High-energy-efficiency optical system for an LED-based headlamp architecture Aiming Ge,1,* Wei Wang,1 Zhengqing Du,2 Peng Qiu,1 Junwei Wang,1 and Jinlin Cai1 1

Department of Light Sources & Illuminating Engineering, School of Information Science and Engineering, Fudan University, Shanghai 200433, China 2

Jiangsu Hongchang Science and Technology Co., Ltd. Danyang 212322, China *Corresponding author: [email protected]

Received 26 August 2013; revised 26 October 2013; accepted 29 October 2013; posted 30 October 2013 (Doc. ID 196074); published 25 November 2013

This paper proposes a low-beam system for an LED-based headlamp architecture, which is composed of an elliptical reflector, a baffle, and a faceted reflector. Using a single device with high brightness LED of merely 6.00 W, two devices total 12.00 W. With a low beam 55 W traditional halogen light source compared to 78.18% energy savings, the specified illumination requirements for the headlamp low beam can be achieved, according to the ECE regulation “Addendum 111: Regulation No. 112 Revision 2.” As we have expected, on the test screen at a distance of 25 m, the simulation results as well as the testing results for the prototype can reach the illuminance distribution requirements, including all specified regions and key points. Moreover, this faceted low beam system enjoys the features of high compactness, high energy efficiency, and feasibility of manufacturing. © 2013 Optical Society of America OCIS codes: (220.2945) Illumination design; (220.4298) Nonimaging optics; (220.4830) Systems design. http://dx.doi.org/10.1364/AO.52.008318

1. Introduction

Solid-state lighting technology has been developed dramatically in recent years, since the luminous efficacy of a single high-brightness LED device can reach more than 180 lm∕W in mass production [1,2]. This encourages the wide lighting application of high-brightness LEDs, especially in automotive lighting and even for headlamps. Compared with traditional headlamps, such as halogen headlamps and Xenon headlamps, high-brightness LEDs only require a power consumption of about 10 W to achieve a total flux of more than 1500 lumens, which is sufficient for a low-beam module [3]. Previously, the luminous flux of a single LED device was limited. Reflector arrays or lens arrays are adopted to fulfill the luminous flux requirement of an automotive headlamp [4]. Now high-power LEDs characterized by high luminous flux of a single 1559-128X/13/348318-06$15.00/0 © 2013 Optical Society of America 8318

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LED device makes it possible to design a more integrated and compact headlamp by adopting only one single LED device without the preceding combing and mixing structures. To make use of the high luminous efficacy and to fulfill the requirements for a low-beam module according to United Nations Economic Commission for Europe’s (UNECE, with its abbreviation ECE) regulations, a specialized optical system based on LEDs is developed [5–8]. In this paper, a composite optical system is proposed to achieve these required specifications. The system is composed of an elliptical reflector, a baffle, and a faceted reflector. By using a single high-brightness LED device as the light source, we get a satisfactory beam pattern on the measuring screen at a distance of 25 m, with a clear cutoff line, a smooth illuminance distribution, and an emphasized hotspot. 2. ECE Regulations for Low-Beam Specification

According to the recently published explanatory UNECE regulation “Addendum 111: Regulation No. 112 Revision 2” [9], the light output of the

low beam module should meet the following specifications: a. The beam shape should have a wide horizontal spread, which can reach 4000 mm on the measuring screen for both sides; b. The beam shape should have a clear cutoff line, which means a horizontal cutoff line at the left side on the H-H line and a tilted cutoff line with a tilting angle of 15° at the right side; c. There must be a hotspot around Point 50R and Point 75R; d. Smooth transition is also required to ensure the visual comfort; e. To constrain the glare for right-hand traffic, the illuminance at Point 50L should be no larger than 0.4 lux, while the illuminance at Point 50R and Point 75R should reach at least 6 lux, to guarantee enough luminous intensity onto far field. To meet the required specifications, we design the compound optical system made up of an elliptical reflector, a baffle, and a faceted reflector, as shown in Fig. 1. The design details will be discussed in the next section. 3. Design of the Faceted Low-Beam System A.

