I wrote a paper on forward ray tracing and general daylight systems simulation - louvers, prisms, that sort of thing. The paper proof is at
It should appear in about 2 weeks in LEUKOS at www.iesna.org.
Anyway, I was quite surprised how many rays it takes to get stable results. We traced up to 7 million rays through a specular microlouver sold by www.Siteco.de. We took the exiting ray distribution towards the room and converted it into an IES photometric format file in 1-degree vertical and 22.5 degree horizontal steps for use in Radiance or other lighting calculation programs.