Message Board Thread - "Why pointing the IR camera at the sky gets very low reading ? "

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Why pointing the IR camera at the sky gets very low reading ? Jason Carbury 5/29/2007
When I take thermal images of overhead transmission lines, the background (sky) gives very very low reading. Do you know why?
 
Re:Why pointing the IR camera at the sky gets very low reading ? Top Gun 5/29/2007
On cloudless days with low absolute humidy (dewpoint) the atmosphere is mostly transparent to infrared energy, so you see the coldness of space with little attenuation by the atmosphere itself. If you are from the Southern US, when the humidity climbs, you will see much warmer backgrounds on cloudless conditions since there is much more water vabor and particles that reflect heat from the ground back downward. Of course, clouds will radiate their thermal energies, lower clouds will be relatively warm, and high clouds will be colder, say -20 to -60 degrees F. Thanks for asking
 
Re:Why pointing the IR camera at the sky gets very low reading ? Top Gun 5/29/2007
Oh Oh, I forgot - the camera must be elevated to at least 30 degrees or more above the horizon for the very cold readings. If you look nearly horizontal, the sky should read very close to air temperature withing a few degrees of the horizon. I hope this helps.
 


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