Ray Tracing

FIGURE 5.17 - Rotate Source dialog.

Tracing Rays

Raytracing is the fundamental tool permitting the simulations in TracePro. This next section covers the three types of raytracing, Standard, Reverse and Luminance/Radiance.

Standard (Forward) Raytrace

Standard or Forward raytracing is the normal method used to simulate the flux propagation in a TracePro model. Rays will emanate from all of the sources defined in the model as described above. Initiating the raytrace is done by selecting Raytrace|Trace Rays or pressing the Trace Rays button on the toolbar.

Most sources are defined as using “discrete wavelengths”. Surface sources defined as Blackbody or Graybody sources will use “calculated wavelengths.” Surface Sources that use a Surface Source Property may use either discrete wavelengths or calculated wavelengths.

The raytrace begins when Raytrace|Trace Rays is selected, or the toobar button is clicked. During the raytrace a progress dialog box opens to indicate progress of the raytrace. You can interrupt or pause the raytrace at any time by clicking Cancel on the Raytrace Progress dialog box and TracePro will finish tracing the current ray. You can resume the raytrace by selecting Raytrace|Resume Raytrace. The raytrace resumes starting at the next consecutive ray number in the raytrace.

Reverse Ray Tracing Standard Expert

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.

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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.

Specifically, in forward ray tracing, the flux of the emitted ray is

	dΦs =	LsdAsdΩs	,	(5.1)
		-----------------------N

where Ls is the source radiance, dAs is an element of projected area on the source surface, dΩs is an element of solid angle, and N is the number of rays

emitted from the surface element into the solid angle. When the forward ray reaches the output plane, the resulting irradiance is

TracePro 5.0 User’s Manual

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