From Wonder Woman’s Invisible Jet to the cloaking devices used by the Romulans and Klingons of the Star Trek universe, science fiction and popular culture are full of examples of completely invisible flying craft. Despite having its roots in fiction, the act of making an aircraft less visible to the naked eye has been an ongoing, but shadowy area of research and development for both private and military laboratories since the early days of military aviation.

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Evading radar systems, infrared sensors, and other sensors is the main strategy behind today’s modern stealthy aircraft, yet the ability to also avoid or delay visual detection remains near the top of the list of strategic aerospace technologies even in an age of increasingly advanced integrated air defense networks. Being able to detect an aircraft on radar is one thing, and even being able to hear it is another, but the ability to actually see an aircraft remains a huge vulnerability. Tight rules of engagement and the premium placed on using stealth and electronic warfare to penetrate into enemy airspace and even persist there for long periods of time mean that visual detection is still a major Achilles heel of many modern combat aircraft. Thus, the need for the ability to make an aircraft invisible as possible remains as pressing as ever.

While the world has yet to be shown evidence of an "invisible" aircraft or a high-end adaptive aircraft camouflage system that comes close to achieving such a goal, there is more than enough documentation originating from both the Department of Defense and associated private contractors to suggest that these technologies may be very much real. Aircraft like the B-2 already possess systems to reduce their visual signatures by minimizing the contrails they produce, and there is a documented pattern of research and development of other visual stealth technologies dating back the better part of a century that point to the real possibility that such capabilities do actually exist today, at least to a certain extent. The Yehudi Lights​​​​ Since the earliest days of mechanized warfare, militaries around the world have tested different methods of applying camouflage paint to structures, ground vehicles, aircraft, and naval vessels in order to reduce the distance at which they could be detected and positively identified with the naked eye. In recent years, computer-generated and more complex camouflage patterns have improved these techniques, although the effectiveness of these specialized paint jobs still depends on lighting and weather conditions and the angle at which an observer views the aircraft or ship, along with many other factors.

USAF A US Air Force F-16C aggressor in "BDU" splinter camouflage.

To get around these issues, aviation researchers in the midst of World War II tested methods of using light to camouflage aircraft, whether through controlling the light reflected off of aircraft using non-reflective paints or through a technique known as counter-illumination, which involves projecting light from the aircraft itself to help it blend in with its surroundings. It was in these early studies that the search for an invisible aircraft began in earnest. The counter-illumination concept is fairly simple. When a bright light is projected against the aircraft's skin, it is much harder to detect against the daytime sky as it no longer appears as a dark silhouette and literally blends in with the bright sky. Starting in 1943, the United States and Canada began testing a somewhat crude technology in which light bulbs were attached to the noses and leading edges of aircraft to reduce the distinctiveness of their silhouettes. The concept required setting the lights’ luminosity and color as close as possible to the appearance of the daytime sky. This tactic made it more difficult for opponents to visually detect and identify the aircraft from the critical head-on viewing angle. These Yehudi Lights, as they were known, could reduce the distance at which aircraft could be visually detected significantly. The U.S. Navy began a program to test the efficacy of these lights first on Grumman TBF Avengers, intending to enable them to better reach German U-boats before they could dive after spotting the aircraft, as well as improve the survivability of the aircraft, in general. According to a 1943 report commissioned by the National Defense Research Committee’s (NDRC) Office of Scientific Research and Development titled “Visibility Studies and Some Applications in the field of Camouflage”, the Yehudi Lights were fairly effective in many circumstances. “Under conditions such that an uncamouflaged plane was visible at about 12 miles,” the report said, “the plane equipped with Yehudi camouflage could approach to within 3,000 yards without detection, even when its approximate location was indicated by an accompanying uncamouflaged plane.”

DTIC A scale model of a B-24 fitted with Yehudi Lights.

That report also devoted a whole chapter to the “Perceptual Capacity of the Human Observer,” which detailed the extensive testing the NDRC conducted to determine the effectiveness of the Yehudi Lights and other luminous camouflage systems. The chapter outlined numerous research programs conducted by the Army-Navy Office of Scientific Research and Development (ANOSRD) Vision Committee, including lengthy studies into how well human observers could perceive and detect minute changes in luminosity and color or detect a luminous object against a similarly-lit background. Extensive testing was also conducted in the field that required researchers to develop custom spectrographs for measuring the luminosity and color of natural environments and erect 100-foot-high towers from which to suspend model aircraft fitted with various experimental luminous camouflage systems. Eventually, researchers developed a full-size model of a B-24 Liberator bomber to test the Yehudi Lights system. While these studies found that luminous camouflage systems could indeed reduce the distance at which an aircraft could be detected, these light bulb-based systems were highly dependent on the environments in which aircraft were flown and were only fully effective from certain viewing angles and distances. Slight changes in atmospheric conditions or weather could significantly impact the effectiveness of luminous camouflage, and the weight, aerodynamic penalties, and power requirements of the light bulbs made them impractical for many applications.

DTIC

It’s still not known how widespread the use of Yehudi Lights was and the 1943 NDRC report notes that, despite the fact that the Naval Air Station at Patuxent River in Maryland was asked to make the necessary changes to its aircraft to allow for installation of the lights, the project was still not completed at the time of the document’s publication. More Recent Versions Of Luminous Camouflage Obviously, the Yehudi Lights and other luminous camouflage techniques are most effective during daytime conditions, but there are ways of camouflaging aircraft at night, as well. That same 1943 NDRC report discussed several different variations of black paint that were either non-reflective or otherwise designed to help an aircraft blend in with the night sky, and similar paint schemes are still in use today. Modern stealth aircraft, such as the F-117 Nighthawk attack jet and B-2 Spirit bomber, are painted in matte black or dark grey and are flown at night to limit their visual signature. It’s unclear exactly what research pertaining to luminous camouflage the U.S. military conducted in the years following the Second World War, but during the Vietnam War, the U.S. Air Force tested light bulb-based systems similar to the Yehudi Lights, which could reduce the distance at which an aircraft could be visually detected. F-4 Phantom II aircraft were fitted with lights on their bellies and sides and painted sky blue and white in order to make them more difficult to be visually identified by enemy forces in what was known as the Compass Ghost project.

USN The Phantom was anything but discreet visually speaking.

The F-4 was large and its two General Electric J79 turbojet engines left a large, dark exhaust trail in its wake, making it easy for observers to spot and making the desire for visual camouflage a particularly pressing one in its case. While the Compass Ghost system was found to reduce the range at which the F-4 could be seen by up to a third, advances in enemy radar technology and the system's limitations meant the Compass Ghost configuration never made into widespread use. Decades later, in the early 1990s, Boeing's Phantom Works began testing the radical Bird of Prey aircraft in secret, which was designed to produce as little signature as possible across all forms - radar, infrared, acoustic, and visual. The aircraft remained in the black world throughout the '90s and was only revealed to the public in 2002.

Public Domain Bird Of Prey.