Guest researcher John Jendzurski prepares the NIST electromagnetic phantom for passage through the walk-through metal detector behind it. The carbon-polymer blocks of the phantom are arranged in a form that simulates the mass and height of the average A
Photo credit: Credit: M.E. Newman, NIST
In the comics, the Phantom is a masked crimefighter who protected the innocent from pirates, hijackers and other evildoers. While not as dashing or exciting as its costumed namesake, this electromagnetic phantomâ€”a carbon and polymer mixture that simulates the human bodyâ€”is being readied by the National Institute of Standards and Technology (NIST) for its upcoming role as a different kind of protector. The NIST phantom serves as a mannequin in a standardized performance test for walk-through metal detectors or WTMDs such as those used at airports.
Metal detectors currently are evaluated by using â€œclean testersâ€ (human subjects) who walk through the detector adorned with different types of innocuous metal objects, such as eyeglasses, belt buckles, watches, jewelry and coins, or by a piece of plywood pushed through the metal detector with the same items mounted on it. The disadvantage of using human subjects is that person-to-person variability in physical makeup and walking style and changes in a particular personâ€™s gait or position at each pass makes standardization impossible. The second method is reproducible, but it canâ€™t tell evaluators how a human body may impact the WTMDâ€™s ability to discriminate between weapons and innocuous objects.
The solution for both problems came from the lab. With funding from the U.S. Department of Justiceâ€™s National Institute of Justice (NIJ), researchers in NISTâ€™s Electromagnetics Division mixed a polymer with carbon blackâ€”a fine powder made almost entirely of elemental carbonâ€”to yield a low-cost, easily molded compound that can mimic the average electrical conductivity of the human body (which includes blood, bone, fat, organs, muscle and skin). The material is shaped into brick-like blocks and then arranged on a non-conductive fiberglass frame in a form that simulates the mass and height of the average American adult male.
Once assembled, the NIST phantom is placed atop a low-friction nonmetallic cart and passed through a WTMD at a speed of 0.5 meters per second by a computer-controlled actuator. This speed was selected because it is a common walking pace for an adult male. Engineers in NISTâ€™s Office of Law Enforcement Standards used the data from recent trials to design and support a reproducible process incorporating the phantom that will evaluate a walk-through metal detectorâ€™s ability to discriminate between threatening and non-threatening objects, such as the simulated eyeglasses or belt buckle. Plans call for the testing protocol to be considered in a future revision of the NIJ standard on metal detector performance.