Re: RBT Mounts "Back in Black"

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Tue, 3 Dec 1996 19:11:26 -0600
>From: P3D Larry Berlin  <lberlin@xxxxxxxxx>
>Subject: Re: RBT Mounts "Back in Black"

>*************  This is what I guessed from Dr. T's experiment when the black
>melted before the white even though it faced away from the lamp. The
>infrared goes right through the white to a significant degree, making the
>use of black on the far side even more likely to cause problems. They need a
>far more reflective surface in front of the slides to prevent transmission
>of the heat. The white plastic doesn't reflect enough of it away and it
>isn't opaque enough by itself to be a good slide holder in projection.
>Still, opaque pigments other than black should be available. The use of
>black is asking for trouble.

Judging from the CRC Handbook tables (see earlier posting), you should use
polished silver! Of course then you have to make sure it doesn't tarnish,
because that would turn it black. :-)

As far as blocking transmission of light (in contrast to reflecting it),
any color of opaque material either doesn't stop everything, or else it
heats up just as much as the black (e.g. the heating of the black on the back
side of the mount).

>Only adequate protection with
>heat filters and air circulation can protect the slides and their mounts.

I think that's the key - in the previous posts, when these protective measures
were working correctly, there didn't seem to be a problem even with black
facing the lamp.

>An additional note about the film chip itself and it's dark or shadowed
>regions. The film substrate is itself a different material than the plastic
>mounts, and it's very thin which allows better ventilation and cooling
>directly to those points that will be absorbing the heat. The black plasic
>side of the mount will likely build up a greater degree of heat than the
>film chip partly due to it's thicker mass. Obviously if the mount is
>experiencing melt-downs, there is a problem which puts the film at risk too.

Being thinner doesn't mean the film would have a lower equilibrium temperature;
it means that the film would reach equilibrium temperature more quickly than
the mount. Actually that could be a slight disadvantage - for instance, if
the primary cooling mechanism is air flow, and if the desired air flow pattern
for cooling takes a few seconds to be fully established once the slide is
moved into place, some parts of the film could potentially get a brief "jolt"
of heat that would have less effect on the mount (since it has greater thermal
mass). Of course, the material of the slide could have a higher melting point
than the material of the mount...

I think Gabriel raises some good points - we've probably gotten about as far
as we can go without some actual numbers and calculations. There are some
significant differences between near infrared and "thermal infrared", and
the appearance of a substance at visible wavelengths doesn't necessarily
reveal everything we need to know about its behavior at the various infrared
wavelengths. Thermal/radiation modeling involves calculations of heat flow
into and out of all the things in the system, with many of the factors
non-linear - it's *very* complex if you want a reasonable level of accuracy.

John R


------------------------------