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Take care interpreting thermal imaging camera output

The bandwidth is very limited: lenses are made with germanium which has its own response curve

You will see that the camera response encompasses little or no CO2 emission band. So what do you see if you heat air to 450°C

- not a lot unless the hot air hits something

What do you see of emissions from co2 - not a lot!

 molecules:between the hot air gun and a sheet of paper there appears to be no heat but the paper is 400°C

Air does not radiate much at 8 to 13um so the camera sees nothing (a thermal imaging camera would be pretty useless if all pictures were fogged from the air radiation)

How about water vapour

note steam is not visible

A video of same

http://www.youtube.com/watch?feature=player_embedded&v=l3t9XeTIA1w

So air is invisible and water vapour is invisible in some thermal imaging cameras. So all these people who keep measuring sky temperatures need to know the IR bandwidth of their thermometers before they can state there is no measurable downward radiation.

AIR sky picture with some spots added. Note that the camera bottom limit is -40°C 

Note clouds contain water droplets so do radiate within the bandwidth of the camera.

Another important item about thermal imaging is you cannot just point a camera at an object and measure its temperature. you need to consider emissivity and reflectivity. Watts points a camera at a MMT air temperature unit and says "look its at the same temperature as the wall". This is not true. It may be simply reflecting local temperatures.

Emissivity affects the measured temperature of a non reflective object but the temperature shown will change with the objects temperature

A reflective object will mainly reflect the temperature of objects around and temperature shown may have nothing to do with the actual object temperature

In the my industry we simply spray all items with a thin coat of matt acrylic undercoat to remove reflections and equalise emissivity.

Comments

  • Here's a video of mine showing reflectivity and emissivity which should always be considered when interpreting thermal images.

    The device used is a power semiconductor with the flat heat sink (~2mm thick copper) this was polished then half of it sprayed matt grey with acrylic primer (matt black does not necessarily have a better emissivity than other colours)

    The first part shows the reflective nature of the heat sink. The lower half simply reflects the IR from a moving hot object. The temperature of the grey painted surface barely changes as the reflection moves.

    The second part shows the effect of using the semiconductors on the reverse of the heat sink to dissipate power. The whole of the heat sink increases in temperature however the uncoated side only rises by about 1°C whilst the grey side increases by about 20°C. The uncoated side is actually reflecting room temperature and barely reacts to the actual copper temperature.

    And finally check out this wuwt article for errors

    https://wattsupwiththat.com/2009/03/04/looking-at-thermometer-placement-and-heat-in-the-infrared/

  • Thank you for this. Terrific visual demonstrations.
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