A Fresnel lens is a plano convex lens that has been collapsed on itself to form a flat lens while retaining its optical characteristics . The resulting lens is thinner and has lower absorption losses, and it is the most extensively used lens for a wide range of detectors. The white plastic window often associated with motion detectors is, in fact, the flat side of a Fresnel lens. Among its advantages, this type of lens offers low cost and eliminates the need for a separate window for most applications. Detectors using Fresnel lenses are normally short- to medium-range devices, detecting objects up to about 30 m/100 ft. away.