Hello I’m trying to implement importance sampling in a model in order to increase the amount of scattered light toward a certain region.

I’ve read the documentation and the tutorial online as well as looked at the sample file provided https://support.zemax.com/hc/en-us/articles/1500005488461-How-to-use-importance-sampling-to-model-scattering-efficiently

I’ve also looked at the sample file in: Samples\Non-sequential\Scattering\Importance Sampling Demonstration.ZMX

In both cases above, the amount of scattered rays in a region of interest increases as is desired. However I have encountered the opposite where the amount of rays decrease. I’ve attached a toy system in which this is observed. Could someone please help me clarify what is going on?

In this model I have two systems in one separated by a barrier. Each system as a source that scatters on a rectangle object modeled as a Lambertian scatterer. In the upper system I use Importance Sampling with a target sphere at the detector matching the semi-diameter of the aperture of the detector. In the bottom system, I’ve not enabled Importance Scattering.

Contrary to what I would expect, I observe lots more rays on the detector in the case where Importance Sampling is turned off. If anyone would have a clue what I am doing wrong, I would be very grateful.