[Radiance-general] making my own ies file

[Rob Guglielmetti]

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