Photon Engineering - FRED

Sequential/Non-sequential Raytracing

The fundamental raytrace mode in FRED is non-sequential; that is, FRED determines the trajectory
of each ray based upon the system’s geometry and the ray’s parameters. (“Non-sequential” raytracing is sometimes referred to as “physical” raytracing because the rays follow physical trajectories that are not in any way constrained by predetermined intersection orderings.)

Non-sequential raytracing is extremely useful in such applications as illumination systems where rays may intersect a reflector or integrator several times before reaching, for example, the digital mirror device (DMD). It is also useful in fiber optic and telecommunication applications where the rays may experience thousands of reflections as they propagate. Non-sequential raytracing is also a valuable check in sequential systems to verify that there are no unintended “leakage” paths or blocked paths in the system.

Non-sequential raytrace through a reflector and integrator
(Red rays do not intersect the reflector.)

Left/Center) Sequential and non-sequential paths through triplet
(Right) User-defined sequential path illustrating caustic formed between two surfaces

However the user can specify any number of specific sequential and mixed sequential/non-sequential paths and override the default action (reflection, transmission, absorption) on a surface-by-surface basis.

FRED also has a very extensive set of raytrace controls implemented during the raytrace and ray manipulation utilities for post-raytrace analyses. These controls allow the user to
• Allow/disallow reflected, transmitted, TIR rays
• Apply a relative/absolute flux threshold
• Move rays to a constant optical path length
• Reverse ray directions
• Redraw ray trajectories
• Recolor rays after transmitting, reflecting, diffracting, or scattering from a surface