If you are measuring the temperature of a static surface, whether or not its temperature is changing, all is good. But what happens when you are changing the surface and its emissivity is also changing? Your pyrometer inaccuracy can increase substantially.
The solution? Emissivity compensation. Done properly and at a fast enough update rate, you can maintain very high accuracy in your thermal measurement.
Take a look at the graph below. The two lines overlapping at 1350C are the desired temperature profile, and the emissivity compensated (actual) temperature. In this case, the inaccuracy suffers only by about 2%. If you are depositing a film on a substrate and the surface is constantly changing, you can see inaccuracies of up to 50%.
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