An optical pyrometer allows one to measure the temperature of an incandescent source by comparing the appearance of light radiated from the target to the known appearance of a calibrated lamp. Both radiators are assumed to be black bodies.

In the instrument shown a rotating optical wedge functions as a variable neutral density filter. In 2002 the cost was in the 3-5000 US$ range. Three years later, similar instruments can be found for 6-800 US$. We have ordered one, and look forward to being able to confirm Stefan's law with a light bulb.

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