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Fresnel Light (pronounced "fre-nel") - a focusable spotlight used in film, television, and theater lighting, which can be adjusted via a knob on the back of the light from "spot" for a narrowly focused beam, to "flood" for a wider beam. This type of focusable lighting instrument is called a Fresnel because it features a Fresnel Lens, a glass lens with concentric ripples that is visible on the front of the light, casting soft, even illumination across the light's beam.
The Fresnel Lens is named for its inventor, the French physicist Augustin-Jean Fresnel (1788-1827), who designed the Fresnel Lens to efficiently project beams of light from lighthouses. Fresnel is also known for observing the Fresnel Effect, a phenomenon simulated today by Fresnel Shaders in computer graphics.
Source: Digital Lighting & Rendering by Jeremy Birn, definition and 3D images adapted from the book by permission. This page © 2001 by Jeremy Birn.