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First Surface Mirrors - For Projection & Planetarium Applications

What's the difference between a First Surface and Second Surface Mirror?

Your normal every day mirror is manufactured so that the  reflective coating is deposited onto the rear side of a clear substrate (usually glass or plastic) and protected with a backing paint on the rear. These are 2nd surface coated mirrors. They are perfect for every day applications as the clear substrate protects the reflective coating from any damage and allows for an image to be seen by the eye. The rear side is also protected by the backing paint to prevent scratching and oxidising of the fragile reflective surface. These mirrors are tough and robust, simple to clean and maintain and last many years. They make up nearly 100% of all mirrors manufactured.

First surface mirrors as the name implies has the reflective coating on the  first surface (top exposed surface) of the mirror. This reversal  is necessary whenever a mirror is to be used to project an image onto a screen. If you use a normal mirror to project an image, the light rays must travel through the clear substrate before reflecting back through the material so as to project an image. This causes a major problem as the light rays will be reflected twice at slightly different angles (caused by the thickness of the substrate). When an image is projected onto a screen the observer will see the main image (from the reflective coating) and also a secondary and duller image caused by reflection from the top surface of the substrate. This makes the process unsatisfactory for projection work. By placing the reflective coating on the top of the substrate no secondary image can occur and the result is a sharp precise reproduction. 

Since the reflective coating is now exposed to the physical world proper care and handling is absolutely essential. Our mirrors use a deposit of aluminium to create the reflective surface. This coating is extremely fragile and cannot be touched or rubbed. The surface is very soft and absolute care is required at all times if the mirror is to remain in its original state. To help protect the surface we apply a polymer coating to the surface which provides an air barrier to reduce oxidising but this is not a physical barrier. If you wipe the surface then you wipe away the polymer coating! The less the mirror is handled the better. Treat a first surface mirror as you would an extremely delicate instrument and a long life will be obtained. 

University of Western Australia
Paul Bourke from the University of WA is the designer of projection systems employing first surface mirrors of a hemispherical shape and it is through his efforts that an affordable alternative to expensive projection lenses is now a reality.  We gratefully acknowledge his work in this field and refer you to him for details on methods and precise technical information. Please see the following links for additional information, more pictures and installation methods.

http://local.wasp.uwa.edu.au/~pbourke/
http://local.wasp.uwa.edu.au/~pbourke/projection/domemirror/
http://local.wasp.uwa.edu.au/~pbourke/exhibition/domeinstall/
 

**Images supplied courtesy - Paul Bourke, University WA

Maintenance of First Surface Mirrors
Firstly if you don't need to handle the mirror - don't! If you must; then always use either non-powdered surgical gloves or gloves used for handling photographic film and negatives. If you touch the surface with your hands you will leave a finger mark which will be extremely difficult to remove without damaging the surface. If *finger marks are present, they may be unsightly but not necessarily causing problems to the projected image. Similarly dust particles may build up on the surface which may look unattractive but may not necessarily cause any image degradation. If dust needs to be removed use filtered compressed air. Photographic shops supply special pressure pack's designed for removing dust from lenses etc. Use this or similar products. Do not blow across the mirror with you mouth as you will leave small deposits of saliva! Unfortunately if you have scratches on your mirror there is no way of removing them. You will either have to put up with them or replace the mirror.  These mirrors require very special care and although the above may sound daunting, simple but strict rules for handling will see the mirror last a very long time. As the mirror surface is exposed it will eventually oxidise so the life of the mirror will depend on many factors but the main one is proper handling! The mirror should be considered  a disposable item (like a projection lamp) and it life is dependent on many factors but handled correctly you can expect to get good service from it.

*If you wish to attempt this please email us so that we can advise the best method.