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How Fresnel Luminaires Work

 
Fresnel Lens

The Fresnel Lens

The fresnel (pronounced: 'fren-el') lens is unique in its design and is easily identified by the stepped concentric rings that form the surface of the lens.

The fresnel lens is named after its inventor Augustin Fresnel (1788-1827) who developed the lens for use in lighthouses to solve the problems presented by the basic plano-convex lens which was less efficient, too heavy and prone to cracking.

The fresnel lens has since become one of the most popular lenses used in luminaires for stage lighting, largely for the same reasons.

Fresnel Spotlight Optical System

The Fresnel Luminaire

The fresnel luminaire is easily identified by looking at the lens from the outside of the luminaire where the concentric rings are easily seen.

Fresnel luminaires produce a soft edged beam of light that is brightest in the centre and gradually darkens toward the edges. This characteristic makes blending the light beams between adjacent fresnel luminaires into a continuous pool of light of even brightness quite easy.

The diagram (left) shows the basic optical system of a fresnel luminaire.

Fresnel and Plano-Convex (PC) luminaires belong to the same family of "focus spots" with the only difference between the two being the type of lens that is fitted. Both types use a spherical reflector which, in conjunction with the single lens, provides a low cost optical system albeit a little less efficient than the more complex optical systems used in profile luminaires. The beam angle of focus spots is adjustable over a wide range, typically from a narrow spot of 6-10 degrees to a flood of 60-65 degrees. This adjustment is achieved by moving the lamp and reflector relative to the lens which is fixed in position on the body of the luminaire. Some models achieve this movement by means of a locking knob at the bottom of the luminaire that you loosen then slide back or forward while other models have a screw thread system with a knob at the back and.or front of the luminaire that is rotated to move move the lamp tray. Moving the lamp closer to the lens increases beam width towards its widest flood setting while moving it away from the lens reduces the beam width towards its narrowest spot setting.

An accessory called a barndoor is usually fitted to the front of focus spots to provide a means of controlling the edges of the beam.

For safety, most good quality fresnel luminaires have an electrical safety switch that automatically disconnects the power to the lamp when the lamphouse if not locked fully closed. An additional safety option is a lens safety mesh that prevents pieces of the lens falling out of the luminaire if the lens breaks.

Typical Uses

The fresnel luminaire is the workhorse of all theatre luminaires. Fresnels are very versatile luminaires that are often used for stage colour washes, as well as for selective highlighting. The ease of blending the light beam from one fresnel with that of an adjacent fresnel makes them quick to point and focus onto the stage ready for use.