[Radiance-general] under water light fixtures

[Ignacio Munarriz]

"A dielectric material is transparent, and it refracts light as well as 
reflecting it. Its behavior is determined
by the index of refraction and transmission coefficient in each wav 
elength band per unit length.
Common glass has a index of refraction (n) around 1.5, and a 
transmission coefficient of roughly 0.92 over
an inch. An additional number, the Hartmann constant, describes how the 
index of refraction changes as a
function of wav elength. It is usually zero. (A pattern modifies only 
the refracted value.)"

dielectric is needed to simulate water refraction

The problem with dielectric in Radiance is that direct light doesn't 
know how to return to light sources when it cross dielectric, when you 
are seeing a point under the water radiance tries to illuminate it with 
direct light connecting it with direct lights with a stright line but 
when it traces the ray to the direct light the ray deviates with 
dielectric and it probably doesnt arrive to the light source(depending 
on its size and angle of incidence with dielectric).
Probably you will get better results using photton mapping of roland 
with dielectric surface,

example a 1.3 refractive index water

void dielectric water
0
0
5 .8 .8 .8 1.3 0

water polygon planoagua
0
0
12
0 0 .5
3 0 .5
3 3 .5
0 3 .5

> Dear Radiance colleagues:
>  
> One of my students is interested in modeling an under-water light 
> fixture for a swimming pool in Radiance. I have modeled a water 
> surface in the past, but I am not sure how to model light traveling 
> through water. I vaguely recall hearing that to do so one must model 
> the water volume as a dielectric. Could anybody explain to me how this 
> is done? An example file would be very much appreciated.
>  
> Christoph
> Christoph Reinhart, Ph.D.                       
> Associate Research Officer                      National Research 
> Council Canada          Institute for Research in Construction
> Adjunct Professor                                     McGill 
> University                                      School of Architecture
> 1200 Montreal Road M-24, Ottawa          Ontario K1A 0R6, Canada
> tel: (613) 993-9703                                  fax: (613) 954 
> 3733                                 christoph.reinhart at nrc-cnrc.gc.ca 
> <blocked::mailto:christoph.reinhart at nrc-cnrc.gc.ca>
>  
> Lightswitch Wizard (initial design)            www.buildwiz.com 
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> DAYSIM (expert software)                        www.daysim.com 
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