Attached Link: http://www.renderosity.com/mod/forumpro/showthread.php?message_id=3817130&ebot_calc_page#message_3817130

att lnk may not work in all browsers, but cspear showed that the curve of render time vs IDL IC looks like this (the curve varies between versions):

when IDL was introduced in poser 7, stefan said to use IDL intensity = 0.8, as this is a good fall-off value per bounce, giving a curve that is approx. inverse linear for large surfaces in an enclosed room.  however, if all diffuse values on all posersurfaces are <=0.8, then IDL intensity can be left at 1.0.

when IDL intensity is >1 and some posersurfaces have diffuse value 1, this causes IDL to multiply (rather than falling off) such that some areas that should be occluded become intensely bright.

there was a bug in earlier versions where intersecting orthogonal surfaces and narrow triangles converging on a polar vertex would show artifacts at IDL IC<100, but this may have been fixed in one of the last or next releases.