I am using Zemax for spectrograph Stray Light analysis, but I got some issues with the grating model. I used two objects (the front object is rectangular Volume, the rear one is Diffraction Grating). When I tried to find the strongest scatterings, one path is shown as the below figure. The light bumps several times within grating. It seems the reflection happens on the inside surfaces of grating, which is not reasonable in the real world.
I am not sure the grating model is correct in NSC. Please give me some suggestions about this. Thanks.