In non-sequential ray-tracing in ZEMAX, optical components are modeled as volume or surface entities called objects. There are 2 major categories of objects in ZEMAX; parametric and non-parametric.

Most non-sequential objects in ZEMAX are parametric, that is, based on an underlying equation. For example, the Standard Lens object is defined by parameters like radius of curvature, conic constant, center thickness etc. Parametric objects can be easily modified just by entering new parameter data in the Non-Sequential Component Editor (NSCE). These objects can then be recreated on-the-fly, with data changes entered by hand, via the slider tool, by a macro or extension program, and -most importantly- by the optimizer.

Parametric objects are represented by their equation in ZEMAX to the full 64 bit (double precision floating point) precision. Some parametric objects support a user defined specification for the number or radial, angular, or x/y facets in the NSCE. It is important to note that the facets are only used to render the object on the layout plots. For ray tracing, the faceted representation is used only to get the rays close enough to the actual surface so that iteration works efficiently. The actual surface shape of the object is modeled exactly by its equation ; the accuracy of the ray trace is not limited by the faceted representation used for rendering. In other words, most objects in ZEMAX are not tessellated or approximated with linear segments, they are accurate to the full numerical precision of modern computers.

To illustrate this point, let's look at a Even Asphere object with 16 angular facets and 8 radial facets specified in the NSCE.




The animation on the right shows the difference between specifying 32 radial segments versus 8. Notice that the ray refraction point is not affected by the number of segments. Regardless of the number of segments, the ray refracts at the exact intersection points calculated by the even aspheric sag equation.

 

ZEMAX also supports non-parametric objects, like the polygon object or imported CAD objects (IGES or STEP). These objects are ultimately represented by a string of data. There are some real advantages to using such objects in some applications. For example, in opto-mechanical stray light simulations, lens mounts and other mechanics can be simply and easily imported.

Some non-parametric objects such as STL, Polygon and Tabulated objects are genuinely faceted both in the layout and for ray-tracing. Other non-parametric objects such as IGES or STEP have faceted representation in the layout, but are smooth when ray-tracing. Nevertheless, they are different from parametric objects since they are not described by parameters (no equation), but instead by a collection of low order (usually 3rd or 4th) polynomial spline segments. Unlike parametric objects, the shape of the object cannot be optimized since there are no parameters in the NSCE that control the shape of the object. For more information about CAD objects, refer to the following knowledge-base article.

http://www.zemax.com/kb/articles/45/6/How-to-Import-CAD-Objects

Generally, parametric objects have greater advantages over the imported CAD objects, in terms of memory usage, ray-trace speed and accuracy. The number of drawing facets should be set as low as possible to get the fastest ray-tracing, and can be easily increased if required for presentation-quality graphics.

This article has explains how parametric and non-parametric objects are represented in ZEMAX both in the layout and for ray-trace. In summary:

• Most non-sequential objects in ZEMAX are parametric
• The facets are only used to render the parametric objects. For ray-tracing, the objects are not tesellated but instead modeled exactly by their equation
• Unlike imported CAD objects, parametric objects can be optimized
• In general, parametric objects are more accurate, ray-trace faster and requires less memory than CAD objects.