Hi Sichen,
If you want to do a glare evaluation, in my point of view the core part to be modeled as correct as possible is the peak rather than the diffuse transmittance. Meaning I would rather sacrifice to model a “correct” angular diffuse transmittance than to sacrifice the accuracy of the angular beam transmittance since it is mainly the beam causing glare for such shadings. As Greg mentioned in the other post, it is super tricky to reproduce the real luminance distribution around the peak correctly, especially when using BSDF. (this is one of the reasons to have the peak extraction in aBSDF). For that reason (aBSDF was not available at that time) I used used BRTDfunc to calculate transmittance thresholds for fabrics in EN14501 and EN17037, described in “Annual glare evaluation for fabrics”. It uses an modified Roos model for the angular direct-direct transmittance and has the normal-normal transmittance and the cut-off-angle as input. I would guess the model works reasonably for typical fabrics used to control glare, which means with rather low (<10%) normal-diffuse transmittance (=dark colors).
The glare evaluation should then also include the blurring the peak to mimic the scattering in the eye (that happens similarly when you capture an HDR image). The importance of this (and how this can be done) is mentioned in Greg’s paper and in my presentation at the radiance workshop in Bilbao 2021.
Jan