Well, the shadow is there because the light that would normally be taking up
the shadow space has been refracted and is instead landing elsewhere,
creating the caustic patterns. You can see why this is done by making the
ior of your disc equal to 1. Then the photons should almost exactly cover
the shadow, cancelling it out, except for innacuracies. If the disc had no
shadow, that area would be brighter, which is physically unrealistic.

Simple workaround: Render the scene with the shadow, and save the photons
with save_file. Then remove the disc from the scene, and use the saved
photons with load_file.

 - Slime
[ http://www.slimeland.com/ ]

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