How do you tell the difference between low-E and regular glass?
- Emissivity and reflectance
- Geometrical optics
"Low-E" means low emissivity. Low-E materials don't radiate thermal energy as efficiently as most materials do for a given temperature. Infrared (IR) thermometers gauge temperature by measuring the intensity of emitted thermal radiation, assuming the emissivity is very high (usually 95%) at these wavelengths (typically 8-20 μm), which good for most materials. Low-E glass has a coating on one side to reduce the emissivity, thus reducing heat loss due to radiation. Therefore, a comparison of the ambient temperature with the reading on an IR thermometer when pointed at the glass should register a large difference on the low-E side. However, one has to be careful; low-E also means high reflectance, so in an environment of uniform temperature, all you will "see" with the IR thermometer is a reflection of the surroundings and little or no difference will be seen.
Here is how both sides of a sample of low-E glass appear in a room of fairly uniform temperature:
As you can see, side A, side B and the room itself all appear to be more-or-less the same temperature.
Go outside, where the ground and the sky have radically different radiation temperatures:
Now look at a reflection of the cold sky in both sides of the low-E glass:
Et voilà! A huge difference. The B-side is plainly the low-E side. The A-side IR temperature is dominated by the ambient temperature; the B-side IR temperature is dominated by that of the sky seen in the reflection, and the warm glass itself is not contributing much to the IR radiation.
Thanks to Super Glass of Vancouver for the low-E sample used here.
© Physics and Astronomy Outreach Program at the University of British Columbia (Chris Waltham 2012-08-17)