observer, his angle of view, the position of the sun, and the presence of haze or clouds. Altitude is extremely important. A jetliner at its cruising height always appears brightly lit in the sky, because dust and moisture in the air beneath
the aircraft scatter light onto
its underside. There are
relatively few particles of
dust and water in the thin
air above the airplane.

So the higher the plane
flies, the more light is
scattered onto it,
and the darker the
sky behind it. The
dark color that
absorbs as much
light as possible
provides the best
camouflage for
a high-flying
airplane.

So the higher the plane flies, the more light is scattered onto it, and the darker the sky behind it.

The dark color that absorbs as much light as possible provides the best camouflage for a high-flying airplane. But even the jet-black Blackbird and U-2 spyplanes look brighter than the skv when seen from below as they cruise at 80,000 feet. At lower altitudes, there is less light-scattering atmosphere below the aircraft, so lighter colors provide the least contrast.

For Have Blue, Lockheed devised a scheme of graduated grays, lighter on the bottom and darker on top. The aircraft's designers also planned to test light apertures, which would be installed on the sides and undersurfaces of the airplane, about two feet apart. (Seen from a distance, the individual lights would blur into a single image.) The apertures would be connected to a central light source by fiber-optic lines, and controlled by sensors on the upper side of the aircraft. The sensors would "read" the background light and adjust the skin's luminance to mirror it. This system never flew on Have Blue, possibly because the

first aircraft was lost in an accident. Work on visual stealth continued, however. In 1980, the Air Force tested a small aircraft, probably unmanned, under a project

known as IMCRS (what the acronym stands for is not known). The aircraft's lower wing skins incorporated slit-like Fresnel lenses to beam light ahead of and below the aircraft, in the direction of the most likely threats.

The IMCRS experiment may have been related to a Defense Advanced Research Project Agency program known as Active Camouflage. Under that program, a small, powered drone was fitted with fluorescent lamps and tested at the White Sands Missile Range with so much success that the project has since been reclassified as Top Secret.

Neither of these lighting systems were adopted for stealth aircraft in the 1980s. They were complex to install, and their effects were difficult to predict and test. Carefully designed conventional camouflage worked well enough under most circumstances to ensure that an aircraft would not be visible before a radar could detect it.

So why were the first F-117s painted soot black instead of a toned gray scheme that would provide better camouflage? One Lockheed engineer recalls that the commander of Tactical Air Command "didn't believe that real fighter pilots flew pastel-colored airplanes." One Air

Force source close to the program says that some senior officers doubted the F-117 could survive in daylight, and wanted to ensure that nobody would try it.

Color Counts
Light colors would be optimal for the un- derside of the future Joint Strike Fighter. which will fly relatively low for ground at- tacks. Some experts say the best color for a fighter Is pink, but pilots may object.

The higher an aircraft flies, the darker it should be to hide from enemies. The F-117 was originally painted a dark black, but has recently been seen in gray.

Black is one of the least stealthy colors for daytime flying at medium altitudes. In fact,the British Roval Air Force is painting its trainers black to make them more visible and reduce the risk of collisions. Black isn't much good at night either, because there is nearly always some light from the moon. That's why the latest F-117s have been seen in a more sensible gray color.

The B-2 stealth bomber's underside is a very dark gray. Many people think that it is designed to attack only at night, like thc F-117. This is unlikely, because thc B-2 was designed to bomb Russia, and the most direct route from the United States lies smack across the Arctic Circle, where thc sun shines 21 hours a day for a large part of the year.

The B-2's underside is dark because it cruises at altitudes as high as 50,000 feet, where a dark gray blends into the sky. It does not use an "active camouflage" lighting system, but it may have an upward-facing light sensor that tells the pilot when to increase or reduce altitude to match the changing luminance of the sky. It appears likely that active camouflage will make a comeback in the 2000s.