Blackening optical surfaces, the process of making them non-reflective, requires both surface preparation for roughening and surface treatment for darkening.
Surface preparation techniques
- Bead blast -- The surface is roughened by blasting with glass beads propelled by compressed air.
- Grit Blast -- Blasting with sand or other grit.
- Thread -- Cylindrical surfaces can be roughened by turning fine threads with a lathe.
- Black nickel plate -- a good choice where plating is practical.
- Anti-reflection coating -- This involves evaporative coating the surface with dielectric layers of precise thickness. It is expensive, requires very careful maintenance, and rarely used except on the most sensitive instruments.
- Carbon black -- this is soot from a carbon flame, like a candle. It is very black, but is easily rubbed off most surfaces. A good technique for experiments 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 paint from the hardware store, or professionally applied paint like Sherwin Williams Polane Dead-Flat Black. For do-it-yourselfers, some people recommend applying several coats of flat white followed by several thin coats of flat black.
- Black anodize -- generally, anodized aluminmum is not considered optically black. It has a reflectance in the range of 5 to 10%. The Oregon branch of Pioneer Metal Finishing, 19005 SW 125th Ct., Tualatin, OR 97062, (503) 692-4202, claims to have an anodizing process using a different black coloring agent that gives a reflectance in the range of 1 to 2%.
- Light absorbing foil -- an adhesive backed foil with good anti-reflection properties is available from Acktar out of Israel. Two products are Spectral Black and Spectral Black HP.
- Flock paper -- this velvet-like paper is available in black, and has a very 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 storing integrated circuits is rough and has low reflectance. It is commercially available in large 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.