Ideal paints (was More experimental ATM)

[Anthony Stillman <atmer@flash.net>]

>What we
>would like is a paint or finish (which is) black in visible light and
>>reflective in infrared.
>Nils Olof



Paint, as a particulate suspention in a medium, derives is opacity and
reflectivity from two principals.  First the differential index between the
particulate and the medium and second the absorption characteristics of the
particulate.  The medium and the medium-air interface also play rolls, but
these effects are often less important. Further, the transparency of a
particulate suspention is wavelength dependent as a function of partical
size.  Specifically, large particles interact with long wavelengths more
than small particles.  These principals can be put to use in defining a
surface finish which is reflective at long wavelenghts while absorbing at
short wavelengths.

To maximize the desired effect, the differential index between the medium
and the particulate should be large in the blue and low in the infared.
Further, it would be desirable to select a pigment which poseses poor
infared absorption charateristics(1).

With out ready access to index profiles of avalible medium and absorption
profiles of avalible pigments, this aspect of an ideal solution alludes me.
However, the additional varable of particle size selection can still be put
to use.  Namely, by first applying a highly reflective layer and then
covering that layer with a finely ground pigment mixed into a medium, it
would be expected that infared(2) light would trancite the pigment-medium
layer with little scatter and reflect off the backing layer.  Shorter
wavelengths would however be scattered and hence absorbed.

Of course, the ideal solution would make use of, and balance, all of these
effects.  Short of a whole lot of research, I would suggest a finely ground
pigment, perhaps a lovely prussian blue(3), mixed with shellac and applied
to the aluminized side of a milar film.

Anthony




1) The key word here is anomalous dispersion and its the reason for placing
infared telescopes in orbit.

2) Grey bodies at 100 C radiate at and around 7um, I think this can be
thought of as the blue end of the infared spectrum of interest here.

3) Prussian blue has a typical particle size of 1000nm

Books to which I refered:
"Optics" Hecht & Zajac   Addison-Wesley  ISBN 0-201-02835-2
"Cooling Techniques for
 Electronic Equipment" Steinberg  Wiley&Sons ISBN 0-471-04403-2
"Light and Color in Nature and Art" Wiley&Sons ISBN 0-471-08374-7