just did some tests in a very splotchy IDL setup, and I cannot determine noticeable differences between 100% transparent and fully invisible. Of course, only unchecking Visible to camera leaves the shadows intact. But - in my test - there are no ghost splotches floating around on an otherwise absent surface.

but given your lighting: transparency only works on (alt_)diffuse. Specularity is not affected (unless plugged into alt_diffuse as well). Does it make a difference when the specularity channel is blacked out on the direct lights? Just guessing.

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though