[Radiance-general] Properly defining a roller shade fabric as a TRANS material type

[kkonis at berkeley.edu]

Dear list,

I am trying to describe a nominally "white" roller shade fabric with a
3% openness factor as a trans material type.

I don't have a real fabric on hand to measure, I'm just trying to make
sure i understand the material definition parameters and then to do
some studies varying the openness factor of a hypothetical woven
roller shade material.

1) I'm curious to know if by setting parameter A7 to 1 i am achieving
my desired goal of representing a shading fabric that acts as an
opaque sieve (e.g. a cheese-grater: light passes only through the 3% of
the surface
which is void
(my interpretation is that diffuse transmission requires some portion
of incident light to pass through the threads)

-any comments on the below process and resulting trans description
would be appreciated.......


# USING THIS FORMULA from RWR and previous email discussions:

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) )

# INPUTS

Td = 0          # ASSUME no diffuse transmission (e.g. fabric threads are
totally opaque)
Ts = 0.03               # ASSUME that (3% openness factor of shade "weave")
means fraction of light transmitted as beam (e.g. not scattered
diffusely) = 0.03
Sr = 0
Rs = 0
Cb = 0.7                # Nominally "white" fabric surface (reflects 70% of
incident visible light?)
Cg = 0.7
Cr = 0.7

# CALCS

A7 = (0.03) / (0 + 0.03) = 1
A6 = (0 + 0.03) / (0.7 + 0 + 0.03) = 0.0411
A5 = 0
A4 = 0
A3 = 0.7 / ( (1 - 0)*(1-0.0411) ) = 0.73
A2 = 0.7 / ( (1 - 0)*(1-0.0411) ) = 0.73
A1 = 0.7 / ( (1 - 0)*(1-0.0411) ) = 0.73

# RESULT

void trans my_shade
0
0
7 0.73 0.73 0.73 0 0 0.041 1


thanks,

Kyle Konis
PhD Student
UC Berkeley