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

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Thu, 10 Feb 2011 09:25:03 +1100

Matthieu DUBAIL wrote:

From what I experiment, I first can tell that the Argyll 's "Apparent flash
duration" results are falses.

It has very limited accuracy, due to the spectrometer sampling rate.
Generally a flash is too fast for it to be able to measure its duration
accurately (that's why I've termed it "apparent").

So, according to Nikon manual, flash duration should go from 0,00096s (M1/1)
to 0,00005s (M1/32). Argyll gave me the exact two same flash duration,
either 0,021504s and 0,028672s, all along the tests. Since integration time
is far higher than flash duration, is there some kind of ambient light noise
in the flash measurements?

Shouldn't be. The flash is extracted from the background samples.

Ambient light was pretty low but rather constant.  However, "Ambient" data
gaven by Argyll seems rather high (from 570 to 400 Lux-Seconds) and
inconstant. Does it depends on total measurement time?

If you're doing an ambient measurement, the results are in Lux.
A single reading will be taken.

How can I correctly transform cd.m-2.seconds (Y) to cd⋅s? Should I use the
integration area of the Colormunki (6mm or so) ?

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 

If you measure the actual flash output, you should then be able to use
the distance scaling to compute the guide number.

I recap the formulas and excel interpretation of it formula I'm using (cause
I made to much erratum posts):
Niso/gn = √ ( k ∫ I(t)dt )
Niso/gn   is the Guide Number in meters for a given ISO sensitivity of 100
k            is a constant equal to 0,51 lm –1⋅m2⋅s –1
∫I(t)dt      is the time integral of the luminous intensity of electronic
flash equipment, expressed in candella.seconds (cd⋅s = lm⋅sr –1⋅s).

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.

Graeme Gill.

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