Radiance and blinds

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It's a little difficult to tell exactly what you are comparing and what you even did (i.e. how you simulated blinds with Radiance). But to answer the last part of the post, if you have a model with no shades and another one with shades applied but set to always up (i.e. not deployed) and simulate them both with the OpenStudio Radiance measure, you will definitely get different results because the "no shades" model will have used the single phase method (rays traced from the POI thru the glass to the sky) whereas the shades model -- even if shades are always up -- will use three-phase method, which uses an "air BSDF" for the transfer matrix at the window. All the transfer matrices used by OpenStudio's Radiance measure are Klems basis BSDFs which are of pretty low resolution.

Slat reflectance is not accounted for in the OpenStudio-Radiance BSDFs either, it's a generic blind.