[Radiance-general] Re: Directional reflectance of glass

[Greg Ward]

Hi Chris,

Radiance doesn't find the illumination of the sun off the glass  
material because the sun is too small to locate with random (Monte  
Carlo) sampling.  The "mirror" type is the one to use, and you can  
account for the effect of angle on reflection by including an  
appropriate brightfunc pattern.  Assuming standard 88% transmittance  
glass, this would be:

void glass glass_alt_mat
0
0
3 0.96 0.96 0.96

void brightfunc glass_angular_effect
2 A1+(1-A1)*(exp(-5.85*Rdot)-0.00287989916) .
0
1 0.08

glass_angular_effect mirror glass_mat
1 glass_alt_mat
0
3 1 1 1

The Fresnel approximation for unpolarized light was lifted out of src/ 
rt/normal.c.  To change the reflectance of your glazing, you have to  
change both the glass_alt_mat definition and the 0.08 normal  
reflectance value in glass_angular_effect.

Hope this helps.
-Greg

> From: Christian Humann <chumann at broadviewnet.net>
> Date: March 17, 2008 6:51:09 PM PDT
>
> Hello all,
>
> I'm trying to simulate the occurrence of direct beam convergence;  
> where sun hitting a concave, east facing, glass facade is  
> redirected to form a "hot-spot" on the ground plane.  I have a  
> photograph, and the time taken, of the building in question forming  
> a hot spot as well as a 3D model. Using the "mirror" material type  
> I can get an approximate facsimile, but not with the "glass"  
> material. I've tried the glass using "mkillum" and setting -ar 0  
> but I still can't "fry" those ants.
> I'm under the impression that Radiance only considers the glass'  
> reflected diffuse component, and not the direct (although I can see  
> the sun's reflection in the glass)?  While close, I don't believe  
> the "mirror" material will give the accuracy we're looking for due  
> to the difference in effect the angle of incidence has on the  
> amount of light redirected off the two materials.  Any thoughts,  
> advice or observations would be greatly appreciated. Thanks.and  
> regards.
>
> Chris Humann