0
$\begingroup$

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:

enter image description here

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

$\endgroup$
3
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ 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 '17 at 20:05

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.