Coatings/platings for visible and infrared light mirror
January 13, 2010
I am developing a light source for a machine vision application, that includes a metal reflector surrounding a quartz-halogen lamp. This lamp holder/reflector assembly will most likely be manufactured from an aluminum alloy, as yet an unknown composition. I need to create a mirror surface on this reflector that creates as much light reflection as possible, across a very wide range of wavelengths: visible to near-infrared light (400 to 2500 nm).
I know polished aluminum has a very good reflectance spectrum in this region, but will it degrade with time? (Oxidation and exposure to a very hot light source). Would it be better to coat the aluminum to provide a flat and long lifetime spectral reflectance? Silver is good, but what about nickel, nickel+chrome? Any other good reflection materials?
This mirrored surface will be on the inside of a long tube reflector with a 'C' like cross section. But this assembly will also produce a lot of excess heat that will be absorbed by the reflector. What can I coat the OUTSIDE of the reflector with to provide the greatest heat radiation? Most computer / electronic heat sinks are black anodized aluminum I think. I'm not sure about the manufacturing process yet, but if for example I electroplate the entire reflector, will the mirror coating on the outside reduce excess heat radiation? Would it be optimal to have a black outside, and a mirror inside?
Any advice, suggestions appreciated.
image scientist - Riga, Latvia
January , 2010
Hi, James. Nickel and nickel-chrome are not good reflectors -- they absorb too much heat. Rhodium plating should be excellent and tarnish-free because it is a precious metal.
If you stick with aluminum, the purer the better, and electropolishing is a bit better than brite dipping. Then a thin anodized layer.
I think a black outside would be good, but there are high emissivity coatings that would be better than black oxide. Please search our site for emissivity. Good luck.
Ted Mooney, P.E.
Pine Beach, New Jersey
Striving to live "Aloha"
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