Re: [ATM] undercorrecting?

[Mike Lockwood <melockwo@uiuc.edu>]

Mitch,

Mitch wrote:
> As I am still the young kid on the block, could someone please explain
> a bit about what "emissivity" is, or just point me to some simple, clear
> text on the net.

Excellent question.  Surprised no one else has asked it.

Emissivity is a measure of how easily a material gives up heat by 
emission of radiation.  Put a warm piece of metal outside on a cold 
winter night when there's no breeze, and most of the cooling is likely 
through radiated heat.  (Better yet, put it in the cold vacuum of 
space - then ALL cooling is through emission.)  The material emits 
photons which have a wavelengths in the infrared region of the 
spectrum.  By giving off energy like this, the material cools.  Higher 
emissivity means the material can more easily give off the photons.

The word emission is used to refer to something (radiation, air 
pollution) that is emitted, and thus the word emissivity.  For more on 
emission of energy, go Google "blackbody radiation".

It's different if you blow cold air across warm objects.  Then the air 
itself gets warmed by the material, and heat is transferred from the 
metal to the air molecules directly.  This is *convective* cooling, or 
cooling done by moving air.  It's different.

The question that is being kicked around is - if a coated mirror is 
facing the sky, does the emitted heat cool the front face of the 
mirror enough to cause it to warp significantly?  The mirror is 
looking at the cold vacuum of space, so there is a large temperature 
difference between it and space, so a significant amount of infrared 
radiation can be emitted, cooling the front of the mirror more than 
the back or sides.  (This is why metal can get colder than air 
temperature on a clear night - it keeps emitting radiation into the 
very cold sky.)

 From your later message:
> The point is, I may be overcomplicating things, but with such small
> tolerances of temperature difference how can we be sure even 4 fans, each
> blowing at the mirror 90 degrees apart is enough?

We can't be sure.  I typically run the fans for a while to get the 
mirror temperature close to ambient, and then shut them off.  Then 
over time the heat in the mirror will redistribute itself on its own 
so that the mirror is all about the same temperature.

Boy am I posting too much today.

	Mike L.


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