B Type Fresnel Lens

Fresnel lenses are a type of lens that has only the curvature flattened to a plane.　Fresnel lenses, though plane, can act as spherical lenses. Since they are essentially flat, Fresnel lenses offer tremendous space- and weight-saving, and also enable focal lengths shorter than their aperture, a feat impossible with spherical lenses.
These Fresnel lenses are designed with the plano side facing the parallel light source. They are used as a light concentrator in solar photovoltaic generation systems, focusing sunlight on high-density solar cell elements to consolidate the solar energy. These lenses are also used in solar furnaces for processing materials at high temperatures.
Because they were designed to condense parallel light, condensing Fresnel lenses with short focal distances can be combined with a sensor, etc. for light-detecting applications.