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.
They are widely used as field lenses for cameras, condenser lenses in overhead projectors, rear projector screens, for optimal gathering of solar energy, lenses in LED lighting, in optical sensors, etc.