[Radiance-general] making my own ies file

[Gregory J. Ward]

 From Rob Guglielmetti, who isn't able to get through to the list at  
the moment...

Subject: 	Re: [Radiance-general] making my own ies file
Date: 	Thu, 03 Aug 2006 10:12:00 -0600
From: 	Rob Guglielmetti <rpg at rumblestrip.org>
To: 	Radiance general discussion <radiance-general at radiance-online.org>

Chris Foster wrote:


> Hi Dave
>
> yeah i know what type of lamp, it was in the last email i wrote to  
> christian, its a 4000w fresnal lens spotlight from Arri. how do the  
> mean and initial lumens value help me? do i use one of them in the  
> 'lumens per lamp' value for the ies file
>
>

Hi Chris, et al.:

Chris, the fresnel lens has nothing to do with the lamp.  Just want  
to be clear on that.  The lens is part of the luminaire itself and  
its behavior should be accounted for in the candela values you got  
from the website. Arri makes luminaires, not lamps.  (And that 4K  
fresnel is a kick-butt luminaire!!  I used their 2K spots in my  
theatre days; those 4K's must _really_ throw some light. The lamp  
socket for the 2K was nuts, with its own mechanical clamp that held  
it in place.  I still remember the lamp code: CYX.  Don't ask me why  
that tidbit remains in my brain sixteen years after the fact, when I  
can't even remember what I had for dinner last night.  Anyway...)

The lumens/lamp field really should contain the initial lumens of the  
lamp used in the luminaire when it was photometered, per the  
specification.  That's just good practice in case someone else ever  
uses that file and they have a different lamp; that way one can  
tailor the output to reflect a different lamp output using the  
multiplier (see below).  The fact of the matter is that in Radiance  
(and I'm pretty sure this is true of AGI as well), it really doesn't  
matter what you put in the lumens/lamp field in the ies file, as the  
candelas are what they are, regardless.  Like I said, it's just good  
practice to follow the spec.  That said, in Lightscape, if you  
changed the lumens in that field, you changed the output of the  
luminaire, but this is not really the way things are supposed to  
work.  That's what the multiplier is for.  The multiplier field  
allows you to adjust the luminaire output to reflect a different lamp  
output, as well as the effect of all the other elements that go into  
the Light Loss Factor (LLF).

This brings me to mean vs. initial.  Lamp catalogs usually have both  
values, the mean value quantifying the lamp's lumen depreciation over  
time.  It's nice to have both because then you can compute more  
accurate LLD multipliers (mean lumens/initial lumens=LLD), as they  
vary from lamp type to lamp type, and manufacturer to manufacturer.  
Of course LLD is only one element of the total LLF.

Note also that ies2rad has its own multiplier (-m), so you could  
leave the multiplier in the ies file at 1 and use the -m to apply  
your multiplier (this is what I do).
- Rob Guglielmetti
www.rumblestrip.org