was detected at a range of about . It was noted at the time that this would force the enemy either to give up radar silence, making submarines easy to locate but harder to approach, or for observers to use binoculars continually. Since 8x binoculars at the time had a field of view of only 5 degrees, whereas enemy submarines at the surface kept watch with three observers each assigned a 120 degree arc, the camouflage was considered effective. Later developments The ability to approach a target unseen was rendered obsolete by advances in radar in the 1940s and 1950s. Since the development of stealth technology, Yehudi lights have attracted renewed interest, first in 1973 when McDonnell Douglas researched a "Quiet Attack" aircraft for the Office of Naval Research, modifying the F-4 Phantom with Yehudi lights on its underside, and later in the 1970s when Lockheed Martin's Skunk Works was contracted to develop a stealth aircraft prototype Have Blue, which helped to guide the development of the F-117A stealth attack aircraft and the B-2 stealth bomber. 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. References Aircraft external lights Military camouflage Prototypes Military research