Yehudi lights are lamps of automatically controlled brightness placed on the front and leading edges of an aircraft to raise the aircraft's luminance to the average brightness of the sky, a form of active camouflage using counter-illumination. They were designed to camouflage the aircraft by preventing it from appearing as a dark object against the sky. The technology was developed by the US Navy from 1943 onwards, to enable a sea-search aircraft to approach a surfaced submarine to "within 30 seconds of flying time" before becoming visible to the submarine's crew. This in turn enabled the aircraft to engage the submarine with depth charges before it could dive, to counter the threat from German submarines to allied shipping. The concept was based on earlier research by the Royal Canadian Navy in its diffused lighting camouflage project. Yehudi lights were unused in the war and were made obsolete by advanced postwar radar. With 1970s improvements in stealth technology, they again attracted interest. Etymology A US National Defense Research Committee report on the history of the project explains in a footnote that the name "Yehudi" in then-contemporary slang meant "the little man who wasn't there". The slang may perhaps allude to the popular catchphrase and novelty song "Who's Yehudi?" or "Who's Yehoodi?". The catchphrase is said to have originated when violinist Yehudi Menuhin was a guest on the popular radio program of Bob Hope, where sidekick Jerry Colonna, apparently finding the name itself humorous, repeatedly asked "Who's Yehudi?". Colonna continued the gag on later shows without Menuhin, turning "Yehudi" into a widely understood late 1930s slang reference for a mysteriously absent person. Canadian origins The use of Yehudi lights to camouflage aircraft by matching their luminance with the background sky was developed, in part, by the US Navy's Project Yehudi from 1943 onwards, following pioneering experiments in the Canadian diffused lighting camouflage project for ships early in the Second World War. A Canadian professor, Edmund Godfrey Burr, had serendipitously stumbled upon the principle when he saw an aircraft coming in to land over snow suddenly vanish. He realized that the reflected light had increased its brightness just enough to match the background sky. The ships were fitted with ordinary projectors mounted on small platforms fixed to their sides, with the projectors pointing inwards at the ship's side. The brightness was adjusted to match the brightness of the sky. The Canadian experiment showed