There are 2 distinct ray-tracing modes in ZEMAX; sequential and non-sequential. The key differences between sequential and non-sequential modes are:

Sequential mode

  • It is mainly used for designing imaging and afocal systems 
  • Surfaces are defined in the Lens Data Editor
  • Ray can only intersect each surface once and has to do it in a specified -sequential- order (i.e. surface #0 then #1 ,#2 …) and hence the name sequential ray tracing
  • Ray can only reflect if the surface material type is MIRROR. Partial reflections from refractive surfaces (Fresnel reflections) are accounted for to the extent of calculating the correct refracted energy, including the effects on dielectric or metallic mirrors
  • Each surface has its own local coordinate system. The position of each surface along the optical axis is referenced to the previous surface. In other words, the “Thickness” column in the Lens Data Editor refers to the distance from current surface and not from a global reference point

Non-sequential mode

  • It is primarily used for non-imaging applications such as illumination systems and/or stray-light analysis 
  • Surfaces or volume objects are defined in the Non-Sequential Component Editor
  • Mechanical components may be easily imported from CAD programs, so that full Opto-Mechanical analysis may be undertaken
  • A ray can intersect the same object more than once and can intersect multiple objects in any order; hence the name non-sequential
  • Each object is referenced to a global coordinate, unless specified otherwise
  • Partially reflected rays can be generated and traced from a refractive interface, in addition to tracing the refracted ray. This is referred to as ray splitting.  Hence both the reflected and refracted rays can be traced.
  • Imaging-system properties such as stop location, entrance and exit pupil, field, system aperture etc.that exist in sequential systems may not be meaningful in non-sequential systems
  • The main analysis feature in non-sequential mode is the detector ray-trace, which gives spatial and angular data on incoherent or coherent rays.

In addition, a hybrid mode ("non-sequential with ports" or "mixed-mode") exists in which sequential and non-sequential ray-trace are used in the same system.

Full details of non-sequential ZEMAX capabilities are given in chapter 12 of the user manual.