System Configuration

Considering that the LED is a small Lambertian emitting surface, we first collect all the light emitted from the LED by an elliptical reflector and then use a faceted reflector to project rays onto the measuring screen. We build our system under the U-V coordinate system, as shown in Fig. 1. The system is composed of an LED light source, an elliptical reflector, a baffle, and a faceted reflector. The origin point of the U-V coordinate system is coincident with the first focal point of the elliptical reflector. Since the LED device should be carefully contacted with the heat sink, we make use of the configuration shown in Fig. 2. The LED device S is set at the first focal point F1 of the elliptical reflector and oriented along the z axis. Therefore, heat management can be approached by attaching the heat sink onto the back of the LED without blocking the optical system.

Fig. 2. Profile of the low-beam system.

The elliptical reflector is generated by revolving the profile on Y–Z plane about the y axis. To avoid blocking rays projected by the faceted reflector and to achieve system compactness, the profile is an ellipse covering v angles from 0° to 150°. The flux efficiency can be calculated according to R π∕2 R 5π∕6 p I 0 sin2 v cos ududv 3  5π∕12 Fc −π∕2 0  R π∕2 R  π 2 π F 0 I 0 sin v cos ududv −π∕2

 96.8%;

(1)

which only results in a 3.2% flux loss. Compared to the traditional PES (Projector Ellipsoid System) design, the system efficiency is mainly dependent on the solid angle that the elliptical reflector covers corresponding to the LED light source. In this context, the elliptical profile on the Y–Z plane determines the concentration while the profile on the X–Y plane collects rays from all angles in that plane. Therefore, there is not much difference whether it is a varifocal elliptical reflector. Rays emitting from the LED device at the first focal point F 1 will be refocused at the second focal point F 2 , as shown in Fig. 2. In this way, the LED with the elliptical reflector can be substituted by an imaginary point source S0 at the second focal point F 2 , with a Table 1.

Parameters for the Ellipse Profile

Long-Axis Length a

Focal Length c

66.00

18.00

Value

Table 2.

Fig. 1. Low-beam system under U-V coordinate system.

Value

Parameters for the Reference Parabolic Profile

Length in z Direction

Focal Length c

Radius of Aperture

30.00

30.00

120.00

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Fig. 3. Tailoring relationship between the faceted reflector and beam pattern.

new luminous intensity distribution transformed by the reflector. Given the focal length c and the longaxis length a of the ellipse profile and the focal points F10; 0; 0 and F20; 2c; 0, the transformation from ⃗ can be dethe incident ray I⃗ to the reflected ray O scribed by the following equations: 8 > > I⃗  I x ; I y ; I z  > > > > ⃗  Ox ; Oy ; Oz   F 2 − F 1  ρ · I ⃗ > > > > > > > :

v0  a sin u0  a tan

Oy ⃗ ‖O‖

B. Design and Optimization for Faceted Reflector

;

2

  Ox Oz

where ρ  a2 − c2 ∕a − c sin v is the radii length. The new luminous intensity distribution is mainly dependent on the focal length c (means the distance between the first focal point F1 and under vertex of the ellipsoid) and the long-axis length a of the ellipse profile. To achieve system compactness and to make a more uniform luminous distribution, we select a group of parameters for the elliptical reflector listed in Table 1. However, the real LED device is not an ideal point source. We select a CREE MK-R LED device as the light source, working at 500.00 mA under 12.00 V, with 2 × 2 mm emitting area, which is not negligible compared to the size of the reflector. This results in a beam extension at the second focal point F 2 and also makes the imaginary light source S0 an extended source. The exact position for the LED device is discussed in more details in the following paragraphs. The faceted reflector is based on a reference parabolic reflector, covering the entire solid angle Ω of the beam from the imaginary light source S0 . Basic parameters of the reference parabolic are listed in Table 2. To get the desired beam pattern and illuminance distribution on the test screen, we divide the reference parabolic reflector into three functional divisions D1 ∼ D3. Each of the divisions is responsible for respective function of the projected beam, which corresponds to three beam patterns, shown as diagonal zones in Fig. 3. The division D1 makes use of the left half of the flux of the imaginary light source and is responsible for extending the beam horizontally from 4000L to 1500R; the division D2 takes use of the higher half of the right part to extend the beam 8320

from 1500L to 4000R. The division D2 makes use of the rest of the right part, and creates the cutoff line with 15 tilted angle. Because of the beam overlapping by D1, D2 , and D3 , there can be a hotspot around 50R and 75R as depicted in white in Fig. 3. All three divisions are made up of subdivisions, called facets. More details about facets design are discussed in next section.

