Fresnel lens

A conventional lens bends light through curved surfaces, but much of its thickness can contribute weight and absorption without being necessary to establish the desired refraction. A Fresnel lens divides the curved surface into concentric sections, each preserving an appropriate portion of the refracting profile. The resulting stepped structure can approximate the focusing action of a thicker lens with less material. Lighthouse Fresnel optics are assembled from shaped glass elements; many modern compact Fresnel lenses are molded as a single piece, often from plastic. The groove pattern and material affect efficiency, image quality and the intended use.

In the early 19th century, lighthouse optics were constrained by the weight and optical quality of thick glass lenses. French physicist Augustin-Jean Fresnel worked on improvements to lighthouse illumination and completed a design for a flashing lens in 1822. Its thin, shaped panels refracted light to form a more powerful directed beam. Fresnel lenses became an important advance in lighthouse technology, helping navigators distinguish lights and identify their characteristic flash patterns. Fresnel was not the first person to conceive of dividing a lens surface into rings, but his lighthouse design brought the approach into influential practical use.

A lighthouse Fresnel apparatus may be fixed to show a steady light or mounted to rotate around a lamp so that successive panels create a repeating flash. The characteristic pattern depends on the arrangement and rotation of the optical panels. Lenses were made in different sizes, commonly called orders: larger optics served prominent coastal lights, while smaller ones were suited to harbors, rivers and other shorter-range settings. Historically, the size and optical design of a lens helped match a light to its maritime role. Many lighthouses have since adopted modern lighting equipment, while historic lenses have been preserved in some towers and museums.

The same lightweight, thin-profile design can be useful wherever light needs to be collected, concentrated or directed without a large solid lens. Fresnel lenses appear in searchlights, signals, cameras and projectors, as well as in optical setups for magnification, projection or solar-light collection. Their performance involves trade-offs: groove geometry and manufacturing quality influence light loss, distortion and image resolution. They are therefore valuable in many illumination and light-gathering tasks, although conventional precision optics may be preferable when high image quality is the priority.