Problem using the BSDF modifier with .xml file

I’m quite new to Radiance and for the past couple of days I’ve been having some problems using BSDF’s.
Just to try it out, I tried to compute a BSDF from a glass polygon using genBSDF which gave me a .xml file. Then I tried to define another material with the BSDF modifier using the .xml file as an argument. However, rendering the same picture, one with the original glass polygon and another with the BSDF-glass polygon, produces quite different outcomes. My question now is: Should the rendering outputs be different, and if so, why? Or should they be the same, and if so, is there something obvious I’m missing?
Thankful for help,

Welcome, Isak!

The results are expected to be different, because the default representation produced by genBSDF is a Klems matrix, which has only 145 directions per hemisphere. This equates to an angular resolution of 10-30° depending on the polar angle. Glass of course has a pure specular component, which then gets spread out by this crude representation. If you use the aBSDF type, most of the issues associated with the through component will resolve, but the reflections will still be very crude.

You can use the tensor tree representation to get better angular resolution, but in any case, a BSDF is not the best way to model glass, unless you need it for some other purpose, such as an annual simulation.

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