[argyllcms] Re: Absolute light meter calibration?

  • From: Marwan Daar <marwan.daar@xxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Thu, 13 Aug 2015 12:15:04 -0400

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, I think you've misconstrued my intentions here. I've just noticed
that there hasn't been any discussion of emissivity here, which, as I
naively understand it, indicates the efficiency at which a substance
radiates energy as a function of input energy.

Here is my understanding:

A true blackbody will emit 100% of input power as radiant flux. Therefore,
if you know the input power, you know the total radiant flux.

Tungsten is not a true black body. It has an emissivity of about 0.35, and
is therefore a gray body. Therefore, you must take this into account when
calculating total radiant flux.

Now, I may be completely off base here in my understanding of emissivity.
For example, the emissivity may only be relevant when considering the
emission of radiant flux vs *input radiant flux*, rather than the emission
of radiant flux vs input of *electrical current*. Perhaps tungsten really
does convert 100% of electrical current into radiant flux.

But until you, or someone else, clarifies this objection, which, to me
seems a perfectly reasonable objection, then it's impossible for me to be
"convinced".

Marwan

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