What is a fresnel lens?
A fresnel lens replaces the curved surface of a conventional lens with a series of concentric groves molded into the surface of a thin, lightweight plastic sheet. The grooves act as individual refracting surfaces, like tiny prisms when viewed in cross sections, bending parallel rays in a very close approximation to a common focal length which if designed properly can be more efficient at gathering light then a standard lens. This efficiency can be an advantage but there is a loss of image quality due to the visibility of the concentric rings in the lens surface. Higher grooved density can allow for higher quality images however. Because of these factors, fresnel lenses are a compromise between efficiency and image quality.
