Marksmen in police raids need to know for sure who the good guys are. The University of Akron, Ohio, has been working on an invisible marker that is woven into clothing and gives a telltale sign when seen through a gun sight (WO 2004/19115).
The marker is a nanofibre spun from polydiphenoxyphosphazene and coated with erbium nitrate hydrate. When heated, the erbium emits infrared light. The fibres have a very small volume compared to their surface area, so their temperature rises fast enough for them to emit IR even when warmed by a human body. Viewed through a night…
