Fresnel lens

The Fresnel lens was developed by the French physicist Augustine-Jean Fresnel for use in lighthouses. It was based on previous work on development of a large burning lens. The advantage of the Fresnel lens is that it has a large aperture and a short focal length, allowing the light from the lighthouse to be visible to mariners at long distances without excessive weight for the lens or excessive luminosity for the light source.   The lens assembly consists of a set of concentric annular sections referred to as Fresnel zones, which function as prisms to redirect light from the source to a horizontal band. The first Fresnel lens was installed on the Cordouan lighthouse at the mouth of the Gironde estuary in France in 1823. This lens soon became the standard against which all other lighthouse lenses were judged. Classic Fresnel lenses for lighthouses were grouped into six orders, based on their focal length. A first order lens had a focal length of 920 mm and an optical area 2590 mm high. The entire assembly was about 3.7 meters tall and 1.8 meters wide. The sixth order lens (the smallest) had a focal length of 150 mm and an optical area of 433 mm. Fresnel lenses were difficult to produce and, hence, expensive. The US Lighthouse Service, like various other lighthouse authorities worldwide, acquired their lenses directly from the source – the Fresnel factory in France. The US Coast Guard continued the practice when the Lighthouse Service was absorbed in 1939. Eventually, though, the Coast Guard transitioned to using cheaper and lighter aero-beacons in its lighthouses (they are easier to maintain and automate than traditional lights). The Coast Guard still has a number of classic Fresnel lenses in use along the coasts and on the Great Lakes. The retired Fresnel lenses are valued – and valuable – collector items.