Properly defining a roller shade fabric as a TRANS material type

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.