Properly defining a roller shade fabric as a TRANS material type

Hi Kyle,

I am not at all the materials measurement/definition guru on this list,
but I do have some comments. First off, you should not use .03 for
specular transmittance based on the openness factor. You should specify
the transmittance. Have a look at the product specs for the shadecloth;
somewhere there should be a reporting (however roughly it may have been
estimated) for the transmittance of the shade.

Next, even though in theory the holes are allowing light to pass straight
through, there will be some scattering at that interface due to the weave
(especially in the case of your white colored shade), so there will be
some diffuse component to this material. And therein lies the rub:
figuring out how much of a diffuse component...

I recommend finding out the transmittance of the cloth, and than taking
some fraction of that and putting that into the diffuse transmittance
portion of the material definition. For example, if the shade has a
transmittance of 20%, maybe specify Td = .05 and Ts = .15. I'd also
recommend getting a sample; even if you can't measure it, you could at
least observe it under different lighting conditions and then mix and
match Td and Ts until you achieve L^ar (looks about right). =8-)

- Rob

···

On Thu, January 21, 2010 10:45 am, [email protected] wrote:

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

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===================
Rob Guglielmetti
www.rumblestrip.org

You're right Chris, at least that is probably a good starting point.

···

On Thu, January 21, 2010 12:10 pm, Christopher Rush wrote:

I would have guessed if your total measured transmission was 20%, and
you know the openness is 3%, the 3% would correspond to specular
transmission and the other 17% must be diffuse transmission, no?