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"
Canadian origins
thumb|left|upright=1.4|A maritime patrol [[Consolidated PBY Catalina|Catalina, painted as bright as possible—white—to minimise visibility against the sky, still mainly appears dark. Yehudi lights match brightness better by generating light.]]
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 that such counter-illumination camouflage was possible, arousing interest in both Britain and America, but the equipment was cumbersome and fragile, and neither the Royal Canadian Navy nor their allies brought it into production.
US Navy research project
Goal
Yehudi lights were developed by the US Navy to help counter the "menace" of German submarines to Allied shipping in the North Atlantic. The United States Army Air Force's Director Of Technical Services (DTS) asked the camouflage section of the National Defence Research Committee (NDRC) to develop a camouflage method that would allow a radar-equipped, sea-search aircraft to approach a surfaced submarine to within 30 seconds' flight time before being seen. This was to enable the aircraft to drop its depth charges before the submarine could dive.<!--paraphrased PD text; or use quote below-->
The Have Blue prototype was disruptively camouflaged to disguise its shape from casual onlookers, as well as being constructed of angled facets to reduce its radar cross-section. The use of any form of active camouflage, whether Yehudi lights or microwave emissions, was rejected.