I Why use Stefan's Law to measure temperature?

[sophiecentaur]

TL;DR

Why is radiant energy flux used to estimate temperatures in thermographic cameras

Measuring the temperatures of bright (visible spectrum) cosmic objects would use spectral analysis. But temperatures of IR (warm / hot) radiators is done using Stefan's Law with radiometric cameras. There seems no reason why suitable filters couldn't be used to find the black body temperature using the ratio of two measured intensities after the fashion of colour TV cameras. Why?
Could it be to do with the actual size of IR filters to mount on a two channel image sensor array? I'm sure there is a good reason for this but I can't think of one.

 

[Charles Link]

I don't know that I can answer what you are asking, but for a laboratory type blackbody at approximately 1000 degrees Centigrade, one of the standard methods to measure its temperature about 30-40 years ago was to use an optical pyrometer where the color/brightness was matched between an internal heated filament and the source being measured.

The commercially available calibration blackbody sources also often had internal thermocouples to monitor their temperature around this same time.

Back about 40 years ago, I was successful at determining the temperature with an alternative measurement where a pyroelectric detector with a very broad and uniform spectral response measured the total energy collected, using Stefan's law. The pyroelectric detector was an instrument that was sold commercially, (my company had purchased the one that I used for my experiment), and was calibrated electrically with a resistor in contact with the pyroelectric detector. It was known as the ECPR=electrically calibrated pyroelectric radiometer. There was minimal atmospheric absorption over the one foot path from source to detector. The measurement had an accuracy comparable to that of the optical pyrometer, plus or minus 3 degrees or thereabouts.

Since that time, I think such a measurement might have become fairly routine, where cameras with pyroelectric sensors can use Stefan's law to measure the temperature.