than those 20 seconds. Backscatter machines rarely produce false alarms. According to one British study, their false alarm rate was about 5 percent [source: Grabell and Salewski]. Millimeter wave scanners don't perform as well. They can get fooled by folds in clothing, buttons and even beads of sweat. 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].