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topic 54083

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.

James Burger
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.

Regards,

Ted Mooney, finishing.com
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Ted Mooney, P.E.
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



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