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 fourth 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.

My clients often ask me What's the big deal about replacing that reflective mirror in their Overhead Projector? A mirror is just a mirror, right?"

Well in reality it is a very big deal. There is a huge difference between a front surface which is what your mirrors are in your Overhead Projector and a standard mirror which is what you would find in your home. A front surface mirror is just how it sounds. The mirroring surface is on the front, where there mirroring on a standard mirror is on the back side. If a standard mirror is used in your Overhead Projector it will shatter from the heat. The reflective surface of a front surface mirror does not absorb the heat like a standard mirror does. Generally there are two types of mirrors in your Overhead Projector.

Body Mirror: The Body mirror in most cases is the mirror found in the base (body) of your overhead projector. The purpose of this mirror is to reflect the transmitted light from the projection lamp to the mirror and then through the Fresnel lens and on to the focus head assembly.

Head Mirror: The head mirror sometimes referred to as the reflective mirror, is located in the head of the Overhead Projector. The purpose is to take the light that is reflected from the base, reflect it through the exit lens of the head and of course on to your wall or screen where you are projecting the image from your transparencies.

So the answer to the question "Can't I just have a mirror cut at my glass shop?" is a resounding no. Front surface mirrors in most cases will need to be purchased from an authorized parts distributor or directly from the manufacturer. The cost of these mirrors will likely be higher because they are a specialty mirror that is not easily sourced from a local glass shop.