Blackening optical surfaces, the process of making them non-reflective, requires bothsurface preparation for roughening and surface treatment for darkening. Surface preparation techniques - Bead blast -- The surface is roughened by blasting with glass beadspropelled by compressed air.
- Grit Blast -- Blasting with sand or other grit.
- Thread -- Cylindrical surfaces can be roughened by turning finethreads with a lathe.
Surface treatment - Black nickel plate -- a good choice where plating is practical.
- Anti-reflection coating -- This involves evaporative coating the surfacewith dielectric layers of precise thickness. It is expensive, requires very carefulmaintenance, and rarely used except on the most sensitive instruments.
- Carbon black -- this is soot from a carbon flame, like a candleIt is very black, but is easily rubbed off most surfaces. A good technique forexperiments where expediency is more important than durability.
- Duracon black -- a coating which can be applied by spin-coat,drip-coat, spray, or paint. Produced by: Materials Technologies Corp., 57 Maryanne Dr.,Monroe, CT 06468, (203) 874-3100, fax (203) 876-0700.
- Flat black paint -- the most common. The paint can be spray paintfrom the hardware store, or professionally applied paint like Sherwin WilliamsPolane Ultra-Flat Black. For do-it-yourselfers, some people recommend applying severalcoats of flat white followed by several thin coats of flat black.
- Black anodize -- generally, anodized aluminmum is not considered opticallyblack. It has a reflectance in the range of 5 to 10%. The Oregon branch of PioneerMetal Finishing, 19005 SW 125th Ct., Tualatin, OR 97062, (503) 692-4202, claims to havean anodizing process using a different black coloring agent that gives a reflectancein the range of 1 to 2%.
Some alternatives - Flock paper -- this velvet-like paper is available in black, and has avery low reflectance.
- Black wet or dry sandpaper -- this is not nearly as good as the flock paper,but sometimes it is more readily available.
- Antistatic black foam -- the black plastic foam used in shipping and storingintegrated circuits is rough and has low reflectance. It is commercially available inlarge sheets.
- A stack of razor blades -- the old fashioned thin, flat, single edged,injector kind can be bolted together to form a surface that will reflect almost no light.No surface treatment is required. Be very careful not to get cut.
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