window glass treatments

Hi all,
I figured this august group of knowledgable people could solve a
mystery for me.
As I was listening to yet another window salesman's pitch, he
mentioned that the coating on the glass would let light pass, but
would minimize heating in the summer. Now that made sense to me, but
then he followed up by saying that in the winter, it would let the sun
help heat the room.
I'm familiar with bandpass optical coatings that will reflect IR and
pass visible (and if they're good, also block the UV), but not with
any that change based on temperature. To work as advertised, I would
guess that the lower cutoff would have to be temperature dependant and
rise in frequency with temperature.
Having never created one like this myself, I was wondering - is it
possible? and if so, what technique creates variable coatings? Sadly,
all of mine are stable over a wide range of temperatures. Has my
little grey cells become even more disfunctional with age than my wife
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Listening to salesmen talk about technical stuff can wear your mind down.
Did he say "LOw Emissivity Windows" or "Low -e glass"? Or did he say anything at all that you can remember that relate to the name of the technology involved?
You could provide whatever you otherwise remember of the discussion.
In case he mentioned low emissivity windows, which have been around for over 20 years in various forms, go to :
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or do your own google search for "low emissivity windows theory" or some similar terms, if you want to knoow the theory of UmpidySquat Windows, try "UmptidySquat Window theory" as search terms.
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mark, You're right about dichroic, (Bandpass), coating. The glass reflects most of the I.R. keeping summer heat out. The gain in winter though, comes from retaining internal heat, not solar gain.
Pragmatist - "It won't fit? - Use a bigger hammer!"
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Did I forget to mention that it was low e (guess I thought it was understood). The part that unusual was the change in cut-off frequency from near IR to thermal IR in the winter. Had to do with outside pane coating since it was the one that saw the temperature extremes. I thought about maybe they used a multi-layer coating with a layer of high expansion material that would shift thickness (as measured in wavelengths) appreciable over only few degrees to reflectance/emissivity.
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Apparently something like that is actually possible, see:
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Note that New Scientist is not the most authoritative source!
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Marc 182
What you described seemed to be just the standard sales pitch for Low Emissivity glass windows, that has been repeated for many years. It wasn't that well understood by the salesmen, and so it wasn't _illiminating_ to the technically inclined listerer.
Thermal expansion changes in length between 10C and 30C _might_ be as much as 0.01 percent of the 'natural' length or thickness. And since the coating is quite thin, then the change in thickness would be quite tiny. Possibly too tiny to shift the wavelength of cutoff significantly.
Besides, it isn't likely that the original coating thickness is controlled so precisely as to be uniform and repeatable to 0.01 percent or less. So, the performance of the windows probably doesn't have that fine a functinality based on such tiny changes in film thickness.
So, this perspective would inply that thermally induced film thickness changes aren't a worthwhile wavelength tuner (transmission/reflection cutoff) for the stated window application.
But, maybe with multilayer coatings, one could imagine such functionality. Then one would sk about cost.... fabrication precision... and so on.
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If one looks at the power spectrum of sunlight, the peak of radiant power is near green, in the visible. So, quite a lot of the actual heat input from direct sunlight is visible light.
The IR fraction of total solar radiated energy could be looked up to serve as an index of just how effective IR rejection could be to total transmitted solar radiation.
It could be as high as 30%, which is a useful heat rejection for reducing room heating, nevertheless.
The most evvective way to control heating of through windows in summer sun is to use a opaque sunshade. A "sunshade" that only blocks infared and transmits visible thermal radiated energy (visivle light) would still fade your carpet, curtains and upholstry, and would very possibly still heat up the room quite nicely.
Reflective windows are OK too, as are properly designed reflective curtains.
In the winter, sunshine still feels nice and warm, even if you do filter out the IR. Because most of the radiant energy from the sunshine is in the visible.
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