A rear lamp can meet its luminous intensity targets and still show bright spots, dark bands, or a dim far end. Rear lamp light guide design therefore needs two linked evaluations: the light sent toward each test direction and the appearance of the emitting surface.
TracePro optical design and analysis software lets engineers evaluate source coupling, extraction, and losses in the complete lamp assembly. Comparing candela distributions with luminance maps helps the team improve appearance while preserving the output required for each signal function.
Start with the lamp function, category, reference axis, and applicable amendment series of UN Regulation No. 148, often searched as ECE R148. Use those requirements to establish the intensity limits, angular test points, and color boundaries. Vehicle installation requirements also need their own review.
Record the OEM appearance target separately. Specify viewing positions, measurement patch size, and the uniformity calculation. A maximum-to-minimum luminance ratio of 2:1 is different from a minimum-to-maximum ratio of 0.5, although they express the same limit. An adjacent-patch requirement may also be needed to control visible steps.
These measures answer different questions. Candela describes luminous intensity in a direction. Luminance, in cd/m², describes the emitting surface as seen from a defined viewpoint. Neither result alone establishes that the lamp meets every requirement.
Import the guide, outer lens, bezel, and housing from the CAD assembly. Include bends, mounting interruptions, exit-face draft, and the LED-to-guide gap because they can change coupling or redirect the output.
Use a supplier ray file where available, with flux and spectrum appropriate to the operating condition. Otherwise, represent the emitting area and angular distribution using documented source data. Check hot and cold conditions and relevant LED bins using supplier measurements rather than a generic derating percentage.
Assign wavelength-dependent material transmission and relevant surface properties. The guide and outer lens can affect both the amount of light transmitted and its color. Keep the same source assumptions when comparing design revisions.
Light available for extraction decreases as it travels through the guide. A constant extractor pattern can therefore produce excessive brightness near the LEDs and insufficient output farther away. Adjust extractor spacing, size, or facet geometry to redistribute the remaining flux.
TracePro supports RepTile surface microstructures for repeated extraction features. Select a representation that captures the intended geometry, and check feature availability in the chosen edition. Include local interruptions explicitly where a repeated pattern cannot represent the part adequately.
Evaluate bends and fixing features alongside the extractors. A dark band may come from missing extraction, poor coupling, or light lost upstream. Increasing extraction in that region will not correct every cause and may reduce light available farther along the guide.
Align the candela analysis with the installed lamp reference axis and compare the angular distribution with the applicable photometric requirements. Inspect off-axis regions as well as the reference direction. A strong central peak can conceal a shortfall elsewhere.
Create luminance maps from the specified observer positions and evaluate the defined patches. A guide that looks uniform straight on may show banding from the side. Use consistent map resolution and sufficient ray sampling so numerical noise does not become a false bright or dark patch.
After each extractor or geometry change, repeat both analyses. Ray and path inspection can help connect an output change to its source, such as bend leakage, an altered exit angle, or absorption in the outer lens.
Vary LED alignment, extractor geometry, exit-face draft, and material transmission over their expected production ranges. Compare the resulting intensity margin and luminance uniformity to identify dimensions that need tighter control.
Correlate the final model with prototype photometry and appearance measurements. Simulation supports design decisions and test preparation; approval still depends on the applicable regulatory process.
Request a TracePro trial to evaluate your rear lamp light guide with the intended LED, optical materials, and appearance targets before tooling.