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

• From: Matthieu DUBAIL <matthieu.dubail@xxxxxxxxx>
• To: argyllcms@xxxxxxxxxxxxx
• Date: Sun, 6 Feb 2011 03:58:42 +0100

```Hi everybody,

I've been trying to determine the guide number of several flash light using
For each measure, Argyll give me Luminance value Y, in cd.m-2.

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 excel transcription would be:
GN = k * Y * D^2

D           is the measuring distance, squared according to the inverse
square law

Problem is using that formula, I got a wrong GN, way greater than it should
be. Where am I wrong?