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TracePro has the capability to perform reverse ray tracing. Tracing rays in reverse
is useful in situations where importance sampling in the forward direction is
difficult or impossible. This is true, for example, in the design of a reflector coupled
to a source for which only a few points in the output plane need to be sampled,
such as for a low-beam head lamp above the horizon. Note that when source(s)
are listed below, it refers to surface sources. Reverse ray tracing does not apply
to grid and file sources.
Tracing Rays
Specifying reverse rays
In summary, to do a reverse ray trace you must perform the following steps:
1.Set up the model with surface source(s) and exit surface(s) as usual.
2.Define importance sampling to be used by the exit surface(s) during the Reverse Raytrace. This is done using the existing Importance Sampling tab in the Apply Properties dialog box.
3.Define the number of reverse rays to trace for each exit surface. This number is entered in the Exit Surface tab of the Apply Properties dialog box.
4.Select Raytrace|Reverse Raytrace to start the ray trace.
5.The process of analysis is the same as if a forward ray trace has been performed. By selecting one of the exit surfaces you can display the Irradiance or Illuminance Map for this surface. In other words it is as if the rays had been traced in the forward direction. The irradiance/illuminance from all surface sources is displayed on the selected surface.
6.To display sorted rays, select the surface of interest - usually an exit surface - and the rays at that surface will be displayed as though the rays were traced forward.
7.The Incident Ray Table and the Ray History Table are displayed in the same way as for forward rays.
8.The only options available for Candela plots are rays exiting a surface and rays incident on a surface. The “missed rays” option is not available for Candela plots with reverse ray tracing.
Note: If there is no importance sampling specified for an exit surface, no rays will
be emitted from that exit surface.
Note: Reverse ray tracing does not apply to grid and file sources - only surface
sources are used.
Theory of reverse ray tracing
Reverse ray tracing allows efficient sampling of rays in illumination design when
only a few local points on the output plane need to be sampled. By tracing rays in
reverse from the output plane and importance sampling toward a reflector, only a
few rays need to be traced to get good sampling for the purposes of design.
Designing in this manner consists of choosing a few representative points on the
output plane, and tracing rays toward importance sampling targets. Rays are
“absorbed” by the surface source in proportion to the radiance/luminance of the
source at that direction and position. The absorbed flux is referred back to the
point on the output plane where the ray started.
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