Attached Link: http://www.cgfocus.com/ArticleDetails.cfm?ArticleID=117

Sorry it took so long to get back. I was trying another test render but it wasn't any good. Basically irridescence can be defined in a few ways, but for CG it is a function of something called a BRDF or BiReflective Distribution Function. This is implemented as a special shader in other packages, so I was trying to simulate it in Carrara. Basically you are getting light reflected off of a surface at two angles with different wavelengths (colors). I don't have time to get into it now, but here are some other images that show the concept. I hope to have more samples soon...