Re: 3M flocking paint (again)

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Thu, 21 Aug 1997 05:30:01 -0500
>From: P3D Patrick Boeckstijns <pboeckst@xxxxxxxxx>
>Subject: 3M flocking paint (again)

>So, I apologize for repeating the same question again. Anyone else any idea
>how to get some of the stuff or in case it doesn't exist anymore, a good
>alternative ? All black paints I've tried still reflect when the light hits
>the surface from a low angle. :-(

That's because there's nothing that's opaque which has the same index
of refraction (characteristic impedance) as air. This is best illustrated
by the existence of gloss black surfaces - the material absorbs "as much
light as it can", but some light is reflected because of the mismatch.
If you make two lenses, one of clear glass and one of black glass (with
the same index of refraction), I expect they will both reflect the same
amount of light - but the percentage of light that would be passed through
the clear lens will be absorbed by the black lens. I've heard that the
index of refraction of glycerine is very close to that of "glass" (but
since different glasses have different indices (indexes?) of refraction,
I'm not sure which kind this refers to) - if this is true then if you
were to immerse the two aforementioned lenses in glycerine, the black lens
would go completely dark - no reflection at all, and the clear lens would
go invisible - no reflection, and also no optical effects on the light
passing through.

>I'd greatly apreciate any advice.

In a medium of air, using existing materials, the only ways to get rid of
reflected light are to divert it somewhere where it doesn't cause a problem
(like the slanted instrument cluster cover on an automobile dashboard),
or to find a way to match impedances of the media at the surface where
reflection would be a problem, or to use wave cancellation techniques, or 
to provide for multiple reflections that gradually absorb most of the light.

Impedance matching requires sophisticated manufacturing techniques (i.e.
binary optics), and since it requires extremely fine-scale geometries at
the optical surface, it would probably be seriously affected by fingerprints,
vapors from cooking grease, etc. - I'm not sure how effective it would be
for general amateur photography.

Antireflection coatings are widely used - I don't know whether anyone has
ever tried to put one on an opaque surface. They tend to work best over a
limited range of colors.

Multiple reflections are what make things like black velvet and carbon black
(lamp soot) so black. Flat black paint has tiny particles in it to give the
surface a microscopic roughness and force multiple reflections for most of
the incident light. Since they require a surface roughness, flat black
surfaces are somewhat susceptible to contamination, but much less than
binary optics. The best absorption should be from very "frothy" surfaces,
which could also have a tendency to crumble.

Perhaps you can use black velvet, or mix some rough material with black
paint. Did you mention what you want to use it for?

John R


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