Re: RBT Mounts "Back in Black"

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Wed, 4 Dec 1996 18:17:16 -0600
>From: P3D Larry Berlin  <lberlin@xxxxxxxxx>
>Subject: Re: RBT Mounts "Back in Black"

>>Date: Wed, 4 Dec 1996 00:16:08 -0500
>>From: P3D John W Roberts comments:
>......................
>>
>>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).

>***************  Isn't that why very thin aluminum foil is used for heat
>protection? It would both block and bounce the infrared away rather
>efficiently, without needing polishing! ; -) I wonder if they could embed a
>thin foil mesh in the white side to enhance reflectivity? Just a thought ...

As Dennis Sherwood mentioned, aluminum's reflectivity for radiant heat from
a heat source at 100 degrees F (what we would typically think of as "thermal
infrared") is 95%. This makes aluminum great for things like emergency
blankets (aluminized mylar, or whatever), because they reflect the heat
back to a person wrapped in one.

But a very hot object such as a tungsten filament puts out most of its
energy in "near infrared" (wavelength not too much longer than visible
light). Apparently aluminum absorbs near infrared much more efficiently
than far or thermal infrared.

This leads to an additional problem - to get rid of the energy absorbed from
the near infrared, the aluminum needs to radiate at thermal infrared
wavelengths (since it's not as hot as the tungsten filament). But since
aluminum has very high reflectivity at thermal infrared, it therefore has
very low emissivity at that wavelength. So aluminum exposed to a strong
source of near infrared absorbs heat fairly well, and radiates heat very
poorly, so it gets *hot*. "The Straight Dope" reports that in space, polished
aluminum in sunlight can reach temperatures of 850 degrees F, while surfaces
painted white and exposed to the same conditions may be at -40 F.

It sounds like the "ideal substance" would be opaque, very reflective at
visible and near-infrared wavelengths, and very absorptive at thermal
infrared wavelengths.

(Of course if the HAG and ventilation in the slide projector are working
correctly, the aluminum shouldn't heat up too much anyway, but that's not
because of any merit on the part of the aluminum. :-)

John R


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