MOONWARD
VACUUM COATINGS
MIRROR
COATING SERVICES
Protected
Aluminum (Al+SiO/SiO2)
Reflectivity 87%
Aluminum can be overcoated with certain dielectric films to prolong the reflectivity and durability compared to naturally aged aluminum mirrors. To make an aluminized mirror more durable, both mechanically and chemically, a thin hard film of silicon monoxide/silicon dioxide (SiO/SiO2) is evaporated onto the aluminum. This overcoating is in essence a quartz film which tremendously improves the mar and corrosion resistance of aluminized mirrors.
In return for providing exceptional durability, a slight penalty is paid in reduced reflectivity due to small light absorbing effects in the thin film. To minimize these light absorbing effects while still providing maximum protection, the SiO/SiO2 film deposit is of ½ wave optical-interference thickness optimized for 5500 .
Enhanced
& Protected Aluminum (Al+MgF2+CeO2)
Reflectivity 95%
The reflectivity of aluminum can be improved (up to 95%) when overcoated with multi layer dielectric optical-interference films of alternating low and high refractive indices. By evaporating a ¼ wave optical thickness of magnesium fluoride (MgF2) followed by a ¼ wave of cerium dioxide (CeO2) onto freshly deposited aluminum, an increase in reflectivity can be obtained.
With enhanced coatings, the dielectric films do much of the reflecting leaving less light to be reflected and therefore absorbed by the base metal (aluminum). A constructive interference effect is produced at each film's interface producing a total of three reflections that add in phase together to give an overall reflection greater than the bare aluminum. Typically, the gain for enhanced aluminum can peak as high as 95% for a denoted portion of the visible spectrum.
Enhanced coatings have served well particularly in instruments where multiple reflections ( 2) occur.
Over coated mirrors, either protected or enhanced, will show a faint colour when viewed at a glancing angle under white light. These colour(s) are the result of constructive interferences produced by the dielectric thin film(s), and are usual for overcoatings. When viewed face on, the residual colour will disappear. This is due to the coating thickness(s) being optimized for maximum reflectivity at 5500 where the human eye is most sensitive.
The dielectric films (MgF2 and CeO2) used to enhance
aluminum provide similar protective attributes (both mechanically
& chemically) as protected aluminum (SiO/SiO2) mirrors.
Contact Information;
MOONWARD VACUUM COATINGS
P.O. Box 850
Copper Cliff, Ontario,
Canada P0M 1N0
Telephone
Inquires:
Alan Ward or Grant Fleury
(705) 690- 3435
copyright 1999
Moonward Vacuum Coatings
revised: Sep 15, 2003
