IR Thermometer uses

[nicksanspam]

No? This is unusual logic. It both emits and absorbs radiation, but it doesn't absorb IR? :-)

I don't think so. Try it. I've pointed my Raytek at night skies and daytime skies, with clouds, and read similar very cold temperatures. When I point a $1K Exeltech at the same cloudy sky, I read a much higher temp. I think the Raytek measures emissive power in a narrow wavelength band that excludes the strong water vapor absorption bands around 1, 1.4, and 1.8 microns, but the Exeltech looks at more of the spectrum.

Nick

[nicksanspam]

A pane of glass transmits about 90% of the terrestrial solar spectrum (which has very little power at wavelengths longer than 2.5 microns) and blocks almost all of the IR spectrum longer than 3 microns. Wien's displacement law says an 80 F (282K) black body has a spectral peak at 2897.8/282 = 10 microns... 966K (1279 F) has a 3 micron peak.

Windows transmit a lot more beam sun power than the IR they reradiate, since houses scatter and absorb incoming sun over a large interior surface and do not contain IR beam suns of their own.

Glass blocking IR makes cars hot in the sense that without glass, they would be cooler. Polycarbonate also blocks IR. Polyethylene doesn't. My sunspace with 256 ft^2 of poly film glazing seldom reaches 100 F on sunny winter days.

Nick

[Richard J Kinch]

Right, it both emits and absorbs (and transmits as well), but is a net emitter at certain times.

[nicksanspam]

When it's also absorbing IR.

Nick

[Richard J Kinch]

Right, it both emits and absorbs.