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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, robust, simple to clean and last many years. They make up nearly 100% of all mirrors
manufactured.
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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 fragile and cannot be touched or
rubbed without leaving marks. The surface is 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 but this is not a physical barrier as such. The less the mirror is handled the better. Treat a first surface mirror
as you would a delicate instrument and a long life will be obtained.
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Cleaning & Maintenance
Cleaning needs to be performed very carefully. Handle the mirror only with
plastic gloves to prevent further fingerprints or marks. Remove surface dust
with compressed air; photographic suppliers have pressure cans for cleaning of
camera parts & lenses; these are ideal. Air supplied motorised compressors will contain oil & contaminants
and blowing over the mirror with the mouth may leave saliva droplets!
Fingerprints and other marks are harder to remove. Never ever use paper
towel or any abrasive on the mirror and definitely never rub/wipe any cloth or
material onto the surface with any pressure. Marks may look unattractive but if
they are not disturbing the projected image then you are better leaving the
mirror as is. We have found the best way to clean is by 'flood cleaning' with a
mild liquid detergent and warm water. The same solution that you would use to do the
household dishes is fine. Completely wet the surface with the solution and use a
dish mop or other soft cloth and very very gently (use no pressure on the
mirror at all) wipe the surface so as to keep a film of detergent between the
mirror and the cloth. Let the solution dissolve and wash away the marks but
don't rub or bear down on the mirror. Complete the process with warm clean water
to rinse & remove traces of detergent. (Distilled water is best and will dry
clear) Most of the water will bead off the mirror but give the mirror a shake to
assist removal. Finally use compressed air or a hair dryer to dry out any
remaining droplets. Some water marks may appear but usually these can be removed
by a second clean or by very carefully and gently dabbing the spots with a micro
fine cloth before they dry. Remember; never rub or wipe the mirror
surface using physical pressure - this will only result in permanent scratches
which can never be removed. If the mirror has been scratched no amount of
cleaning will remove it. Trying to polish out scratches will result in only more
damage.
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
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