Article
First-order rotating catadioptric Fresnel lens, dated 1870, displayed at the Musée national de la Marine, Paris. In this case the dioptric prisms (inside the bronze rings) and catadioptric prisms (outside) are arranged to concentrate the light from the central lamp into four revolving beams, seen by sailors as four flashes per revolution. The assembly stands 2.54 metres (8.3 ft) tall and weighs about 1.5 tonnes (3,300 lb).
A Fresnel lens (/ˈfreɪnɛl, -nəl/ FRAY-nel, -nəl; /ˈfrɛnɛl, -əl/ FREN-el, -əl; or /freɪˈnɛl/ fray-NEL) is a type of composite compact lens which reduces the amount of material required compared to a conventional lens by dividing the lens into a set of concentric annular sections.