Aviator sunglasses are a style of sunglasses that were developed by a group of American firms. The original Bausch & Lomb design is now commercially marketed as Ray-Ban Aviators, although other manufacturers also produce aviator-style sunglasses. Design Aviator style sunglasses are intended to be worn under headgear and are characterised by dark, sometimes reflective lenses and thin monel, steel or titanium metal frames with double or triple bridge and bayonet earpieces or flexible cable temples that hook more securely behind the ears. The large lenses are not flat but slightly convex. The design attempts to cover the entire field of vision of the human eye and significantly reduce the amount of transmitted visible light and (near) infrared radiation and prevent (erythemal) ultraviolet radiation from entering the eye from any angle. For selecting sunglasses, the United States Federal Aviation Administration has published an aeromedical safety brochure for general aviation pilots, commercial pilots and physicians. Polarized and photochromic lenses are not recommended for use by pilots. The UK Civil Aviation Authority has also provided guidance on the use of sunglasses by pilots. History U.S. Army Air Corps D-1 sunglasses The first aviator style sunglasses contracted by the US military in 1935 were the U.S. Army Air Corps D-1 Sunglasses made by American Optical. They have a conspicuous USAC engraving on the hinged bridge. The D-1 flying goggle assembly was standardized on 13 August 1935, and was actually a pair of sun glasses with a rigid frame and plastic insulated arms. The D-1 sunglasses were superseded by the more comfortable AN6531 flying sun glasses (comfort cable) in November 1941. AN6531 military sunglasses In the second half of the 1930s and early 1940s, a group of American firms kept developing sunglasses. The military "flying sun glasses (comfort cable)" were standardized in November 1941. They were produced in huge quantities (several million pieces) for pilots and sailors. The lenses were made to a joint standard shared by the U.S. Army Air Corps and the U.S. Navy. As a result, the lens carried an "AN" (Army/Navy) specification number: the AN6531. The U.S. Government specified the shape of the lens and the color, which was initially a green tinted lens that transmitted 50% of incoming visible daylight. This AN6531 Type 1 lens proved insufficient to protect pilot's eyes from sun glare so this lens was superseded by the darker AN6531 Type 2 lens in rose smoke. Various