Recent I was asked this “What material has the best emissivity?”.

It is a tricky question to answer as first I need to figure out what is meant by “best”. Best could mean, best highest, or best lowest.

Emissivity it not just a material property, it is affected by other factors.

The object with the highest emissivity is called a “blackbody”, this would have an emissivity of 1. Blackbodies however are theoretical objects. The nearest true object to this would be a blackbody simulator, these are used for calibration and would have a typical emissivity of 0.996, so almost black (not to do with the colour black). This is an object constructed to have a high emissivity, and is typically a box pierced with a hole. Normally the highest emissivity that we come across is paint, which can have an emissivity up to about 0.96.

The object with the lowest emissivity would be a flat plate made of polished metal. Polished metals can have an emissivity as low as 0.01.

2 Comments

  • Bob,

    A few comments on your answer:
    1. Emissivity is an optical property of material, see SpectralEmissivity.com for more details;
    2. Most practical blackbody sources have emissivities a lot lower than 0.996. If you accept a 1% calibration error and require a 4:1 calibration source uncertainty to desired outcome, then a 1/4% error in emissivity of the blackbody would be necessary. That looks like a much higher than 0.996, but not many people or sensor makers talk about it. NMIs usually require a 10:1 ratio and that’s even harder and more expensive to find.

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