Summary
A Fresnel is a lighting instrument that has a front lens element made of concentric circles that disperse light emissions in a directional manner. Most fresnels are capable of widening and tightening (flood & spot) the light beam as well.
Fresnel instrument types
Fresnel Lens Design
Fresnel lights can be found across the full range of nearly all lighting instrument classifications. There are HMI Fresnels, Tungsten Fresnels and LED Fresnels that range in size from the very small such as a Pepper Light, all the way up to the biggest HMI 18K or 24K Tungsten lamp heads. The Fresnel lamp is named after the frenchman who invented the shape of the lens that distinguishes a fresnel lamp from others. If you imagine a rounded glass lens element such as a magnifying glass, you can see how one solid thick piece of lens glass would be impractical for use in a large lighting instrument,
or even a medium or small instrument because of the size and weight of the lens of glass. Fresnel invented a single lens comprised of concentric circles of partially shaped convex glass curvatures. This design allows for a considerably lighter and smaller lens element that causes the shape of light to be concentrated and behave as if it is passing through a traditional fully convex piece of lens glass.