Faced with the need to construct a large lens for a lighthouse of appropriate focal length, but unable to support the large weight of a double convex lens of that size, French physicist Augustin Fresnel (1788-1827) reasoned that it was the surface curvature which gave the focusing power. He reproduced the surface curvature of the thick lens in sections, maintaining the same focal length with a fraction of the weight. The lens strength in diopters is defined as the inverse of the focal length in meters.

 

There are two main types of Fresnel lens: imaging and non-imaging. Imaging Fresnel lenses use curved segments and produce sharp images, while non-imaging lenses use flat segments, and do not produce sharp images. As the number of segments increases, the two types of lens become more similar to one another. In theory, with an infinite number of segments, the difference between curved and flat segments disappears.