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

[Christopher Rush]

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?

-----Original Message-----
From: radiance-general-bounces at radiance-online.org
[mailto:radiance-general-bounces at radiance-online.org] On Behalf Of Rob
Guglielmetti
Sent: Thursday, January 21, 2010 1:59 PM
To: Radiance general discussion
Subject: Re: [Radiance-general] 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, kkonis at berkeley.edu 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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> Radiance-general at radiance-online.org
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>



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


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