[Radiance-general] Re: luminaire modeling Was: set overflow in addobject

[Krzysztof Wandachowicz]

Hi Chas,
You are right, I try to calculate light distribution of luminaires. I can do
it for diffuse luminaires using Radiance. It works for luminaires with
diffuse reflectors and luminaires with diffusers (thanks Greg fur such
wonderful stuff). You can see Desktop Radiance Tutorial How to Calculate
Luminaires Using Radiance (only in Polish):
http://lumen.iee.put.poznan.pl/kw/oprawy/p-opr.html

This tutorial is for my students. They have exercise at fifth year of study
and make project of luminaire using Radiance. Now three of them try to take
Master of Science and their subjects are connected with calculation of
luminaires using Radiance. One of them try to calculate light distribution
of specular reflectors. It is possible to do it with Radiance. When we want
to use backward ray tracing method we look for Figure of Bright Points at
the surface of reflector. This is area where we can see image of lamp (light
source). This is very simple to calculate but the problem is that when we
use Radiance we have to divide reflector's surface into very small
elementary surfaces. Each of these elementary surfaces should be smaller
than dimension of lamp (light source). This is the reason why I build my
reflector with 15750 pieces.
Parabolic mirror reflector produces parallel beam (with point source placed
at focus) and none of polygons can see each other. Generally most of mirror
reflectors are build in such way.

Unfortunately I can't make my calculation with 15750 polygons. Greg, thanks
for your advise. I can start oconv process only at -r 389376 but after 3
hours I get:

Out of memory in emalloc (when I use Desktop Radiance)

oconv: system - out of octree space (when I use Radiance)

Best regards,
Krzysztof Wandachowicz
------------------------------------------------
Politechnika Poznanska, Instytut Elektrotechniki Przemyslowej
Zaklad Techniki Swietlnej
http://lumen.iee.put.poznan.pl/kw/
-----------------------------------------------

----- Original Message -----
From: Charles Ehrlich
To: radiance-general@radiance-online.org
Sent: Sunday, April 20, 2003 11:20 AM
Subject: [Radiance-general] Re: luminaire modeling Was: set overflow in
addobject


Hi Krzysztof,

I'm interested to learn if you are successful at using Radiance to model
parabolic reflectors.  I presume you are trying to get an accurate
distribution of the light fixture.  When I have used Radiance to model light
tubes such as tubular skylights, the ray-tracing method of Radiance breaks
down after about 20 reflections--with only 7 surfaces!  This means that
light will not emit from the tubular skylight at angles between 180 degrees
from nadir (grazing the ceiling) to 160 degrees.

If many of your mirror polygons can "see" each other, then you may have the
same problem.  If you limit the number of reflections, then there is a
certain amount of error introduced into the calculation for those rays that
get "trapped".

What is really needed is a forward ray-tracer.  Have you tried the product
called "Raydirect" that Georg Mischler and friends is providing?  Georg,
would it work for this application?

-Chas