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

• 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.

```