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

[Christopher Rush]

Good point about the thickness. If the shade is 3% open I suppose the
expectation would be slightly less than this in pure specular
transmission due to the thickness of the fabric.

Lars, Does this agree with your goniophotometer experience with woven or
perforated materials?

-----Original Message-----
On Behalf Of Lars O. Grobe
Sent: Thursday, January 21, 2010 9:16 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?
>   
Well, it is still a simplification valid only for normal incident
angles, as the fabric has a thickness and the projected area of the
wholes between the fibres depends on that incident angle. You will have
light hitting the front of the fabric being reflected back, the "sides" 
of the opening, being scattered, light passing through the openings
without touching any material being transmitted as specular, light
hitting the fibres but being transmitted as diffuse.... You can assume
to get a pretty diffuse reflection, but the 3% information will not be
enough to accurately descibe the transmissive properties. This would
require either measurements (we have used a goniophotometer for this
type of materials) or a good model for the fabric where you could derive
the raypaths for different incident angles from.

Cheers,

Lars.

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