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Old July 25th, 2004, 05:50 AM   #1
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difference between Fresnel and Open-Face lights?

what is the difference between 'Fresnel' and 'Open-Face' lights? they seems identical in the specs.
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Old July 25th, 2004, 07:39 AM   #2
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A fresnel lens will focus the light beam via a lens.
The open face will just give a flood or wash of light. Usually, the fresnel can be focussed to give a spot, or widened to a flood.

Robin.
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Old July 25th, 2004, 07:42 AM   #3
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For your further information,
"A Fresnel lens is a type of lens invented by Augustin-Jean Fresnel. Originally developed for lighthouses, the design enables the construction of lenses of large size and short focal length without the weight and volume of material which would be required in a lens of conventional design."

Thanks to Google ;-)

Robin
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Old July 25th, 2004, 08:35 AM   #4
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Robin, you're right basically, but I would just point out that a lens is only one way to focus a light beam. Polished reflectors can also focus/concentrate light. For example a PAR lamp uses a reflector sealed inside the glass envelope to produce a concentrated beam (PAR is an acronyn for Parabolic Aluminized Reflector). In this case different textures are applied to the glass in front of the lamp to provide narrow, medium or wide beam spreads. ETC refined this concept a bit in the past few years with their version of the PAR lighting instrument which has a permanent aluminized glass reflector and interchangeable halogen lamps. You can put a clear glass lens over it to get the narrowest beam, or use several different textured lenses to provide wider spreads.

The beam projector is a more archaic sort of lighting instrument that used a metal parabolic reflector and a moveable lamp to acheive different beam spreads. The large searchlights you've seen in old war movies, or outside of movie theatres were also examples of tightly focused lighting instruments that used only a reflector and no lens.

The advantage to using a reflector as opposed to a lens is one of effeciency, when light passes through glass some intensity is lost and the energy is converted to heat. As you point out, the fresnel lens was a way to make the plano convex lens more efficient and lighter. The disadvantage is that the circular rings produce a pattern in the light. To minimize this effect the back (flat) side of the lens has a pebbled finish. While this makes the light more even and the lines less noticeable, it decreases the efficiency a bit and provides a less tightly focused beam. However this soft effect is generally regarded as desirable for the sort of things fresnels are used.

Anyway, since the topic came up I thought I'd share a bit of stage lighting lore :-) Robin is correct in that you would normally use a fresnel when you want a concentrated, adjustable light beam or an open faced unit to provide a larger, flatter wash effect.
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Old July 26th, 2004, 05:53 AM   #5
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thanx all for the replies. they were very helpfull.
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