Wednesday, August 20, 2008

What Is A Fresnel Lens

Writen by Mark Boehm

Over the past 25 years I have had the unique opportunity to talk directly with many of the professionals and instructors who use Overhead Projectors as an integral part of their profession. Through these interactions I have accumulated notes and information that has inspired me to write these articles that pertain to some of the most common problems experienced by owners of today's and yesterday's Overhead Projectors. This is the seventh article in a series of articles that will be written from a professional Electronics Technicians point of view in regards to some of today's most common Overhead Projector problems and questions.

What is a Fresnel Lens?

If you have ever looked at the lens of your Overhead Projector (the lens that sits under the glass where you lay your transparencies) you may be reminded of a magnifying glass. And in some ways you are right. The idea of the Fresnel Lens in your Overhead Projector is to focus the beam of light produced by the projection lamp, projecting it to the optics in the head assembly and on to your wall or screen. Unlike a conventional magnifying lens that is thick in the middle and tapers down to nothing at the edges, the Fresnel lens is a single thickness all the way across.

This thin piece of plastic magnifiying lens found in your Overhead Projector is called a Fresnel Lens. It is flat on one side and ridged on the other. In most cases there are two pieces of this thin acrylic lens placed together often referred to as a dual element Fresnel Lens. Fresnel lenses were first used to focus the beam of light in lighthouse lamps. These plastic lenses are used where a concentrated light is required. The Fresnel lens serves the same purpose in your Overhead Projector where a concentrated beam of light is required to project your image on to a screen or wall.

How do I clean my Fresnel Lens?

The typical overhead projector has a top piece of glass (stage glass) and just below that is a plastic lens with lots of circular lines on it (the Fresnel lens). Most Fresnel Lenses are marked on one side indicating which side goes toward the lamp. If you know how to remove it, just take it out and reinstall it the correct way. You should never use glass cleaner on the Fresnel Lens,it can destroy the plastic lens. To clean your Fresnel Lens a soft cloth dampened with water or cleaner approved by the manufacturer should be used. A word of caution here: handle the Fresnel lens with care to avoid scratches or other types of damage,this lens is a very expensive part of your Overhead Projector.

After you re-install the Fresnel Lens and you suddenly get a blurry round spot instead of a clear image from your Overhead Projector, you will find that the Fresnel Lens is probably installed upside down. No need to panic, simply remove the lens and re-install the correct way. This is a common problem for those who have removed this lens while cleaning the dust out of their Overhead Projector.

Where can I purchase a new Fresnel Lens?

Fresnel Lenses can be purchased from the manufacturer or from an authorized parts distributor.

Mark Boehm is the president of M-B Electronics. He has over 25 years of experience in the Audio Visual and Electronics Industry. You can contact M-B Electronics at 800-872-9456 or at:

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