Fresnel Lenses (for Rear Projection TV)
This is an explanation of the fresnel (pronounced "fur-nell" or "frenell") lens panel in a rear projection TV.
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In a Nutshell
The screen of a rear projection TV set (RPTV) has two or three layers. The panel with a concentic circular panel, seen only if you look inside, is the fresnel lens. It redirects the light rays to all be parallel, directly out from the screen.
If you ever dismantle the screen of an RPTV, you must be sure to re-install the fresnel lens with the ridged surface facing forwards. Also the fresnel lens must be behind the lenticular lens (ribbed) panel or frosted (diffusion) panel.
Just before reaching the screen, the light rays from the projection unit down below are ever spreading out (diverging). The purpose of the fresnel lens is to aim, or redirect, all of the light rays to be parallel, directly out of the TV set.
An ordinary convex lens will do this job. But it must be as large as the screen and it would be thick, heavy, and expensive.
A fresnel lens has the same curvatures as an ordinary lens, redirects (refracts) the light the same way, but is collapsed down. For the RPTV, the fresnel lens has thousands of ridges in a circular pattern and is a panel about one eighth to one quarter of an inch thick overall.
An ordinary lens can have the curved surface facing either way, requiring only minor calibration differences for focusing. A fresnel lens must be positioned so the ridged surface is on the side of the parallel rays, which means outwards for an RPTV.
Note that on the fresnel lens, some of the surfaces of the ridged side are perpendicular to the flat surface and the other surfaces are not.
The light rays will miss the perpendicular surfaces when the flat side is facing inwards, towards the projection units. This ensures that all of the light rays come out parallel.
With the fresnel lens reversed, light rays will hit the perpendicular surfaces. (An ordinary lens does not have these perpendicular surfaces.) When this happens the rays will go off in many different directions (scatter). The rays don't have to go far (just another 1/8 to 1/4 inch) before reaching the front screen surface but you will see excessive haloing.
The lenticular lens is also a panel 1/8 to 1/4 inches thick. It takes some light rays from each spot on the screen and redirects them to each side while directing less light upwards and downwards. This gives a more even brightness for viewers sitting off to the sides. The diffusion panel (a frosted panel optionally used instead of the lenticular lens) does not have the graininess caused by the rib spacing of a lenticular lens. It allows more light to travel upwards and downwards necessitating more brightness from the projection unit to give the viewers an equivalent picture.
Fresnel lenses come in different shapes for different purposes. Some are equivalent to concave lenses as opposed to convex lenses. Click here for more information: http://www.3dlens.com
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All parts (c) copyright 2000, Allan W. Jayne, Jr. unless otherwise noted or other origin stated.
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