[argyllcms] Re: Spotread -f and flash Guide Number calculation problem,

  • From: Matthieu DUBAIL <matthieu.dubail@xxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Fri, 11 Feb 2011 14:32:40 +0100

> It has very limited accuracy, due to the spectrometer sampling rate.

That's what I tought. Do you know if an Eye-One has a better sampling rate?
Appart from that, the -h high rez is a pretty badass feature for testing
light sources.

 It seems to me that the brightness measured by the instrument will depend
> on the distance to the flash. That distance should be in your formula
> somewhere.

So somewhere there should be a target of a given number of Lux seconds
> to get proper illumination at a given distance, f number and ISO 100
> sensitivity.
> If you measure the actual flash output, you should then be able to use
> the distance scaling to compute the guide number.

Actually Ngn is the distance. So there should be a simple multiplication
factor of the whole formula.

> Typically there is a factor of pi to convert between cd and Lux (due to
> the assumption of 180 degree angle). What exactly is the constant k's units
> ?
> You need to get the units to match.

I reread the whole paper, pointing out several aspects that doesn't seam
very clear. I made several jpg of this reading as attached files.
I'm really stucked on this calculation. Damn...

Attachment: GN-1.jpg
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Attachment: GN-2.jpg
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Attachment: GN-3.jpg
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Attachment: GN-4.jpg
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Attachment: GN-5.jpg
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Attachment: GN-6.jpg
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