The challenge
When the target area — a detector window, a coupling aperture, a measurement plane, an observation target — is small relative to how widely the source emits, forward tracing wastes most of its rays: only a few actually hit the target. The result is a noisy illuminance map and an uncertain angular distribution, and smoothing them out requires tracing enormous numbers of rays — which significantly lengthens the computation. Characterizing how much flux actually reaches the target, and how it's distributed, becomes expensive.
The solution
In reverse ray tracing, rays leave the target (an observation surface set as an exit surface) heading toward the source. The direction and solid angle of the rays define importance-sampling targets in TracePro, from which étendue is calculated — so that almost exclusively the rays that actually contribute at the target are traced. In the documentation example, 1000 reverse rays were specified with a ring-shaped sampling target split into 4×4 = 16 cells, giving 16,000 effective rays.
The performance difference is striking: reverse tracing to the observation target delivered about 29,000 rays, producing a smooth illuminance map. Forward tracing with 100,000 rays — a much longer run — delivers only about 4,000 rays to the same target, giving a much noisier result. A single run yields the illuminance map, a polar iso-candela plot, and the collected flux at the target.
Illuminance map on the target from reverse ray tracing — smooth with about 29,000 rays reaching the target.
Polar iso-candela plot — angular distribution and collected flux (about 0.40 lm) on the target surface.
Results
- A clean result on a small target — a smooth illuminance map and angular distribution where forward tracing produces mostly noise.
- A fraction of the computational cost — a comparably clean result with far fewer rays than forward tracing.
- Collected flux and angular distribution — illuminance (lux), a polar iso-candela plot, and total collected flux (lm) read at the target surface.
- Faster iteration — cheaper checks on how much light reaches a detector or aperture as source and reflector geometry changes.
Why TracePro
Dlaczego to narzędzie. Reverse ray tracing with importance sampling is a native TracePro mechanism for efficiently collecting flux at a chosen target — that is, a direct analysis of optical efficiency and illumination. It's part of the energy-balance and light-distribution domain, without needing diffraction or aberration analysis, which are the domain of other tools.
Want to quickly check how much light reaches your detector, aperture, or measurement target? Request a TracePro trial or get in touch with us — we'll show you how to set up reverse ray tracing for your system.
Uwaga. An illustrative example, based on the reverse ray tracing functionality in TracePro (TracePro 2026 documentation, "Example Using Reverse Ray Tracing"). The values given are for illustration only.