# under water light fixtures

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
[email protected]
<blocked::mailto:[email protected]>

Lightswitch Wizard (initial design) www.buildwiz.com
<blocked::http://www.buildwiz.com/>
DAYSIM (expert software) www.daysim.com
<blocked::http://www.daysim.com/>

"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
5 .8 .8 .8 1.3 0

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

···

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 [email protected] <blocked::mailto:[email protected]>
Lightswitch Wizard (initial design) www.buildwiz.com <blocked::http://www.buildwiz.com/> DAYSIM (expert software) www.daysim.com <blocked::http://www.daysim.com/>

_______________________________________________
[email protected]

Hi Christoph,

how to model light traveling through water

If you mean seeing the beam that results from scattered light, then you'll need to use the mist material. Check out Section 8.4.2 (p 457) in RwR. I recall seeing somewhere that the mist material is also implemented in pmap.

Cheers,

-John

···

-----------------------------------------------
Dr. John Mardaljevic
Senior Research Fellow
Institute of Energy and Sustainable Development
De Montfort University
The Gateway
Leicester
LE1 9BH, UK
+44 (0) 116 257 7972
+44 (0) 116 257 7981 (fax)

Hi Christoph,

John is right. If you want to see the light beam in the water, the mist material is for you. Follow the examples and stop the mist volume right before the light source, and preferably just inside the walls and water surface of your pool. List the underwater light source(s) in your mist's string arguments. You should specify a high scattering albedo (probably 0.97 0.99 0.99) and a fairly high H-G constant (0.85 or so).

The water surface needs to be a dielectric as Ignacio indicates. If you apply a texture for a natural wavy appearance, you will see the distorted bottom of your pool and the scattered beam of your source, but you will be missing caustics from the water's surface on the pool bottom and the surroundings above water, as Radiance doesn't know how to calculate those properly. Even without a texture, you won't get proper illumination of the bottom from lights above, I'm afraid. The best you can hope for is sky contributions from the ambient calculation.

Attached is an example description of a 10 by 5 meter pool with two (diffuse) lights.

-Greg