[Radiance-general] IES file fun

[Greg Ward]

I can attest that ies2rad doesn't know about these "tricks," as I 
didn't know about these tricks.  I don't think I'll have time to fix it 
in the immediate future, and given that I still don't understand the 
subtleties involved, I wouldn't want to attempt a quick hack in this 
case.  It sounds like Schorsch is definitely onto something, though...

-Greg

> From: Georg Mischler <schorsch at schorsch.com>
> Date: June 12, 2004 3:49:18 AM PDT
>
> Rob Guglielmetti wrote:
>
>> IESNA LM-63-95 implies (but does not specify) that the 0?180
>> degree  vertical plane of the photometric web is oriented parallel to
>> the lamp axis of linear  fluorescent luminaires.
>
> As I understand my copy of the standards document, this is not
> just "implied" but clearly spelled out. At least the phrasing
> left me with no doubt about the intended goal (I'm not at the
> office, so I can't cite the original right now). nevertheless,
> I consider this requirement a conceptual defect in the
> specification.
>
> The practical result is that with asymmetric linear fixtures, the
> C0 plane will show the least interesting curve of the complete
> dataset. For this reason, many manufacturers "cheat". They create
> their files so that the width is given a larger value than the
> length. That way, they can place the most interesting
> (asymmetric) curve in the C0 plane, without violating the letter
> of the standard (of course, they still violate its intention).
>
> Most software reading IES files knows about those tricks, and
> will apply the necessary corrections. I do the same in Rayfront,
> but I didn't check if ies2rad does. In either case, I don't know
> if this is the reason for the troubles described here, but it
> might...
>
> FWIW, the Eulumdat format specifies the orientation of the C0
> plane in the sensible way, so that it will contain the most
> interesting curve for asymmetric distributions.
>
> -schorsch