Virtual strip search scanners won’t catch key explosives

The TSA is working to put full body visual scanners that see through clothes in the world’s airports. But a British newspaper says what I had thought myself, that they will just see through low-density chemicals such as those used by the Underwear Bomber:

Since the attack was foiled, body-scanners, using "millimetre-wave" technology and revealing a naked image of a passenger, have been touted as a solution to the problem of detecting explosive devices that are not picked up by traditional metal detectors – such as those containing liquids, chemicals or plastic explosive.

But Ben Wallace, the Conservative MP, who was formerly involved in a project by a leading British defence research firm to develop the scanners for airport use, said trials had shown that such low-density materials went undetected.

Tests by scientists in the team at Qinetiq, which Mr Wallace advised before he became an MP in 2005, showed the millimetre-wave scanners picked up shrapnel and heavy wax and metal, but plastic, chemicals and liquids were missed.

If a material is low density, such as powder, liquid or thin plastic – as well as the passenger's clothing – the millimetre waves pass through and the object is not shown on screen. High- density material such as metal knives, guns and dense plastic such as C4 explosive reflect the millimetre waves and leave an image of the object.