Intro
Optics for Hire was engaged by a European cycling technology company to develop a custom lens for a compact dynamo-powered bicycle front light. The goal was to create a beam pattern compliant with the German StVZO TA23 standard while working within a very small optical package.
The project required a custom injection-molded PMMA lens, ZEMAX non-sequential optical simulation, CAD development, prototype sourcing, and physical test validation. This case study highlights the challenge, the optical strategy, and the final development path that brought the system to a certification-ready state.
The Challenge
The client’s lighting concept used front-facing lights mounted on the wheel rim, harvesting energy from the rotating wheel without batteries or external wiring. That created an unusually tight form factor for the optical system.
The entire system, including LED, lens, and housing, needed to fit within an envelope of roughly 20 mm diameter and approximately 11 mm height. At the same time, the front light needed to comply with one of Europe’s most demanding bicycle lighting standards: German StVZO TA23.
Application Context
This project involved a wheel-integrated dynamo bicycle light for a European cycling technology company. Unlike traditional battery-powered or spoke-mounted systems, the client’s product generated power directly from wheel motion.
That concept offered clear product advantages, but it also introduced packaging constraints that made optical performance especially difficult. OFH was tasked with designing a custom lens that could work within those constraints while supporting a compliant beam pattern for road use.
What the StVZO TA23 Standard Requires
The German StVZO TA23 standard defines a specific illuminance distribution pattern measured at a 10 meter working distance. It is not enough for the light to be bright. The beam must also be carefully shaped to illuminate the road while limiting glare for oncoming cyclists and pedestrians.
Among the key requirements:
At least 10 lux at the HV point
No more than 2 lux in the anti-glare zone above HV
Adequate horizontal spread across the required field
Minimum illumination in lower zones for useful road visibility
These requirements must all be met at the same time, which makes the optical design significantly more demanding than a typical compact bicycle light.
Why This Was Optically Difficult
There were several reasons this problem was especially challenging.
First, aperture size was limited. A 20 mm lens collects far less light than the larger optics often used in bicycle lighting, which makes it harder to achieve sufficient peak intensity.
Second, the anti-glare cut-off had to be sharp. The design needed to place light precisely where it was needed without allowing excessive spill above the beam boundary.
Third, LED die image artifacts had to be controlled. Because the design relied in part on how the LED image was formed and projected, the orientation of the LED itself became an important variable in beam quality and compliance.
Optical Design Approach
After evaluating multiple concepts, OFH developed a dual-zone asymmetric lens architecture tailored to the application.
In the final design, different regions of the lens performed different optical functions:
The outer and side lens surfaces created the bright HV line required near the top of the beam
The central inner features produced a wider and dimmer light distribution that filled the lower required zones
This combination allowed the beam to achieve both sharp cut-off behavior and useful spread below the main peak.
Asymmetric Lens Geometry
A key design evolution was the shift from a rotationally symmetric lens to an asymmetric profile. One side of the lens was slightly wider than the other, helping create the needed beam asymmetry and providing a defined orientation reference for LED placement.
The final lens design fit within the client’s mounting envelope, used PMMA V825 material, and included mounting features to support reliable PCB registration and repeatable positioning relative to the LED.
LED Selection and Orientation
LED selection played a critical role in meeting the optical goals. OFH specified the Cree XQE-HI because its smaller die size supported tighter beam control within the limited aperture.
During design, OFH also identified that LED die orientation affected beam symmetry. In one orientation, the HV region split into two bright spots. Rotating the LED by 90 degrees produced a cleaner and more uniform bright line.
That insight became part of the manufacturing specification and was important to achieving the desired beam shape consistently.
ZEMAX Simulation Results
The final design was validated in ZEMAX non-sequential ray-trace simulation using the selected LED model and expected operating flux.
The simulation showed strong performance against the target requirements, including:
Peak illuminance at HV: 10.67 lux at 10 m
Lens efficiency: 87%
Cut-off above HV: 5.7° with less than 0.02 lux above it
Illuminance below HV at 5°: about 3 lux
Simulation results indicated that the design met the key StVZO TA23 requirements by design, including minimum HV intensity, controlled glare above the cut-off, and adequate illumination in the required lower zones.
Tolerance and Alignment Sensitivity
OFH also used simulation to evaluate how sensitive the beam pattern was to LED positioning errors.
This turned out to be important. Lateral positioning errors in the vertical direction had a noticeable impact on beam placement and the anti-glare boundary, while small axial shifts were less critical. Based on this work, OFH recommended a lateral positioning tolerance of ±0.1 mm and machine placement rather than hand assembly.
That tolerance analysis directly informed the manufacturing specification and reduced downstream risk during production.
Prototyping and Physical Testing
OFH arranged machined and hand-polished PMMA prototypes for early beam testing. These prototypes were useful for validating beam shape and confirming the basic optical concept before mold tooling.
Prototype testing also revealed an important mechanical sensitivity: the LED-to-lens axial distance needed to be reduced by 0.25 mm compared with the initial design assumption. Once that adjustment was made, the measured beam more closely matched the ZEMAX prediction.
The testing phase also confirmed that machined prototypes were suitable for beam characterization, but not ideal for final quantitative lux validation because prototype surface quality reduced performance relative to a molded optic.
Following prototype validation, the client moved forward with injection mold tooling using tighter production tolerances.
During mold development, one change to flange thickness was reviewed to simplify manufacturing. OFH simulated the modification and confirmed that the optical penalty was negligible.
The finished system was later submitted to TÜV Berlin for independent StVZO measurement. The beam distribution closely matched the design intent. The result fell just outside the Zone 1 boundary requirement, but follow-up investigation showed that a small vertical adjustment to LED mounting position would shift the beam upward into the required position.
This aligned with OFH’s earlier sensitivity analysis and confirmed that full compliance was achievable with a minor PCB design modification.
Mold Development and TÜV Testing
Project Summary
This project pushed the limits of what is physically achievable from a 20 mm bicycle optic while aiming for regulatory beam compliance.
Key outcomes included:
Development of a dual-zone asymmetric lens architecture
Identification of LED die orientation effects on beam quality
Quantitative tolerance analysis to define manufacturing requirements
Prototype sourcing and physical validation
Delivery of production-ready documentation including CAD, tolerance drawings, LED orientation guidance, and photometric data
From optical concept through simulation, prototyping, and test analysis, OFH delivered a complete illumination optic development program for a highly constrained application.
About Optics for Hire
Optics for Hire is an optical engineering consultancy based in Arlington, Massachusetts. Since 2002, OFH has supported clients ranging from startups to Fortune 50 corporations across a wide range of optical engineering applications.
Relevant capabilities include illumination optic design, injection-molded lens design, tolerance analysis, LED system optimization, photometric simulation, CAD development, prototype sourcing, and physical beam testing.
If you are developing a custom illumination system for bicycle lighting, medical devices, industrial products, or other constrained optical applications, Optics for Hire can help with design, simulation, prototyping, and validation.
