How do emissivity and absorptivity affect thermal imaging results?"

[gmason85]

TL;DR

Can these be different where the temperature inside is different than outside a building

Hi all,

I’ve run into a number of paints that are sold as “thermal” paints, or insulation paints. I know it’s mostly bs, but want a clarification on how emissivity and infrared absorption relate.

The salesperson says the paint surface has an emissivity of 0.91, and reflects 99.5% of infrared energy (95% of the solar energy infrared, visual and UV). He claims with thermal images, that the paint can reduce the surface temperature of concrete by about 20 degrees on a hot day (taking it from about 47C down to 26C when the ambient temperature is about 31C). While the white colour would reflect the visual spectrum, it would assist a little, but the additional ceramic additives would largely be useless and unnecessary.

My understanding of radiation is if it is reflecting 99.5% of infrared energy (absorbing only 0.5% of the energy), it effectively has an emissivity of 0.005; which is would make a thermal image completely irrelevant to use if the emissivity setting is 0.91. Even if in reality, the paint surface is absorbing 20%, it would still effectively have an emissivity of 0.2.

However, he seems to think that one can have one can have a surface with a high emissivity, and low absorptivity. My understanding is that this is impossible. Or even if it was possible, the thermal camera would mostly be reading the reflected infrared energy, rather than the emitted energy, making the thermal image, based on an emissivity of 0.91, incorrect.

Am I out to lunch, or is he?

 

[hutchphd]

Mostly he is. One needs to always be comparing apples to apples. For instance, the specular reflectance can be much more directed than the "diffuse" Remission from a surface at temperature It is the diffuse emission , thermodynamically constrained by the principles of detailed balance, that drive most of reciprocity. One cannot spontaneously cool an object using paint alone. This would violate the Second Law. But there are coatings that can reduce the influx of directed radiant energy at some wavelengths and promote the emission at others (into cold interstellar space) in a way that minimizes the Temperature of the object. Check this out

Not magic but clever.

 

[gmason85]

Thanks. Interesting stuff. I assumed that there was a grain of truth behind the claims. The video was fantastic; but mostly backed up what I suspected. His homemade paint needed 20-30 thin coats to be effective and took the entire day to apply to a small square. Not very practical for applying to a roof. Shortcuts that make it more practical would reduce the efficacy of the coating.

The video you showed really only talked about visual light, and used thermometer, rather than thermal camera to get the readings. I wished that they took a reading from a thermal camera as well to show the difference that coatings can make to the temperature reading.

Assuming that the salesperson claims of the paint reflecting 99% of the infrared spectrum of solar energy is true, the thermal camera reading of the surface temperature would be reading the reflected ambient infrared, rather than the surface temperature itself? And the camera would need to be adjusted to account for that reflection, regardless of what the stated emissivity of the surface is?

 

[hutchphd]

All light carries energy (heat). There is nothing "magic" about infrared. We simply feel (see) it with our backside or our outstretched hands but not our eyes. The energy flux from the sun actually peaks in the blue green (~500nm). So a perseveration about IR is counterproductive. There are coatings that will "improve" a situation that involves steady state radiant flux (but cannot change equilibrium detailed balance)
I shall not (re)produce a treatise. Lotsa physics here. What a thermal camera reports can be tricky to interpret.