[argyllcms] Re: Absolute light meter calibration?

  • From: Richard Kirk <richard@xxxxxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Thu, 13 Aug 2015 08:51:25 +0100

On 08/12/2015 09:32 PM, Marwan Daar wrote:

Do you accurately know the surface area of the filament? The
emissivity is only useful if you know the watts per square meter
leaving the surface from the temperature and the emitting area. In
the bulb we have a certain amount of emission which is caused by
the input electrical power; but we would get just the same from
another bulb with half the emissivity and double the surface area.

I think...

Either way, it seems that you can't assume that all the input electrical power is converted into radiant flux.
Why not? Where else does the energy go? The filament is held up by the ends, so there is very little conduction. The bulb is evacuated, though a tungsten-halogen lamp may have some convective and chemical means of energy transport, the actual energy losses should be tiny as a fraction of the whole. And that may be balanced by the warm glass envelope radiating some heat back at the filament when the bulb reaches a steady state.

You seem convinced I cannot do this, even though I did it once, and it seemed to work at the time. I abandoned it for purely practical reasons: they bought me a good spectrometer that I got calibrated once a year, so I could concentrate on the important stuff. I still have the bits, but I don't think I will get back to it anytime soon.

It's not important. I doubt I can convince you, but others might like a go.

Richard Kirk

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