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I have a cheap infrared thermometer that works with a fixed emissivity of 0.95. My understanding is that the thermometer measures the amount of thermal radiation at some wavelength (range) and estimates the surface temperature from that.

What I don't understand is why that temperature is (more or less) accurate for different materials. For example, white paper has an emissivity around 0.7 and rubber has an emissivity around 0.95. I couldn't find out which wavelength my thermometer uses, so I'm guessing it's around 50 micrometers (mid infrared). Then I would expect the thermometer to display temperature differences >50° between paper and rubber for the same real surface temperature:

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But in fact I get very similar measurements. (Less than 5°C differences between all materials I could find)

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When an object has low emissivity, it will have higher reflectivity. If all you're measuring are room temperature objects in a larger space at room temperature, then I wouldn't expect to see much difference. Some of the energy the meter is reading is reflected IR.

Now if you warm the objects above ambient, I would expect the lower emmissivity object to show a temperature below the actual temperature.

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  • $\begingroup$ Ah, I think I get it. So the emitted energy is actually $\text{Emissivity} * \text{Planck Radiation}(\text{surface temperature}) + (1-\text{Emissivity}) * \text{Planck Radiation}(\text{ambient temperature})$ $\endgroup$
    – nikie
    Nov 20, 2017 at 20:05

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