[simpits-tech] Fresnel Lens

[Rob Hommel]

Robert:

A regular glass cleaner should work fine for cleaning this lens, softly 
remove what ever dust and such as may be removed when dry prior to using a 
wet cleaner on the surface. Fresnel lenses should be treat in the same 
manner as any lens for determination of focal length and imaging a small 
amount of experimentation should quickly yield the best distance to get an 
objective or virtual image depending upon the manner of use you wish to 
engage your lens.

Some items you should be aware of when using a fresnel lens is the 
chromatic aberration you will observe near the edges of the lens. This may 
result in the appearance of color changes or you may see image distortion 
near the edges of the lens. The most perfect imaging will be toward the 
center of the lens. A bit of experimentation using the lens prior to making 
he investment of a new television set may save a bit from your bankbook. As 
a rough guess, without knowing anything about your lens I would say that a 
20 inch should give you a good image enough to fill your fresnel lens. You 
may wish to learn a bit more about fresnel lenses by ordering the Edmund 
Scientific optics catalog. A brief glance at some of the items that they 
sell will yield a bit about what you should expect from your lens.

 From the Edmond Scientific catalog
A Fresnel lens replaces the curved surface of a conventional lens with a 
series of concentric grooves, molded into the surface of a thin, 
lightweight plastic sheet.The grooves act as individual refracting 
surfaces, very close approximation to a common focal length. Because the 
lens is very thin little light is lost by absorbtion. Fresnel lenses are 
most often used in light gathering applications, such as condenser systems 
or emitter detector setups (lenses should be mounted with the grooves 
facing the distant light source). Fresnel lenses may also be used as 
magnifiers or projection lenses; however due to the high level of 
distortion, it is not recommended.

A further look at the information supplied by this catalog yields the 
information that the largest lens supplied by them has a focal length of 30 
inches. The lens is 35 inches in diameter.

Another good source of information on lenses and optics in general is the 
JML Direct Optics catalog. JML supplies Fresnel lenses for TV projection 
systems and the Fresnel lenses they sell are designed to be placed at a 
distance of 8 inches from the television projecter

. The focal length of the largest lens they sell is 770 mm from a 203 mm lens.

I highly recommend adding these catalogs to your library optics is not a 
difficult science for the lay person to get a basic grasp of and each 
catalog will give you a very good introduction to optical systems.

Keep 'em Flying
Rob Hommel

At 06:20 PM 4/27/2004, you wrote:
>I have a large fresnel lens, about 27.75" by 37". It's gotten kind of 
>dirty from sitting in the garage and has a few scratches (kind of large, 
>but on the edge of the lens so I don't think they'll cause much trouble). 
>I am planning on building a Simpit soon, and would like to use this lens 
>with it. I don't know anything about fresnel lenses. What type of display 
>should I use? This is a very big lens. I'm thinking of getting as big of a 
>TV as I can, since my video card has an S-video input. How far away from 
>the display should I mount the lens? I'm sure I'll have to work everything 
>out myself, but I could use some ideas.
>Also, what's the best way to clean this thing? It has quite a bit of dust 
>and dirt on it, and I don't want to damage it further.
>
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