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Glossy surfaces look hotter than they are under thermal camera?

opampsmoker

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stenzer

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Hi,

if you set the emissivity to 1 (blackbody) while measuring a shiny polished metal surface, the metal suface appears to be colder. See [1] which describes this kind of measurements and how to achive correct results. Have a look on the last picture in [1] where you can see the effect of measuring a too cold temperature of a metal surface when the emissivity is set to a high value.

[1] https://www.flir.com/discover/professional-tools/how-does-emissivity-affect-thermal-imaging/

BR
 

opampsmoker

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Thanks, as you know, matt black has an emisivity of almost 1. But what would you say the emisivity of gloss black is? I know silver and mirrors are low emisivity, but what about gloss black's emisivity?
 

stenzer

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Hi,

to be hornest I do not know the emissivity of a glossy black surface. But you might overcome this unknown issue by use of an electrical tape, as suggested in the link in my previous post.

BR
 

opampsmoker

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Thanks, from your kindly supplied link, it appears that if the thermal cam has the emissivity set to 1, and then a glossy surface is imaged, then its impossible to say whether or not the temperature recorded would be higher or lower than it really is, because it will reflect the infra-red of nearby objects, which could be higher or lower in temperature than the glossy surface.

So in other words, a glossy surface may actually be cooler or hotter than the thermal camera (with emissivity set to 1) "says"?
 

stenzer

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Hi,

the displayed temperature depnds on the chosen emissivity value. E.g. for the third picture showing the metal light switch i assume a low emessivity value was chosen. But the switch appears to be warmer than the wall behind. For the investigation of the human hand I assume a high emissivity was chosen. As you can see, the ring appears to be much colder.

Have a further look at [1]. In [2] you can find an emissivity table provided by fluke.

[1]https://www.flir.com/discover/rd-science/use-low-cost-materials-to-increase-target-emissivity/

[2]https://www.thermoworks.com/emissivity-table

BR
 

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A possible misconception involved with the question is that the thermal IR emission coefficient is equated with the behavior for visible light. Glossy black does not necessarily reflect thermal IR so much. It's clearly the case for metal surfaces.

It should be usually possible to distinguish between emitted and reflected radiation by moving the camera or shadow possibly reflected hot objects.

We can clearly state that matt black paint, if not containing metal particles, has a reflection coefficient near 1 and a glossy black surface can't emit more thermal IR.
 

stenzer

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Hi,

Glossy black does not necessarily reflect thermal IR so much. It's clearly the case for metal surfaces.
I agree with that. Nevertheless, to overcome the unknown emissivity of glossy black which might have an emissivity around 0.9 (I don't know) I would use an electrical tape with a known emissivity as suggested in link [1] in post #6.

We can clearly state that matt black paint, if not containing metal particles, has a reflection coefficient near 1 and a glossy black surface can't emit more thermal IR
I agree.

BR
 
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