The ability to import CAD objects into ZEMAX is very important, particularly when undertaking complex opto-mechanical stray light simulations, where reflections and scattering from mounts is critical. It is also important in illumination system, where light may be directed down a complex-shaped lightpipe, for example in automotive dashboard design. ZEMAX has powerful and flexible CAD import capabilities, and supports common CAD exchange formats.

Because CAD objects can be of arbitrary complexity, a ray may interact with it many times. As a consequence, we use non-sequential ray-tracing to describe the interaction of rays with CAD objects. CAD objects may be easily included in otherwise sequential optical systems by using hybrid mode non-sequential ray-tracing.

ZEMAX supports four CAD formats: STL, IGES, STEP and SAT. Of these, only STL uses facets to represent the object: the other three model the object as a smooth, continuous surface shape. Facets are only used to draw these objects on screen. Therefore ray-tracing to the continuous, smooth CAD surface is exact, at least to the limit of the accuracy of the CAD model, despite facets being used to draw the object.

Its important to realize that although ZEMAX supports genuinely faceted objects, in most cases facets are only used for rendering purposes, and the real surface shape used for ray-tracing  is exact.