# Why does an infrared thermometer work with a fixed emissivity?

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:

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

• Ah, I think I get it. So the emitted energy is actually $\text{Emissivity} * \text{Planck Radiation}(\text{surface temperature}) + (1-\text{Emissivity}) * \text{Planck Radiation}(\text{ambient temperature})$ – nikie Nov 20 '17 at 20:05