Re: Flat black

[John Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Thu, 7 Mar 1996 16:10:43 -0600
>From: "Paul S. Boyer" <boyer@xxxxxxxxxxxxx>
>Subject: Flat black

>There are spray paint cans of "flat black" paint available at most paint 
>and hardware stores.  On metal, one should use the appropriate primer first.
>Flat black does not wear as well as glossy paint,...

There's an important physical reason for the lower durability of flat surfaces:
Just about any solid substance has a significantly greater index of refraction
than air (or vacuum). Coloring an object black prevents the material of the
object from absorbing and re-emitting photons, but in the same way that an
impedance mismatch in an electrical circuit causes a "reflection" of signals,
the discontinuity in the index of refraction between air and a smooth surface
causes a certain percentage of the incident light to be bounced back in a
specular reflection.

The solution is to make the surface rough, so the typical photon has to bounce
several times before leaving the surface. For instance (using arbitrary
numbers), if a smooth surface of some substance has a specular reflection of
1/10 of the incident light, and if the surface is roughened up sufficiently
so that an average of three bounces is required before the photons get away,
then only 1/1000 of them will do so (and also the reflection will appear much
more diffuse). Flat paints contain structures that make the surface
microscopically rough, insuring many more multiple reflections than a smooth
surface would provide. Loose soot powder produces excellent results, because
of the extreme roughness, and because of its depth - light can penetrate far
into the soot layer.

The problem with rough surfaces is that they are not as durable as smooth
surfaces because of the small-scale geometry. A careless touch can break off
the rough components as a powder, and possibly leave the surface smoother
than it was. Rough surfaces also pick up dirt, dust, and grease (i.e.
fingerprints) very easily, and they are hard to clean.

Black felt/velvet is both rough and durable, so it works well, as long as you 
can keep it clean.

John R.


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