yes, dv, just to concur with ya. I found that a filter setting of 0 gives a blotchy-looking caustic, and a setting of 100 gives a caustic that is blurred to the point of roundness. so the default setting (around 30%) works well. in my case it doesn't crash when trying to do 'em. the "filter" setting appears analogous to something like "photon exposure" (the inverse of ambient occlusion), and increasing photon count increases the sharpness of the caustic edge, as noted by mark in the msg above. I also tried an rgb spotlite, but there was no birefringence. although lowering the filter setting caused colour separation in the caustics, it was in blotches, and not in accord with the laws of optics. antoine is easy enuff to locate, but I daresay he would consider intrusive any questions about a render he did years ago, using software with which he is no longer involved. e.g. a google search using his name, and limited to domain www.daz3d.com, gives no results.