Posted by: THenne1713@aol.com thenne1713
> BTW, I recently discovered that the IFR (Infrared) signal of a trail camera will NOT work if behind glass... Who knew, LOL?
Hi Terry;
Glass is generally opaque to "long wave-length infrared" (the relatively cooler radiant heat in our normal environment, instead of the short wave-length heat radiated from extremely hot sources, like much of the invisible heat coming from the sun. Radiant heat is electromagnetic waves, like visible light, ultraviolet, x-rays, etc, but with longer wave-lengths.
Instead of passing through glass, long wave-length radiant heat is absorbed by the surface of the glass. The heat is then spread into the glass via "conduction" (heat transfer from molecule to molecule) warming the mass of the glass, and heat can then be radiated away from the other side, if that surface is warmer than surrounding surfaces. Heat can also be convected (moved by air, or other fluid) away from the surfaces of glass.
Low-E (low emissivity) glass, which is used in most of today's windows, has a special surface coating (on a side) between its layers) which is literally thinner than the wave-length of much/most of the long wave-length heat that would be absorbed and radiated from its surface, therefore it cannot radiate or absorb heat that has a wave-length that is greater than the thickness of the Low-E surface. This significantly increases the insulation value of the gap between the panes of glass.
Anyway......Motion detectors are designed to see long wave-length infrared light, which will pass through many/most plastic lenses, but can not see through glass.
Laren Corie
Solar Heated and High Efficiency Home Designer and Consultant
Posted by: larencorie@axilar.net
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