Re: Gasoline thrown in the campfire and burning 3-D images!

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Mon, 2 Dec 1996 20:35:15 -0600
>From: P3D George Gioumousis <georggms@xxxxxxxxxx>
>Subject: Re: Gasoline thrown in the campfire and burning 3-D images!

>P3D Gabriel Jacob wrote
>> RBT mounts were made white on the side that faces the light so that it can
>> reflect the radiant heat. Now why is the other side black? It seems (and
>> I am only guessing here) whatever little energy is absorbed from the white
>> side can be more efficiently emitted if it is black! Since the temperature
>> difference between the white side and black side is greater on the white
>> side, the black part will radiate instead of absorb.
>> 
>Good guess. Back in my younger days, when I was in graduate school studying
>non-equilibrium phenomena, I read a book (purely theoretical in those 
>days) on the design of space craft. The recipe was to make the thing
>a sphere, with one side black and the other side silver. Too hot: face
>the silver side to the sun and the black side emits to the stars. Too
>cold: face the black side to the sun so it could soak up the heat while 
>the silver side emits very little to the stars.

In current practice, spacecraft for use in low Earth orbit seem to be mostly
white or reflective wherever possible. I suspect this may be to minimize
thermal cycling and temperature differentials. When not otherwise controlled,
the interior temperatures of such objects seem to be "cold but not bitterly
cold". Objects in LEO are usually in shade part of the time, but nearly half
of their field of view is taken up by the Earth, which is considerably warmer
than the background sky. The Apollo 13 spacecraft was in the sun nearly all the
time, and much further from Earth - I believe they were cold and had problems
with water condensation, but not with ice.

BTW, I seem to recall that about half of the energy of sunlight is "infrared"
(I think that's in space - I don't know the percentage in what reaches the
Earth).

>Related Phenomena:

>1. Frost forms when the air temperature is above freezing.

In the Middle East, that phenomenon has been used deliberately to make ice.

>2. The best rooftop solar collectors are made of a material which
>is black at visual wavelengths and a reflector at 10 microns, which 
>is the peak of the black body curve for something near the boiling
>point of water.

How do you find out the properties at thermal infrared wavelengths?
My CRC Handbook gives reflection coefficients at several visible wavelengths,
reflection coefficients of surfaces for "incandescent" light, coefficient
of absorption of solar radiation [a few surprises - silver=0.07,
aluminum=0.15, white paint=.25-.30, aluminum paint=0.55, black matte=0.97],
emissivity of total radiation, and that's about it.

Regarding the slide mounts:

I doubt that the black side can efficiently radiate very much heat. Even if
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one assumes that low reflectivity at visible wavelengths correlates with
good emissivity at thermal infrared wavelengths, I believe radiative heat
flow is proportional to the difference in the fourth powers of the (absolute)
temperatures of the object and its environment, and the slide just isn't
very hot and its (solid) surroundings aren't much cooler. I would think that
convective cooling would strongly predominate, and the main reasons for the
black side are to block extraneous light and reduce internal reflections.
(Is it gloss black, or flat black?)

John R


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