[Radiance-general] IES file fun

[Rob Guglielmetti]

Ciao Stefano!

On Jun 11, 2004, at 6:51 PM, Stefano Callegari wrote:
>
> First, are you sure the ies data are correct?

It looks fine to me in two different photometric file viewers, as well 
as in plain text...
>
> It seems a symmetric fitting.

It is.  No problems there.  That was just for example.

>> http://www.rumblestrip.org/site-img/rad/assym.jpg
>
> This time it seems an asymmetric fitting. What you meen with
> *directional*?

Sorry, I mean exactly the same thing, asymmetric.

>> Eureka!  I have defied physics!  It was after looking at this image 
>> that
>> I re-checked the ies file, and discovered that it's the LM-63-95 spec.
>> This is a problem, because the spec calls for the 0-180 vertical plane
>> to be oriented parallel to the lamp axis, whereas it's more common to
>> orient it perpendicular.  ies2rad can't even have a check for this,
>
> It's right, the 0-180 plane normally must be oriented parallel to the
> lamp axis. This is a general rule to point right the fitting.

Depends on which version of the IES spec you reference, and this is the 
whole problem.  There is a good bit about it in Ian Ashdown's Lightfair 
presentation "Thinking Photometrically".  A link to the course notes is 
at the bottom of the page referenced by this url (there is also an 
excellent summary of the IES file format in there):  
http://www.helios32.com/resources.htm

I have cribbed from these course notes the following section 3.6.2, 
"Luminaire Orientation and Position":

"Given a luminaire with an asymmetric luminous flux distribution (such 
as a fluorescent  wall-washer), it is essential to orient the luminaire 
correctly in a CAD model.  Unfortunately, this can be more difficult 
than you might expect.

"The first problem is that the various IESNA LM-series documents 
provide contradictory  specifications on how the photometric web is to 
be oriented with respect to the physical  outline of a luminaire.  For 
example, 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. If the luminous flux 
distribution is bilaterally symmetric about the  90–270 degree vertical 
plane (that is, perpendicular to the lamp axis), then the horizontal 
angles must be reported from 90 to 270 degrees.

"However, IESNA LM-41-98 recommends (but does not specify) that the 
photometric web  for such luminaires be oriented perpendicular to the 
lamp axis, with 0 degrees being the “beam side.”  Most (but not all) 
fluorescent lighting manufacturers have ignored IESNA LM-63-95 and  
followed the recommendation of IESNA LM-41-98. This means that the 
lighting design  software program user must manually examine the IESNA 
LM-63 text file to see whether  the photometric web is oriented 
parallel or perpendicular to the lamps axis. If this  information is 
not stated in the file header, the user may have to contact the 
luminaire  manufacturer."
   - From Ian Ashdown's "Thinking Photometrically"

So, I guess we could call it a faulty file, but really it seems to me 
to be a difference in opinion as to how the photometric web is supposed 
to be arranged, between the manufacturer that supplied this IES file, 
and the IES spec. Or am I missing something (else)?

-Rob G.