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Re: Optical pyrometers, was R = V/I ?



I assume, Leigh, that the purpose of the chopper was to measure
temperatures of sources which too high for the pyrometer. I would
expect the frequency to be at least 30 Hz, to avoid blinking.

On a related issue: do your students use an optical pyrometer to
measure the filament temperature? I think this instrument (which may
be a tad expensive) is a marvelous one to introduce students to, and
it fits well into the course when one is studying Stefan's law as
your students are doing. 42 years ago, while I was working at General
Atomic in La Jolla, a colleague devised a method by which the optical
pyrometer temperature range could be extended downward. He used a
chopper wheel running at about 10 Hz to make the dimly glowing
filaments visible. I think it got written up in RSI or somewhere like
that.

Actually a simple automobile light bulb (plus a rheostat and a battery)
can be used to construct a simple pyrometer. I once built one. The
difficulty comes with calibration; one need glowing sources of known
temperatures. If my memory is still good a dark adapted eye starts seeing
a glow at about 450 C. What are other "easy to get" reference points?
Chopper is a good idea to use an instrument calibrated below 1000 C at
higher temperatures. It is a potentially good student project.
Ludwik Kowalski