*From*: Graeme Gill <graeme@xxxxxxxxxxxxx>*To*: argyllcms@xxxxxxxxxxxxx*Date*: Wed, 09 Feb 2011 23:02:23 +1100

Matthieu DUBAIL wrote:

For each measure, Argyll give me Luminance value Y, in cd.m-2.

Actually it's meant to be cd.m-2.seconds for the flash, since it's the integral of the luminance over the flash duration. [Mind you, with nothing to verify it against, it's hard to be 100% sure it works as intended!]

For a given photographique ISO sensitivity of 100ISO, the way to measure the Guide Number (GN) is: Niso/gn = k ∫ I(t)dt 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). If I clearly understood, ∫I(t)dt is the total amount of light emitted by the flash... so = Y, am I right?

The thing to check is whether the units work out correctly. If the instrument is measuring cd.m-2.seconds, and you formula requires cd⋅s, then there is an area that needs to go in there somewhere. [Do you have a source for that formula ? Most of the reference on the web are to do with using GN, rather than measuring it... 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] 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