Non-sequential ray tracing implies that there is no predefined sequence of surfaces which rays that are being traced must hit. The objects that the rays hit are determined solely by the physical positions and properties of the objects as well as the directions of the rays. Rays may hit any part of any non-sequential object, and may hit the same object multiple times, or not at all. This can be contrasted with sequential ray tracing where all of the rays traced must propagate through the same set of surfaces in the same order.

In sequential mode in ZEMAX, all ray propagation occurs through surfaces which are located using a local coordinate system. In non-sequential mode, optical components are modeled as true three-dimensional objects, either as surfaces or solid volumes. Each object is placed globally at an independent x, y, z coordinate with an independently defined orientation.

The non-sequential ray tracing capabilities of ZEMAX do not suffer from the same limitations that sequential ray tracing does. Since rays can propagate through the optical components in any order, total internal reflection (TIR) ray paths can be accounted for. While sequential mode is limited to the analysis of imaging systems, non-sequential mode can be used to analyze stray light, scattering and illumination in both imaging and non-imaging systems. If an optical system can be traced with rays, it can be traced with non-sequential analysis in ZEMAX.

There are many types of optical components which cannot be modeled using the simple sequential surface model. Such optics need to be modeled as real, 3D components. Examples of objects that require non-sequential ray tracing include:  complex prisms, corner cubes, light pipes, faceted objects, objected created in CAD programs and embedded volume objects (i.e. objects located within other objects). For example, here is a liquid crystal on silicon projector, designed in non-sequential mode by Michael Pate of OSCI:

Non-sequential ray tracing can be modeled in ZEMAX using one of two modes:

• Pure non-sequential ray tracing
• Mixed sequential/non-sequential ray tracing

When using pure non-sequential ray tracing, all optics to be traced are in a single non-sequential group. In addition, source and detector objects are setup within the group to launch and capture rays, respectively. The source modeling capabilities of pure non-sequential mode in ZEMAX are far more comprehensive than sequential mode. In sequential mode, you are limited to modeling point sources located on the object surface. Using the image analysis capabilities of sequential mode, planar extended sources located at the object surface can be modeled. Using pure non-sequential ray tracing, sources can be placed anywhere in the non-sequential group, at any orientation, and can even be placed inside of other objects. The source objects themselves can range from simple point sources (like those used in sequential mode) to complex, three-dimensional source distributions. ZEMAX can even import measured source data for real sources from programs like ProSource (Radiant Imaging) and Luca Raymaker (Opsira).

Rays from non-sequential sources, known as NSC rays, can be split and scattered by optical components. These rays can also be diffracted at phase surfaces/objects. The analysis options available when tracing NSC rays include evaluating radiometric data on detectors and the storing of ray data in ray database files. Detectors can be modeled as planar surfaces, curved surfaces and even three-dimensional volumes. Non-sequential detectors support the display of a variety of data types including:  incoherent irradiance, coherent irradiance, coherent phase, radiant intensity and radiance. Ray database files store the history of each ray traced. Ray paths can be filtered to isolate rays that hit specific objects. The filtered ray data can then be displayed in layouts and on detector objects. All of the above makes pure non-sequential ray tracing very useful for ghost analysis, stray light analysis as well as a variety of illumination applications.

When using mixed sequential/non-sequential ray tracing (also known as hybrid or mixed mode ray tracing), a collection of non-sequential objects are setup inside of a non-sequential group. This non-sequential group is part of a larger sequential system. Sequentially traced rays enter the non-sequential group through an entrance port, and exit the group through an exit port to continue propagating through the sequential system. Multiple non-sequential groups may be defined in the same sequential system, and any number of objects may be placed in each non-sequential group. This allows non-sequential components such as faceted mirrors, roof prisms or CAD objects to be included in a sequential design.