Non-sequential ray tracing refers to the ability to compute which object in a list of objects a ray strikes, independent of the order in which the objects are listed. Generally, the order in which a ray intercepts objects depends upon the size, shape, and positions of the objects and the starting angle and position of the ray. Objects which usually require non-sequential ray tracing include faceted objects, prisms, and light pipes.

Non-sequential: objects vs. surfaces

Traditionally, lens design programs that supported surfaces for sequential ray tracing would implement non-sequential ray tracing using the same surface model; the rays would simply intersect surfaces in a possibly out of sequence order.

The disadvantage of using surfaces is that surfaces do not adequately describe many optical components. For example, lenses not only have a front and back surface, they also have edges and outer flat faces for mounting. Light may intercept, and then refract or reflect from these additional surfaces. This is normally ignored by sequential surface ray tracing codes.

Complex objects, such as a general faceted solid generally cannot be described by the surface model at all. Accurate tracing of these types of objects requires the use of 3D solid models. When performing non-sequential ray tracing, ZEMAX uses 3D solid models of optical components, and is not limited to tracing rays just through surfaces.

Non-sequential objects

This screenshot shows three glass rods. The rays  may interact with the rod surfaces any number of times. The rod is defined as a three-dimensional object. Note the bubble embedded in the middle rod (made much larger than real life for clarity)

Each non-sequential object in ZEMAX can be a 3D solid or a 3D surface. Objects may be placed anywhere in a global coordinate space. Object positions and rotations may be linked together; this makes it easy to define complex compound objects and then move the entire assembly as a unit.

The non-sequential group may be placed anywhere within an optical system. When a ray leaves the object, it may enter the non-sequential group, and then is traced non-sequentially until the ray exits the group through an exit port. The ray is then traced sequentially through the remaining surfaces, if any.

This hybrid technique permits the best of both worlds; sequential paths are modeled sequentially for maximum speed, and non-sequential paths are modeled robustly. The two models may be combined to simulate propagation through virtually any component or system.

Object types

ZEMAX supports the following non-sequential objects (list is not exhaustive):

Triangles	Defined by 3 points
Ellipses	May be annular
Rectangles	May be 2D faces or 3D volumes
Cylinders	For modeling tubes or volumes
Faceted objects	Completely arbitrary solids or surfaces, ideal for prisms or objects designed in CAD packages. Supports IGES, STL, and STEP CAD exchange formats
Spheres	Used for bubbles or glass globes
Lens	Conic aspheric surfaces with edges
Even Asphere Lens	Standard lens plus even polynomial asphere terms
Diffraction Grating Object	Object with line gratings on front and back surface
Binary 1 and Binary 2	Lenses with a diffractive optic phase profile on front
Fresnel	Radial and cylindrical Fresnel lenses with grooves, pitch angle etc
Toroidal Lens	Conic and aspheric toroids and cylinders
Torus	For curves light pipes and fibers
Sources: Point, Ellipse, Rectangle, Gaussian, Cosine lamp, Volume Cylinder, Volume Ellipse, Volume Cuboid	For modelling source radiance
Detectors	For detecting power (W/cm2) at any position

All objects may be reflective, refractive, or absorbing. There are no limits to the number of non-sequential groups or on the number of objects in each group, or in the number of facets on any faceted object.

CAD Objects

Faceted objects may be imported in STL, IGES and STEP format from mechanical CAD packages like ProEngineer, SolidWorks and AutoCAD. ZEMAX also has its own built-in ASCII format which is ideal for describing simple faceted objects like prisms and cubes.

Prism library

ZEMAX includes a large library of predefined prisms. These objects may be scaled to any convenient size, then placed anywhere within the non-sequential group. Most common prisms, such as right, dove, roof, penta, pechan, and many others are included.