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.
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.
Diffused lighting camouflage
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.
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Counter-illumination is a method of active camouflage seen in marine animals such as firefly squid and midshipman fish, and in military prototypes, producing light to match their backgrounds in both brightness and wavelength. Marine animals of the mesopelagic (mid-water) zone tend to appear dark against the bright water surface when seen from below. They can camouflage themselves, often from predators but also from their prey, by producing light with bioluminescent photophores on their downward-facing surfaces, reducing the contrast of their silhouettes against the background.
Military camouflage is the use of camouflage by an armed force to protect personnel and equipment from observation by enemy forces. In practice, this means applying colour and materials to military equipment of all kinds, including vehicles, ships, aircraft, gun positions and battledress, either to conceal it from observation (crypsis), or to make it appear as something else (mimicry). The French slang word camouflage came into common English usage during World War I when the concept of visual deception developed into an essential part of modern military tactics.
Diffused lighting camouflage was a form of active camouflage using counter-illumination to enable a ship to match its background, the night sky, that was tested by the Royal Canadian Navy on corvettes during World War II. The principle was discovered by a Canadian professor, Edmund Godfrey Burr, in 1940. It attracted interest because it could help to hide ships from submarines in the Battle of the Atlantic, and the research project began early in 1941. The Royal Navy and the US Navy carried out further equipment development and trials between 1941 and 1943.
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