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2006-01-0104

Automotive Forward Lighting with use of high flux white Light-Emitting-Diodes

Jih-Tao Hsu and Wen-Liang Wang

Automotive Research ＆ Testing Center

Copyright © 2005 SAE International

ABSTRACT

In general, an LED headlamp consists of an optical system, an electrical system and a mechanical system. Conventionally, the optics of a headlamp is a classical illumination set, which includes a light source, a reflective mirror and/or a lens. In this research, the source simulation, a reflector design and the propotype are presented. The application ways of the LED headlamp prototype include fog lamps, low beam lamps and high beam lamps, and overall with a light distribution that approaches the ECE regulation. Presently, the AFS (Adaptive Front Lighting System) is one of the strongest areas of development for the headlamp design, and the LED headlamp with the AFS function will be the leading trend in the next generation. Therefore, the LED headlamp design and AFS function are integrated to produce the prototype of the AFS-LED headlamp. Finally, the study will show that an optical system to provide the applicable possibilities for the adaptive front lighting system (AFS). INTRODUCTION

Forward lighting plays an indispensable role for the vehicle safety. Currently, the discussion regarding for the driving situation and environment is AFS. [1-2] Due to the new technology, the headlamp will not be more than a simple front lighting system. The headlamp can provide different light distributions according to environment change; it allows the drivers to see the roads ahead more clearly. With improvements of the materials and technologies, LED is the revolutionary lighting source in the 21st century. It is a particularly interesting topic because of its potential on automotive lighting. LED advantages over traditional bulbs include faster response time, longer life, lower power consumption and more design flexibility. LED can play an important role in not only interior applications but also in forward lighting. In the past two years, several headlamp designs of LEDs as the light sources have been proposed, including fog, daytime running lamp (DRL), and headlamps [3-11]. The main purpose of this document is to show how the LED headlamp is designed with AFS function. In general, the AFS function, which

includes basic passing beam, town passing beam, motorway passing beam, wet road passing beam and high beam, is achieved by the mechanics principle. [12] However, according to the LED’s physical properties, LED headlamp is designed by a type of the array optical system. It is unlike the common headlamp which has only one source. Thus, the LED headlamp could achieve the AFS function by the optical arrangement but not by complicated mechanism. In this document, the projector-type design and reflector-type design are used for designing LED headlamp modules. Further, the reflector-type module is designed by using the side-emitting LED, while the projector-type module is designed by Lambertian LED. Finally, the LED headlamp is designed for the prototype with the AFS function. The corresponding simulation, experimental demonstration and potential applications are presented. LED LAMP MODELS 1) REFLECTOR-TYPE MODULE Currently, reflector optics has been commonly used for the vehicle headlamp design. The design methodology of the reflector optics lamp is similar between the tungsten lamps and HID. Fortunately, the same optical design methodology can be applied to LED source [13-14]. In general, the common reflector-type optical system is used by the longer focal length to increase the volume of the reflector, because large area of reflectors could enhance light collection efficiency. However, in the LED headlamp design, in order to decrease the volume of the reflectors, longer focal length of the reflectors could not be used. There is a point in how the light is collected by the reflector. In this document, the reflector-type module is designed by the parabolic reflectors with shorter focal length, and incorporates a 3w side-emitting LED. According to the side-emitting LED’s physical properties [15], the LED is oriented towards the bottom of the reflector surface, and the reflector is the multiple-reflector (MR). Those optical arrangements not only to achieve high light collection efficiency but also to create a legal ECE112 beam pattern. The reflector-type design is shown in Fig.1. In simulation, the 3W LED is assumed to produce 60 lumens, and the reflectivity is assumed 85

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percent. The corresponding simulation of the specific reflector-type module is shown in Fig. 2, and the “hot zone” is almost 2 Lux. The dispersed angle is around 25 degree.

Fig.1. Reflector-type module.

Fig.2. Simulation of the reflector-type.

2) PROJECTOR-TYPE MODULE The projector-type module, which is consisted of an elliptical (or compound ellipsoid) reflector, a 3w Lambertian LED, a shield and a projector lens, is shown in Fig.3. In order to increase the light collection efficiency, it is important to take advantage of Lambertian LED’s light emitting characteristics, thus the LED needs to be positioned to face the top of the ellipsoid reflector. These optical arrangements could achieve high light collection efficiency than using the conventional design. In the simulation, the 3W LED is assumed to produce 60 lumens, the reflectivity is assumed to be 85 percent and transmission of the project lens is assumed to be 90 percent. The corresponding simulation and experimental results of the projector-type modules are shown in Fig. 4. The “hot zone” is almost 1.3 Lux, and the dispersed angle is around 20 degree. (Note for that the purpose of this simulation, an “imperfect” surface was used, i.e., consider surface scatter and reflection loss, and it could design by the optical package program). [16]

Fig.3. Projector-type module.

