advanced rendering for AutoCAD
Check this out!
Place a 3D face over the window opening, assign a clear material to it, tag it as thin and daylight opening.
Then choose Interior with Daylight as your set up.
Then render for about 100 passes.
If you post the model here I'll have a look at it for you.
thanks . and the passes have a differently algorithmic pattern ,compared with unpathtracer rendering.