The heart of the device is a block of electronics about the size of a 1990s tower personal computer. It comes housed in a musician's black case, akin to the one Spinal Tap might use on tour. At the front: a large, square white plate, the terahertz camera and, just above it, an ordinary closed-circuit television (CCTV) camera. Mounted on a shelf inside the case is a laptop that displays the CCTV image and the blobby terahertz image side by side.
An operator compares the two images as people flow past, looking for unexplained dark areas that could represent firearms or suicide vests. Most images that might be mistaken for a weapon -- backpacks or a big patch of sweat on the back of a person's shirt -- are easily evaluated by observing the terahertz image alongside an unaltered video picture of the passenger.
It is up to the operator -- in LA's case, presumably a transport police officer -- to query people when dark areas on the terahertz image suggest concealed large weapons or suicide vests. The device cannot see inside bodies, backpacks or shoes. "If you look at previous incidents on public transit systems, this technology would have detected those," Sotero says, noting LA Metro worked "closely" with the TSA for over a year to test this and other technologies. "It definitely has the backing of TSA."
How the technology works in practice depends heavily on the operator's training. According to Evans, "A lot of tradecraft goes into understanding where the threat item is likely to be on the body." He sees the crucial role played by the operator as giving back control to security guards and allowing them to use their common sense.
I am quoted in the article as being skeptical of the technology, particularly how its deployed.