Glass, infrared, emissivity & transparency

Mike T., Swampeast MO

The old timers (including scientists) seemed to believe that glass was transparent to heat as well as light. For a simple example they would say, "Put a piece of glass between you and the sun and you still feel heat from the sun." I agree--that happens.

Now however we "know" that glass is nearly if not completely opaque to infrared (heat in the form of radiation). Stated is that using an IR thermometer you can only measure the temperature of glass itself--not the temperature of objects visible through it.

What gives? Is it because glass is somewhat transparent to short-wave infrared (as comes from the "full spectrum" of the sun) while being opaque to long wave infrared--say from a human body or radiator? Or is it because you feel heat from the sun through glass because it has actually heated the glass via conduction and now the glass itself is radiating to your body. A combination? Granted I [think] I can feel a somewhat warmer spot on glass where the sun is shining through, but not that much warmer...

Now the emissivity thing. I understand that plain glass is quite emissive and extremely conductive. Thus infrared radiation striking it [seems] to pass directly even through as it's actually first being both accepted and then conducted and finally re-radiated.

Lowered emissivity coatings in multi-glazed window units typically have this low-emissivity coating on an INSIDE face of one? both? of the panes--mainly because the coating is rather delicate.

Emissivity is a surface phenomenon yet even "good" explanations of these windows say that they reflect infrared heat energy back into the room. How can this be possible when the lowered-emissivity coating is not the FIRST thing struck by the infrared. Am I still missing something or is it that these windows reduce heat transmission mainly by reducing radiant transmission BETWEEN the panes of glass?

scrook_2

Emmissivity vs transmission vs conduction

I believe because the glass is emmissive your remote IR thermometer reads the glass temp not the temp of that beyond. An experiment comes immediately to mind though: what if you look at something really hot (say a cherry red fire brick, or the sun itself (what, about 10,000-11,000°F (5,800-6,400K on the surface-- though it is a very small target to aim at from here), and see if the IR radiation -- weighted toward the shorter wavelengths then say the neighbor's garage through the window -- is seen by the IR thermometer.

Conductive is a whole 'nother animal -- because cold air (or snow) is touching it on the outside the inside maybe a 1/16" of thichness away gets cold too (but still quite warm in absolute terms so it still is emmissive/radiative. Meanwhile, as you note, some IR from the 10,000° sun is transmitted through, some is absorbed, warming it up more than if it was in the shade, and because it is warmer it re emmits/reradiates (both into the room and out to the outside world) an ever changing balancing act.

As for low-E glass, you got me -- Perhaps it is more accurate to say it is a high IR reflective coating in the longer wavelengths that come from 70° walls, 90-100° humans, and up to 215° radiators, but is less reflective to the short wavelengths from the 10,000° sun so the sun's radiant heat gets in but the room's radianthead doesn't leave?

That make any sense? Can anyone else shed more light on the subject?