[Radiance-general] IES file fun

[Georg Mischler]

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

-- 
Georg Mischler  --  simulations developer  --  schorsch at schorsch com
+schorsch.com+  --  lighting design tools  --  http://www.schorsch.com/