There are several types of lenses
==The Plano-Convex Lens==
Sometimes called a P-C lens it is used in older ERS’s to redirect the spreading rays of light coming from the source of the reflector. It has one flat (plano) surface and a second outwardly curved (convex) surface. It is the simplest and least expensive lens for concentrating spreading rays into a compact and bright beam of light.
- Each P-C lens has a focal point just like a reflector. If parallel light rays strike a lens they will be bent to converge at the focal point. Conversely if a source is placed at the focal point of a lens all the rays of light that emerge from the lens will be parallel to one another.
- The distance from the focal point to the center of the lens is called focal length. Lenses are identified by two numbers. The first indicates the diameter and the second the focal length. So a 6x9 has a diameter of 6”s and a focal length of 9 inches. You can figure out a lens’ focal length by taking it outside holding it plano side down so the suns rays are concentrated on the ground (just like using a magnifying glass to set fires). Then measures from the ground to the lens and you have its focal length.
Pronounced FRE-NEL is named for its inventor Augustin-Jean Fresnel. It was originally created to be used in Light Houses. It has distinctive ridges that are used to create a smooth and soft beam of light.
A PAR lamp uses its lens to redirect the nonparallel rays of light emanating from the front of the lamp. Because of its parabolic reflector the quality of light from a PAR is very harsh. In contrast, the beam edge remains diffuse to the action of the lens. There are typically 4 types of Par lens used.
- Very Narrow
- Narrow- Looks frosted
- Medium- Has long rectangles
- Wide- Has more smaller rectangles than the Medium lens
Other Lenses to be aware of:
- Step Lens: Looks somewhat like a Fresnel lens but creates ugly concentric rings when light passes through it.
- PARNEL: ETC’s version of a hybrid PAR/Fresnel fixture--it consists of two wave lenses that rotate against each other to go from spot to flood.
- Pebble Convex: Similar to plano convex except that the plane side has a pebbled finish. Creates a similar pool of light to a fresnel with slightly harder edges.
- Micro-Fresnel. A plastic lens used for many manufacturers' 10° and 5° ERS units. Also found in the ubiquitous Overhead (transparency) Projector in every classroom.
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