Any rays which fall outside of a defined aperture in sequential mode are terminated.  Using Non-Sequential ray tracing, rays are not simply terminated if they fall outside the bounds of a defined aperture.  After all, although the rays might not hit the aperture object, it is possible that they physically hit another object within 3D space.  So how do we go about creating an aperture such that any rays which fall outside that aperture are terminated, or absorbed? 

The trick here is to create two objects and utilize the nesting rule in ZEMAX, which says that if a ray strikes more than one object at any point in 3D space, the properties of the object which is listed further down in the NSC Editor dominates. 

So, a rectangular aperture which only “passes” rays within the apertured region can be constructed using 2 Rectangle objects: one absorbing and one made of air.  Imagine a smaller rectangle of “air” immersed in a larger rectangle of an absorbing material:

Considering the nesting rule, the inner rectangle of air should be listed after the outer rectangle whose material type is set to ABSORB.





Rays from the source which hit within the aperture continue to the detector.  Those rays which fall outside the rectangle made of air are absorbed by rectangle object number 2 (in grey above).



Of course, the Rectangle object in NSC ZEMAX can be used to create a rectangular aperture, but the same method we have just described can be used with user-defined apertures as well.  Just change the object type to Standard Surface, choose your user-defined aperture, and “nest” the surface with an absorber!