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

[Lars O. Grobe]

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.