[Radiance-general] External 'clear' shading

[Greg Ward]

Hi Per,

If you knew a few more details, such as the type of frit and percent  
coverage, you could use the Radiance glaze script to arrive at a more  
accurate model.  Based on the information you have and a reasonable  
assumption of Rs=0.08, you can use a trans material (also see  
attached excerpt from RwR):

void trans fritted_glass
0
0
7 0.422 0.422 0.422 0.08 0 0.434 0.5

I got this using:

A6 = (Td + Ts) / (Rd + Td + Ts) = 0.23 / (0.23 + 0.30) = 0.434
A5 = Sr = 0
A4 = Rs = 0.08
A3 = A2 = A1 = (Rd - Rs) / ((1-Rs)*(1-A6)) = (0.30 - 0.08)/((1-0.08)* 
(1-0.434)) = 0.422

The only thing that's really a guess is A7, the fraction of specular  
transmission.  This would require an additional measurement or the  
percentage frit coverage.  I guessed 50% coverage.

I hope this helps!
-Greg

Excerpted pages 325-326 from "Rendering with Radiance" <http:// 
tinyurl.com/2wn78b>:

5.2.6 Modeling Transmitting Media
Some luminaires use semitransparent surfaces to cover the bright  
light sources hidden
inside. This can include downlights and other lensed luminaires. In  
addition,
some daylight control products, such as curtains and fritted glass,  
are most appropriately
modeled as a semispecularly transmitting medium. In these cases, it is
important to know how to properly specify the trans material type to  
accurately
model the appearance of the luminaire or other light-transmitting  
architectural
element.
Specifying a trans material can be intimidating. It is one of the  
most confusing
material entities in the Radiance repertoire. However, it is the  
simplest material that
will trace direct source rays through a semispecular surface in order  
to determine
the diffuse and specular transmitted components.
You will have to measure the following surface properties in  
preparation for the
creation of a trans material type:
• Diffuse reflectance (RGB): The color will affect both diffusely  
reflected light (if
there is any surface roughness) and transmitted light. Call the red,  
green, and
blue components Cr, Cg, and Cb. Calculate the photopic average of the  
RGB
and call this Rd.
• Reflected specularity: As with glass, this is the fraction of light  
that is reflected
off the first surface in a mirror-like way. This we’ll call Rs; it is  
equal to floatingpoint
argument 4, A4.
• Surface roughness (RMS): Facet slope as in plastic. Call this Sr.
• Diffuse transmissivity: Fraction of light that passes all the way  
through the surface
diffusely. Call this Td.
• Transmitted specularity: Fraction of light transmitted as a beam— 
that is, the
fraction of light not diffusely scattered. Call this Ts.
The following formulas can be used to calculate the A7 through A1  
parameters
for the trans material:
A7=Ts / ( Td+Ts )
A6=( Td+Ts ) / ( Rd+Td+Ts )
A5=Sr
A4=Rs
A3=Cb / ( (1-Rs)*(1-A6) )
A2=Cg / ( (1-Rs)*(1-A6) )
A1=Cr / ( (1-Rs)*(1-A6) )
The behavior of trans is regulated by the -st specular threshold  
rendering parameter.
The following formula can be used to determine the appropriate -st  
setting for
rview, rpict, or rtrace to ensure that the transmitted (and  
reflected) semispecular
component will be rendered. The variables A1 through A7 are the first  
through seventh
floating-point arguments to the trans primitive as calculated above.
St = A6 * A7 * (1 - grey(A1, A2, A3) * A4)
Unlike most other Radiance material primitives, the trans material is  
neither
intuitive nor straightforward to apply. It is a good idea to keep  
ample notes for later
reference when you are creating a trans material. Backing out your  
assumptions
from an unannotated trans primitive is difficult.

> From: Per Haugaard <perhaugaard at yahoo.dk>
> Date: February 11, 2008 11:21:16 AM PST
>
> Dear Radiance users,
>
> I am new to Radiance.
>
> An existing building is being renovated in order to reduce its need  
> for primary energy and optimize the building envelope daylight  
> penetration.
>
> I order to reduce the need for cooling an external shading (glass  
> with printing) is used. The data for the shading material is:
> Light reflection (for both sides): 30%
> Light transmission: 23%
>
> It is important to include the both parameter (transmission +  
> reflection) in the study in order to clarify possible glare and in  
> order to maximize daylight distribution to the room.
>
> My question is: what material would be the best to use for the  
> shading material in order to model this scene correctly and do I  
> need to include skyfunc and illum for this?
>
> Thank you in advance :)
>
>
> Best Regards
>
> Per Haugaard
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