A Fresnel spotlight is a directional stage light that produces a very soft-edged, controllable beam of light for back, side or down lighting. The theatrical spotlight is identified by a lens, which consists of a series of concentric rings on its surface, fitted at the front of the optical chamber. The stepped lens was named after AugustinJean Fresnel, a French physicist and engineer who developed this cost-effective, lightweight alternative to conventional continuous surface optics for use in lighthouses in 1823. A Fresnel lens is basically the subtraction of material from a plano-convex (PC) lens. The concentrically aligned ring-shaped lens segments act as individual refracting surfaces which bend parallel light rays to a common focal length. As a result, a Fresnel lens is capable of focusing light with effectiveness comparable to that of a conventional PC lens with a corresponding shape or curvature. While breaking the lens into concentric circles makes it considerably flatter, lighter and less expensive. The rear side of the lenses can be structured to mask visible irregularities in the light distribution when required. This distinctive feature of Fresnel lenses allows to soften the beam edge.
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