Question-and-Answer Resource for the Building Energy Modeling Community
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You will need to check whether the backface visibility ("-bv") flag is given to Radiance. If it is, then rays only encounter the surface with normal facing the ray's origin. This is a useful way to give the sides of a surface different reflectance values.
If the -bv flag is not given, the ray might hit either surface and potentially produce different results if the two surfaces have different properties. This should not have a significant effect on calculation time (except for making the octree file bigger) and will not result in duplicate rays (because only one of the two surfaces will be found by each ray).
