*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 sensitivity. 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): *ISO:* 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.

**Follow-Ups**:**[argyllcms] Re: Spotread -f and flash Guide Number calculation problem,***From:*Matthieu DUBAIL

**References**:**[argyllcms] Spotread -f and flash Guide Number calculation problem,***From:*Matthieu DUBAIL

**[argyllcms] Re: Spotread -f and flash Guide Number calculation problem,***From:*Graeme Gill

**[argyllcms] Re: Spotread -f and flash Guide Number calculation problem,***From:*Matthieu DUBAIL

- » [argyllcms] Spotread -f and flash Guide Number calculation problem,- Matthieu DUBAIL
- » [argyllcms] Re: Spotread -f and flash Guide Number calculation problem,- Matthieu DUBAIL
- » [argyllcms] Re: Spotread -f and flash Guide Number calculation problem,- Matthieu DUBAIL
- » [argyllcms] Re: Spotread -f and flash Guide Number calculation problem,- Graeme Gill
- » [argyllcms] Re: Spotread -f and flash Guide Number calculation problem,- Matthieu DUBAIL
- » [argyllcms] Re: Spotread -f and flash Guide Number calculation problem,- Matthieu DUBAIL
- » [argyllcms] Re: Spotread -f and flash Guide Number calculation problem,- Iliah Borg
- » [argyllcms] Re: Spotread -f and flash Guide Number calculation problem, - Graeme Gill
- » [argyllcms] Re: Spotread -f and flash Guide Number calculation problem,- Matthieu DUBAIL
- » [argyllcms] Re: Spotread -f and flash Guide Number calculation problem,- Iliah Borg
- » [argyllcms] Re: Spotread -f and flash Guide Number calculation problem,- Matthieu DUBAIL
- » [argyllcms] Re: Spotread -f and flash Guide Number calculation problem,- Matthieu DUBAIL
- » [argyllcms] Re: Spotread -f and flash Guide Number calculation problem,- Matthieu DUBAIL