A brightness enhancement filter (BEF) is used in LCD backlighting to improve the coupling of light to the outside world. It is made up of a series of prisms on the rear side of a plastic sheet. In this file, the BEF is modelled as a polygon object (POB):

The ray bending produced by the BEF causes light to flow out of the filter on the opposite side, and therefore improved brightness and illumination uniformity. Note that the plastic sheet is being illuminated by a single cylindrical source (inside a parabolic mirror) along one edge only.

Now this is a good case where simple splitting improves ray-tracing speed with little loss of accuracy. Because we are interested only in the out-coupled light, we get a ray-tracing speed improvement of a factor of 6, with no loss of accuracy.

Therefore, in illumination systems, simple splitting can often give big productivity gains. However, the results should always be tested against full splitting, in order to establish confidence. In stray light work, where ghosts of ghosts of ghosts are important, full sampling will be both faster and more accurate.