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

• From: Matthieu DUBAIL <matthieu.dubail@xxxxxxxxx>
• To: argyllcms@xxxxxxxxxxxxx
• Date: Wed, 9 Feb 2011 18:18:00 +0100

```Hi Graeme,
Thanks for answering. I've been struggling on that problem for a few days
now.

Yep, in fact, I was asking myself whether I had misinterpreted the GN
formula or Argyll wrong measurement. I just didn't wanted to formulate the
question like that at first.

From what I experiment, I first can tell that the Argyll 's "Apparent flash
duration" results are falses.
I measured the Nikon SB700 flash at different power settings.
The Nikon SB700 manual specify that  flash duration depends on flash power:
"*
Flash duration (approx.):
1/1042 sec. at M1/1 (full) output
1/1136 sec. at M1/2 output
1/2857 sec. at M1/4 output
1/5714 sec. at M1/8 output
1/10000 sec. at M1/16 output
1/18182 sec. at M1/32 output
*"
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?
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?

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) ?

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).

*excel:*
GN = D * SQRT( k * Y )
GN         is the guide number
D           is the flash-colormunki distance in meter
k            is a constant equal to 0,51 lm –1⋅m2⋅s –1
Y           is the luminance as given by Argyll

I didn't found any GN measurement through internet, I had to buy the ISO
paper.
I'm pretty shure I can't diffuse it so I'll send it to you via PM. Sorry for
those who are reading this post.

Thanks!
Matthieu DUBAIL

2011/2/9 Graeme Gill <graeme@xxxxxxxxxxxxx>

> 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.
>
>
```