The BRDFs at 0 deg and 15 deg in incident angle were measured for the wide angle with the same way as elog2034 and Fig. 1 but finer scaning.
The results for P-pol and S-pol are shown in FIgs. 2 and 3.
It looks that there is no difference for polarization.
The tendency that the BRDF increases from -75 deg is consistent to the previous result of the BRDF measurement of the viewport used in LIGO.
Between -40 deg and -70 deg, we face the sensitivity limit of the PD (PDA100A). (In this range, the estimated power is 1 nW and the power range of the PDA100A is 500 pW in the specsheet.)
For comparing the two BRDF, the BRDF plot in which the BRDF at 15 deg incident angle is shifted by 30 deg is shown as Fig. 4.
In this plot, the BRDF for P-pol is picked.
It looks that there is no difference between 0 deg and 15 deg in incident angle from -40 deg to 40 deg.