Ray tracing can be extremely time-consuming—sometimes taking hours or even days—especially for complex optical illumination systems. This article presents a practical case where ray-tracing speed is significantly improved with minimal loss of accuracy, making it well suited for engineering iteration and performance evaluation.
Brightness Enhancement Filter (BEF) Example
A Brightness Enhancement Filter (BEF) is commonly used in LCD backlighting to improve light coupling efficiency toward the viewer. A BEF typically consists of a plastic substrate with a dense array of micro-prisms formed on one surface.
In this example, the BEF is modeled as a Polygon Object (POB). The prismatic structure redirects light toward the normal direction, allowing more light to exit the filter on the desired side, thereby increasing brightness and improving illumination uniformity.
The plastic sheet is illuminated along one edge by a single cylindrical source located inside a parabolic reflector, a common configuration in edge-lit backlight designs.
Fresnel Reflections and Ray Splitting
When light crosses an interface between two media with different refractive indices, partial reflection and transmission occur due to the change in light velocity. These effects are commonly referred to as Fresnel reflections.
At each interface:
- A portion of the ray’s energy is transmitted
- A portion is reflected
- Additional energy may be absorbed, especially in the presence of metallic or absorbing coatings
Optical illumination software accurately models these effects, including bare surfaces and complex multi-layer coatings.
During ray tracing, when a ray intersects an object surface, the software calculates the reflected, transmitted, and absorbed energy fractions. The ray can then be split into reflected and transmitted rays with the appropriate energy weighting.
Improving Performance with Simple Splitting
In this illumination system, the Simple Splitting option is used instead of full ray splitting.
- Full splitting creates both reflected and transmitted rays at every interface, which is accurate but computationally expensive.
- Simple splitting selectively tracks only the dominant ray paths of interest, dramatically reducing the total ray count.
Because the design goal here is to analyze only the out-coupled light from the BEF, Simple Splitting provides a substantial performance advantage.
Performance Result
By using Simple Splitting:
- Ray-tracing speed is improved by approximately a factor of 6
- No measurable loss of accuracy is observed for the targeted output metrics
When to Use (and Not Use) Simple Splitting
Simple Splitting can deliver major productivity gains, but it should always be validated against full splitting to establish confidence in the results.
Well suited for:
Illumination efficiency studies
Out-coupling analysis
Early-stage optimization and iteration
Not recommended for:
Stray-light analysis
Ghost analysis involving multiple internal reflections
Scenarios where “ghosts of ghosts” significantly affect performance
In stray-light investigations, full splitting is both faster and more accurate overall, as it avoids missing low-probability but high-impact ray paths.
Reference Source