Fresnel lenses are thin optical lenses specially cut and shaped with many concentric rings to function like thicker or heavier lenses in focusing concentrated light through a projector or lamp. They are used in lighthouse beacons and, more commonly nowadays, in cameras and overhead projectors. Fresnel lenses in lighthouses are made of glass, but camera and projector lenses typically are made of lightweight plastic. Cleaning a Fresnel lens in an overhead projector is a simple project that requires only a couple of easily acquired supplies.
Remove the smooth top layer of glass from your overhead projector. Each projector model will have a different method of removing the cap that holds the lenses down. Be sure to refer to your model's manual for specific directions.
Remove the Fresnel lens from your projector. Handle it very carefully to ensure you do not scratch or mar the surface in any way.
Wipe the lens with the soft lint-free cloth. Since it is protected by the covering glass, the worst dirt you should encounter will be dust, and the cloth alone should take care of most of it. If needed, you can moisten the cloth with distilled water and use that to gently wipe the lens clean.
Replace the Fresnel lens and the covering lens and secure them in the projector, following your model's instructions.
Do not use window cleaners or commercially prepared glass cleaners on a Fresnel lens as these can destroy the lens surface.