I guess I fail to see the reason for the diffusion. No 350th ship I have built has portholes large enough for my eye to see any detail behind the porthole- same thing with windows, except maybe bridge windows. I personally flat black the bridge, but don't bother to diffuse the portholes. I use pvc window material and the shape it dries to does kind of distort the image, but I cannot discern any detail anyway.