Comparing reflector-type design with projector-type design, the “hot zone” of the projector-type is less than that of the reflector-type. However, the projector-type module is simple to achieve a sharp cutoff line by using the shield. Therefore, the projector-type module is much better than the reflector-type module for the ECE R112 requests. According to the compared results, the projector-type module is chosen for the LED fog-lamp and LED headlamp in the following section.

(a) Simulation.

(b) Experimental results (25m).

Fig.4. Simulation and experimental results of the projector-

type module: (a) Simulation. (b) Experimental results.

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3) LED-FOG LAMP An LED fog lamp inevitably needs multiple light sources, since one LED can not produce the sufficient lumens. As a result, the fog lamp is made by several projector-type modules. It means that the module size is too big to meet the requirement of lamp size. In order to decrease the system size, the ellipsoid reflectors are designed for the multiple-ellipsoid surface, which are shown in Fig.5. The prototype shown in Fig.6 is made to prove the possibility of the simulation. The size of the LED fog module is 195mm x 60mm x 60mm.

Fig.5. LED fog-lamp

Fig.6. Prototype of the LED fog-lamp.

In the simulation, the 3W LED is assumed to produce 60 lumens and the total lumens is 240. The corresponding simulation and the experimental results are shown in Fig.7. The “hot zone” is achieved 3.2Lux by using four 3W Lambertian LEDs, and those beam distribution are acceptable for the ECE R19. Comparing the simulation with the experimental results, they are slightly different. The first reason is that the size of both the light sources and the optical system are shrunk (shorter focal length), and the tolerance of the optical components would become very small. The second reason is that the current manufacturing processes could cause the errors in reflector accuracy.

(a) Simulation.

(b) Experimental results (25m).

Fig.7. Simulation and experimental results of the fog lamp

module: (a) Simulation. (b) Experimental results. 4) LED HEADLAMP In order to achieve beam pattern for the ECE R112 (low beam and high beam), two different ellipsoid reflectors are designed for different light distributions. Five A1 reflectors could concentrate light distribution, and seven B1 reflectors could extend the light distribution. The LED headlamp is designed for the prototype with 12 projector-type modules, which are shown in Fig.8. When the 7 LEDs are turned on for low beam function and all LEDs are turned on for high beam function. The corresponding simulation of the low beam distribution is shown in Fig. 9, and the “hot zone” is achieved 14 Lux by using seven 3W Lambertian LEDs (total lumens is 420). The high beam distribution is also shown in Fig.10, and the “hot zone” is only 22 Lux by using 12 3W Lambertian LEDs (total lumens is 700). Fig.11 shows the prototype of the LED headlamp on the car, and the experimental result of the LED headlamp for the low beam distribution is shown in Fig.12. The beam distribution is acceptable for the ECE112 class A, and is also close to meet the ECE112 class B. Fig.13 shows the bird eye view of the LED headlamp and common headlamp (H4 light sources). It is believed that when the luminance of LED is higher than the existing filament halogen light sources, the light distribution could be accepted for the ECE112 class B. Using LEDs as light sources for the automotive headlamp could be accomplished.

Fig.8. Prototype LED headlamp included low beam and high beam.

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Fig.9. Simulation of the low beam distribution.

Fig.10. Simulation of the high beam distribution.

Fig.11. Prototype of the LED headlamp.

(a) Low beam pattern in the 10m.

(b) Low beam distribution (25m).

Fig.12. Experimental result of low beam distribution.

(a) LED headlamp. (b) Common headlamp.

Fig.13. Bird eye view of the LED headlamp and common

headlamp (H4 light sources). LED HEADLAMP WITH AFS FUNCTION

The automobile lamp is the important automobile initiative safety part, especially while night driving, it can provide the suitable illumination function at night. If drivers cannot promptly see the coming vehicles run in the opposite side, they might cause serious traffic accidents. Although the lighting distribution of the LED headlamp could only be accepted for the ECE112 class A at present, the LED headlamp with the AFS function will be the leading trend in the next generation. The main point here is how the LED headlamp is designed with AFS function. Fortunately, the LED headlamp is an array optical system. It means that the LED headlamp is consisted of multiple light sources. Based on the principle of lighting superposition, the LED headlamp could achieve the AFS function by the optical arrangement but not by complicated mechanism.

Fig.14 Lighting superposition.

In this document, four kinds of the compound ellipsoid reflectors are designed to achieve beam pattern for AFS drift. A type reflector is for the high beam module, B type is for the wide light distribution, C type is the basic light distribution, and D type is for hot zone distribution. The dispersed angles are around 15, 35, 25, 20 degree, respectively. The principle of the lighting superposition is shown in Fig.14. Therefore, the different AFS beam patterns could be achieved by different reflector module’s arrangement. Besides, the beam distribution is based on the ASF drift which is shown in Fig.15. In order to create AFS light distribution, the shields are designed

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to type α and type β which are shown in Fig.16. Finally, the LED headlamp design and AFS function are integrated to produce the prototype of the AFS-LED headlamp which is shown in Fig.17. In order to meet the headlamp size, there are only 9 projector-type modules could be used. The functions of AFS-LED headlamp include basic passing beam, town passing beam, motorway passing beam, wet road passing beam and high beam.

