The functions of the taillights of the car include: steering indication, brake indication, parking indication, position indication, reversing indication, and rear fog light.
Sometimes you need to:
– The ECE specification stipulates that a brake light is also required at a higher position in the center of the rear of the vehicle.
– License plate lighting
In this briefing, we only consider the brake light design.
Figure 1: Location of various types of lamps
The picture below shows the true effect of the lights (Prius C). For this car, the brake light uses a red LED and the turn signal is amber. In the FMVSS standard, the turn signal should be red.
Figure 2: Various types of lights lighting effect
Related specifications:
– Federal Motor Vehicle Safety Standards and Regulations (FMVSS) – US Federal Standards
– Society of Automotive Engineers (SAE) – specifications, research and statistics for luminaires, but not legally binding, especially important in North America
– Canadian Motor Vehicle Safety Standard (CMVSS)
– United Nations Economic Commission for Europe Inland Transport (ECE)
– Most countries adopt the ECE specification.
– Most of Japan follows the ECE specification, except for daytime running lights.
The following figure shows the important specifications of the ECE taillight design:
Figure 3: ECE taillight design specifications
ECE brake light requirements:
19 test points, each of which must be between the standard maximum and minimum values. In addition, according to the relevant requirements, you also need:
– Vertical direction: some light needs to travel in +/-15o
– Horizontal direction: some light needs to travel in +/-45o
Figure 4: Intensity test point location
Figure 5: Intensity test point allowable range value
Brightness visualization:
The so-called brightness is the intensity perception of the human eye on light, unit: lumens/sr/m^2. In addition to meeting the requirements of the corresponding strength test points, the spatial distribution seen by the observer is also important in this discussion system. Also consider the human eye resolution (we will set the mesh size on the receiver based on the angular sensitivity of 30 arc seconds). Finally, you need to choose a viewing distance (we choose 3 meters here).
The relevant specifications for this design work:
Main specifications:
– Design must pass the ECE S1 brake light intensity standard
– satisfactory spatial distribution
Secondary objective: Efficiency
Light source selection: LED light source provided by the manufacturer
Optimization:
– Optimize spatial distribution using a 45° half cone angle (human eye opening angle)
– Optimize the target to use the average brightness
– also optimize the angular spread of the outgoing light to match the corresponding intensity distribution
Create an optical model using LightTools.
LED light source selection:
– use high power red LEDs to meet design specifications
– Select OSRAM Golden DRAGON, LR W5SM-HZJZ-1-1
– This LED is a single chip with a luminous area of ​​approximately 1mm x 1mm
– Light source output power: 39 – 71 lumens
– Distribution of the distribution curve approximates the Lambertian distribution
– LED manufacturer provides 5M light data source file
Figure 6: LED spectral distribution
Figure 7: LED light distribution curve distribution
CAD structure design:
The initial structural design of the taillights was done by CAD software (this case uses SolidWorks software).
Figure 8: Model parameter description
Figure 9: Lighting area division of the CAD model (for placing different fixtures)
Figure 10: The model after the division is completed
Design method of edge side light-emitting light pipe:
– Try using a long strip light tube or a round light tube for edge illumination
– length is ~150 mm (the length of the brake light placed in the CAD model)
– You can first cut a plane on the surface of the light pipe and then use the LightTools 3D texture function to add the library component texture to the plane just cut out.
– The Backlight Optimization Tool (BPO) can then be used to optimize Z-depths for even spatial distribution.
– Start with a simple 45o wedge angle.
Figure 11: Sawtooth texture description
Optimize the wedge angle:
– Add 90% Lambertian optical properties to the back of the light pipe
– Obtaining a uniform spatial distribution structure requires BPO optimization with an optimum wedge angle of 45°
Figure 12: BPO User Interface
Figure 13: Light pipe model
Figure 14: Simulation results of 45° half cone angle
The intensity distribution of the wedge angle results:
Figure 15: Intensity distribution results (not ideal)
Restructuring:
Adjust the vertical distribution:
– Appropriately reduce the width of the texture element - to 4.1mm.
Adjust the horizontal distribution:
– More complex light extraction structures: done by library component functions and Bezier curves in LightTools.
– Optimize the corresponding parameters of the Bezier curve
– Add draft angle
– Add fillets with a radius of 50 microns
Figure 16: Modified sawtooth texture description
Re-optimize the spatial distribution:
After the optimization is completed, duplicate the optimized light guide tubes and place them in the light box.
Figure 17: Re-optimization results
Figure 18: Lightbox structure
Figure 19: Intensity distribution simulation results
Test point values ​​for the complete system:
– Total power 130 lm (single LED 43.4 lm)
– pass the minimum test point indicator
– There are 15 lumens entering the test point area, +/-20o x +/-10o (11% of total light source power)
– There are 72 lumens in the positive hemisphere (55% of the total power of the light source)
Figure 20: Test point value
Figure 21: Light box assembly structure
Brightness simulation results for observers at different distances:
The simulation uses 15M rays and a 2° half cone angle.
Figure 22: Simulation results for different distances
Figure 23: Multi-view brightness simulation
Figure 24: Real rendering simulation
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