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Theoretically, more facets can achieve more precise control of the projected beam pattern. However, a large number of facets will increase the difficulty of manufacturing. Therefore, we use a limited number of facets to achieve individual functions of three divisions. The optical feature of a parabolic reflector is to collimate rays from its focal point to infinite far fields as parallel rays. As we use the parabolic reflector as our base reflector, we keep using the parabolic optical surface for the facets. We set the targets array on the test screen and make tailoring mapping between facets and the targets. As in Fig. 4, the center facet is generated from the base reflector and is responsible for the center focal point while other facets are generated from the variation of the base reflector, as both optical axis and the conic coefficients vary according to targets and their beam pattern requirements. As shown in Fig. 4, the division D1 is divided into nine facets, the division D2 is divided into eight facets, and the division D3 is divided into ten facets. Notice that the facets are rotated 15° in division D2 to generate the 15° cutoff line. As seen in Fig. 3, to achieve the horizontal cutoff line and the tilted cutoff line, we set the targets distributed along the cutoff lines. For point source assumption, all flux will be concentrated onto the targets array, and the beam pattern at each target can be the same as the corresponding facet shape.

Fig. 4. Beam created by relevant facets.

Fig. 5. Extended source in the system.

However, since the real LED device is an extended surface source, the real beam pattern can be treated as the convolution of the beam pattern generated by the point source and the source image, which spreads the beam pattern and can be useful for beam

extension along the vertical direction while we still have to pay attention to stray lights and optimize the illuminance distribution. To ensure a clear cutoff line, we need to block stray rays that reach above the cutoff line. We set an optical baffle to absorb stray rays, as shown in Fig. 5. Besides, to make use of the source flux as much as possible, we shift the geometry of our real LED source and match the upper bound of the emitting surface with the focal point of the elliptical reflector. In this way, a small amount of the source flux will be blocked, but stray rays above the cutoff line can be eliminated. As is mentioned above, the convolution effect can help to spread the beam around the targets array. The extended beam shape generated by each facet varies according to the facet shape. The facets near the center facet, e.g., facet No. 1, can generate larger beam extension as they take up larger solid angle, and the source images viewed from the facets are also larger than the edge facets, e.g., facet No. 3. The facet of D2, e.g., facet No. 2, can create clear 15° cutoff line on the test screen, as potential stray lights are blocked by the baffle in advance. The overlapping among these extended beams will influence each other and create unexpected hotspots or dark stripes. To eliminate the unexpected phenomenon, we adjust the targets position and conic coefficients and evaluate the beam pattern, until the illuminance distribution can reach the requirements.

Fig. 6. Simulation beam pattern. 1 December 2013 / Vol. 52, No. 34 / APPLIED OPTICS

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Fig. 7. Real prototype model consisted of two theoretical low beam systems and the tested beam pattern. (a) Real prototype model and (b) tested beam pattern.

4. Simulation and Experiment

After iterations and optimization, we build the entire system in CAD software. We introduce the Cree MK-R LED device as the real light source and make ray-tracing simulation, as shown in Fig. 6. Since the source images generated by the facets result in extended beam patterns for all targets, the entire beam becomes a smooth and continuous beam. The horizontal extension can reach both 4000L and 4000R, and clear hotspots can be seen around 75R. As we use a baffle to block stray light, the cutoff lines are quite clear. In this application, we drive MK-R at 500.00 mA under 12.00 V, resulting in a power consumption of 6.00 W. We choose high reflectance Aluminum reflective coating on both elliptical reflector and the faceted reflector with a high reflectivity of more than 95%. Since most of the flux is collected by the compound optical system and projected onto the test screen, we can collect more than 386.40 lumens on the receiver with a flux input of 685.71 lumens for the MK-R device in simulation, meaning a high system efficiency of 56.35%, as shown in Fig. 6. We also make a prototype for performance testing. The reflectors are made of PC material coated with Table 3.