Fig.15. AFS drift.

Fig.16. Two kinds of shields α and β.

Fig.17. Prototype of the AFS-LED headlamp.

a) Basic function module This is the general night lighting function, and the beam pattern does not need special requests. C type ellipsoid reflector could concentrate light distribution. The dispersed angles are around 25 degree. In the simulation, the “hot zone” is achieved 3.22 Lux by using

three 3W Lambertian LEDs. The shield is used by the type α. b) Town function module Based on the AFS drift, the light distribution must more extensive lighting. B and C type ellipsoid reflectors are chosen. The dispersed angles are around 35 and 25 degree, respectively. In the simulation, the “hot zone” is achieved 5.16 Lux by using five 3W LEDs. The shield is used by the type α. c) Wet road function module The wet road function module consists of B and D type ellipsoid reflectors. The dispersed angles are around 35 and 20 degree, respectively. The shields are used by α and β. In the simulation, the “hot zone” is achieved 5.68 Lux by using four 3W LEDs. d) Motorway function module According to the AFS drift, drivers need farther vision on the motorway. Motorway light function module consists of C and D ellipsoid reflectors. The dispersed angles are around 25 and 20 degree, respectively. The shields are used by α and β. In the simulation, the “hot zone” is achieved only 6.97 Lux by using five 3W LEDs. e) High beam module All modules include a combination of A, B, C and D type reflectors. All LEDs are turned on for high beam function. In the simulation, the “hot zone” is only achieved 17 Lux by using nine 3W LEDs. Thus, the four kinds of reflectors could create five types of lighting distribution. This document serves to show examples of how LEDs can functionally be used to create the needed distributions. The different AFS beam patterns are made by the principle of the LED’s lighting superposition. The corresponding simulations and experimental results are shown in Fig.18. The potential on the LED headlamp with AFS function is shown in Fig.19. DISCUSSION Comparing the simulations with the experimental results, there are slightly differences. The first reason is that the size of the LED is smaller than the filament halogen light source, and the reflector is designed especially small. Thus, the tolerance of the optical components become much more important--no matter the LED light source position, the shield position, and the lens position all playe the important roles in the mechanism design. When the relative positions of the optical components are not accurate, the different light pattern would not be overlapped well. It could cause the light distribution and the hot zone to not conform to the original design. Therefore, the accuracy of the relative positions of the optical components is very important for the LED headlamp. In addition to the performance of the module at normal position, an analysis is carried regarding the sensitivity to the source position. LED sources are located in the focal point of the ellipsoid reflector. The LED is moved by 1mm in Z axis direction. Fig.20 shows the effects of this translation of the source in the optical system. The beam pattern is wrong when the LED is not in the right position. The results show that the accurate

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positions of the optical components are very important. The second reason is that the luminance of the LED is not higher, and the experimental results of the lighting distribution are not acceptable for the AFS legal requests. However, it has been demonstrated that using LEDs can functionally be used to create the AFS drift beam pattern by the designed reflectors and the shields. The AFS-LED headlamp provides the applicable possibilities for the adaptive front lighting system. It is used to display its function but not to meet the AFS legal requests. It is believed that when the luminance of LED is higher than the existing filament halogen light sources, the AFS function can be applied to the LED headlamp development in the near future. CONCLUSION

LED is revolutionizing vehicle lighting application. New design considerations will be taken into account, including total flux output, optical design, and system size. The accuracy of the relative positions of the optical component is very important for the LED headlamp. Vehicle with AFS function using LED as light source would be the next generation of LED headlamp. In the near future LED headlamp will be used on the vehicles everywhere.

(a) Simulation and experimental results of basic beam

pattern (25m).

(b) Simulation and experimental results of town beam

pattern (25m).

(c) Simulation and experimental results of wet road

beam pattern (25m).

(d) Simulation and experimental results of motorway

beam pattern (25m).

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(e) Simulation and experimental results of high beam

pattern (25m). Fig.18. Corresponding simulations and experimental

results of the AFS-LED headlamp.

Fig.19. Experimental result of basic beam function (10m).

(a) Correct position in X Y Z axis without shifting.

(b) LED shift along Z axis (-1mm, +1mm).

Fig.20. Shifting selectivity of the LED: (a) Correct

position, (b) longitudinal direction.

ACKNOWLEDGMENTS

This study is supported by Automotive Research ＆ Testing Center of the Republic of China. REFERENCES

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15. Technical Data Luxeon™ Emitter, Lumileds Lighting 16. ASAP Version 2005, Breault Research Organization. DEFINITIONS, ACRONYMS, ABBREVIATIONS LED: Light Emitting Diode HID: High Intensity Discharge AFS: Adaptive Front Lighting System