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The general rule is that when you bounce or diffuse the light, that surface becomes the "source" for discussing fall-off even if inverse square law doesn't technically apply anymore. So if you have the real sun beaming straight into a window, let's say almost horizontally, that has a very gradual fall-off, in fact, the sun hitting someone outside the building is the same intensity as someone's face being hit by the sun inside the room. However, but 216 diffusion over the window and the fall-off inside the room starting at the window is very fast as you move away from the diffusion.
BUT... I've been meaning to test this someday, but I believe in the case of very, very light diffusion (Hampshire, etc.) that what passes through is a mix in terms of fall-off rate, a mix of a far and a near source. I definitely could see that if the diffuser was very porous, like a curtain sheer with a loose weave. However, I've never tested this theory because it would need a stage with a lot of distance so the point source could be very far away so as a base in the experiment (no diffusion) I had a very gradual fall-off rate starting in the center of the room where I would set up the diffusion frame.