Tested Illumination Result Compared with the Corresponding Value of ECE R112 R2

Point on Measuring Screen Point B50L Point 75R Point 75L Point 50L Point 50R Point 50 V Point 25L Point 25R Any point in ZONE I Any point in ZONE III Any point in ZONE IV

Required Illumination in Lux

Simulation Illumination in Lux

Tested Illumination in Lux

≤0.4 ≥12 ≤12 ≤15 ≥12 ≥6 ≥2 ≥2 ≤2 · E

0.34 26.88 14.01 16.85 33.13 27.86 2.97 3.55 21.12

0.28 22.58 11.42 13.92 26.76 23.52 3.79 2.81 16.69

≤0.7

0.57

0.65

≥3

3.88

5.46

E is the actually measured value in point 50R. 8322

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high reflectance aluminum. Figure 7(a) indicates a real prototype model consisting of two theoretical low beam systems to enhance the quality of the low beam pattern. To verify the real performance, we test all key points announced in ECE regulation and record the results. A comparison of the actual tested values and simulation values are both listed in Table 3. The tested beam pattern is seen in Fig. 7(b). All the illuminances on selected test points can reach the requirements according to ECE regulation. The prototype shows outstanding performance as expected. 5. Conclusions

The low beam system for the LED headlamp we proposed in this paper is composed of an elliptical reflector, a baffle, and a faceted reflector. Using a single device with high brightness LED of merely 6.00 W, two devices total 12.00 W. With a low-beam 55 W traditional halogen light source compared to 78.18% energy savings, the specified illumination requirements for the headlamp low beam can be achieved, according to the ECE regulation “Addendum 111: Regulation No. 112 Revision 2.” As we have expected, on the test screen at a distance of 25 m, the simulation results as well as the testing results for the prototype can reach the illuminance distribution requirements, including all specified regions and key points. Moreover, this faceted low-beam system enjoys the features of high compactness, high energy efficiency, and feasibility of manufacturing. This work was partially supported through the Produce-Learn-Research project no. 10245 by Jiangsu Hongchang Science and Technology Co., Ltd. Aiming Ge thanks the China Scholarship Council (CSC) for financial support no. 2010610538 of his research study at Utah State University and University of California, Merced, USA. References and Notes 1. T. Luce, “LED headlamps—the spiny path to a legal headlamp,” Proc. SPIE 5663, 112–121 (2005). 2. A. Domhardt, S. Weingaertner, U. Rohlfing, and U. Lemmer, “TIR optics for non-rotationally symmetric illumination design,” Proc. SPIE 7103, 710304 (2008).

3. J. L. Alvarez, M. Hernandez, P. Benitez, and J. C. Minano, “TIRR concentrator: a new compact high-gain SMS design,” Proc. SPIE 4446, 32–42 (2002). 4. A. Cvetkovic, O. Dross, J. Chaves, P. Benitez, J. C. Miñano, and R. Mohedano, “Etendue-preserving mixing and projection optics for high-luminance LEDs, applied to automotive headlamps,” Opt. Express 14, 13014–13020 (2006). 5. A. Domhardt, U. Rohlfing, and S. Weingaertner, “New design tools for LED headlamps,” Proc. SPIE 7003, 70032C (2008). 6. A. M. Ge, W. Wang, and Z. Q. Du, “Design of an LED-based compound optical system for a driving beam system,” Appl. Opt. 52, 2688–2693 (2013).

7. F. Chen, K. Wang, and Z. Qin, “Design method of highefficient LED headlamp lens,” Opt. Express 18, 20926–20938 (2010). 8. W. J. Cassarly, S. R. David, D. G. Jenkins, A. P. Riser, and T. L. Davenport, “Automated design of a uniform distribution using faceted reflectors,” Opt. Eng. 39, 1830–1839 (2000). 9. UNECE Addendum 111: Regulation No. 112 Revision 2, 20 September 2010, Amendment 1, 27 January 2011, “Uniform provisions concerning the approval of motor vehicle headlamps emitting an asymmetrical passing beam or a driving beam or both and equipped with filament lamps and/or light-emitting diode (LED) module.”

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High-energy-efficiency optical system for an LED-based headlamp architecture.

This paper proposes a low-beam system for an LED-based headlamp architecture, which is composed of an elliptical reflector, a baffle, and a faceted re...
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