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Abstract
In this paper, we proposed and demonstrated a new optical design scheme for vehicle forward lighting with a high-contrast cut-off line. We first presented a design of a reflector to meet that meets one regulation (e.g., K-mark) but fails in the other regulation (e.g. ECE R113 class A). The design starts from a general approach in forming the cut-off line but with a narrower light pattern to fit the first regulation. Then an optimized cylindrical lens array (CLA) is used to laterally spread the light pattern. The introduction of the CLA can increase the linearity of the cut-off line, and most importantly, suppress the illuminance in the dark zone so that contrast across the cut-off line can be effectively increased. As a result, the headlamp can pass the severer regulation without redesigning the reflector. Additionally, the CLA can compensate for the optical effect by lens/reflector deformation in the manufacture process so that the manufacturing tolerance can be enlarged and the design scheme of forward lighting becomes easier and more robust in practical manufacturing.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Light emitting diode (LED) has been extensively applied to not only general lighting but also specific lighting owing to its advantages including fast response, vivid color, compactness, high efficiency, and low cost [1,2]. One of the important applications is vehicle forward lighting [3–7]. A headlamp in a vehicle is different from the other lamp because it not only provides roadway illumination but also needs to equip anti-glare function. Therefore, a regulation to indicate the light pattern of a headlamp is necessary. The related regulations for vehicles, including bikes, motorcycles, and cars, are different but all request a cut-off line, especially in low-beam head lamp. The cut-off line is a boundary to define the bright and dark zone. The gap between the darkest part and brightest part is associated with the characteristic of the vehicle. A slower vehicle could be under a regulation with larger gap. The most rigorous regulation defines a very small gap and causes difficulty in optical design. Even the optical design can perform a short gap in the cut-off line, the manufacture could cause deformation of the optical component so that the real light pattern fails to meet the regulation. Traditionally, a low-beam head lamp is designed with a condenser reflector to focus the light emitted by the light source, and then a block plate with a specific shape is located at the focus spot to form the cut-off line. Then a positive lens is used to project the blocked focused light away the vehicle [8,9]. The advantage of the design is that the cut-off line will be very clear, and the brightest point is near the dark region to have a very short gap. However, this approach will cause long working distance in the optical system. Another approach is to use multi-segment reflector or cylindrical lens to shape the projection light to fit the regulation. In the case of a sharp cut-off line, a larger reflector is necessary [10–17]. The design becomes tough in not only sharpness but also linearity of the cut-off line. These problems become worse in practical manufacture because a slight deformation of the reflector could cause distortion of the cut-off line. In this paper, we propose and demonstrate a new rule of optical design. It is a robust approach even when the manufactured optical component deforms to destroy the cut-off line.
2. Regulation
To demonstrate the novel design concept, two regulations are discussed, as shown in Fig. 1, where the K-mark regulation is for bike and the ECE R113 class A is for motorcycle [18–20]. In the K-mark regulation shown in Fig. 1(a), HV is a specific point along the vertical scanning line, and the illuminance at HV should be larger than 1/1.2 of the brightest point. The minimum illuminance at HV is 20 lx at a distance of 10 m from the luminaire. The spot pattern at L1, R1 and “2” must bear more than half illumination of that at the brightest point. A dark zone (zone 1) at 3.4o above the HV requests maximum illuminance of 2 lx. Then a cut-off line is formed with the gap between the bright and dark zones. The least contrast by the illuminance at HV and the dark zone should larger than 10. The key point in design to meet the K-mark regulation is to increase the brightness at HV and hold the contrast larger than 10.
The ECE R113 class A regulation shown in Fig. 1(b) is a different story, where luminous intensity rather than illuminance is applied because the vehicle is more powerful with higher speed. The dark zone (zone 1) requests luminous intensity lower than 320 cd. A bright zone (the cut-off line) with 1.72o below the zone 1 must reach 1100 cd or above. A second bright zone with 3.43o below the zone 1 must reach 550 cd. Between the zone 1 and the brightest zone, a cut-off line located 0.57o below the zone 1 should be checked with the so-called “sharpness” to indicate the contrast, and a G value is defined
A design example is shown in Fig. 2. The white LED was made with yellow phosphor covering on a blue die with dimensions of 1mm×1 mm. The optical model was built through midfield model [21,22]. The emitting flux was set 370 lm. A downward multi-segment reflector with reflectivity of 80% was designed to form the projection pattern with cut-off line. Through Monte Carlo ray tracing [23] with reaching 100 million ray number, a simulated light pattern on a vertical plane at a distance of 10 m was obtained. In checking the illuminance on the plane according to K-mark regulation, a check table was obtained and shown in Fig. 2. Although the cut-off line is not smooth, the illuminance performance in the check table shows that the light pattern meets the K-mark regulation.
Fig. 2. (a) The structure of the white LED. (b) The downward reflector. (c) The light pattern by the simulation. (d) The check list for the K-mark regulation, and all values meet the criteria in the regulation.
We apply the same design to the ECE R113 class A regulation, and the result is illustrated in Fig. 3. Figure 3(b) shows the minimum of luminous intensity at bright zone near the cut-off line is only 591.1 cd, which is much less than 1100 cd required in the regulation. This shortage is caused by the shorter cut-off line in the design. To enlarge the length of the cut-off line, the multi-segment reflector must be redesigned. Figure 3(c) shows the sharpness of the cut-off line, where the G value is 0.217, which is larger than the minimum requirement of 0.08 in the regulation. Figure 3(d) shows the linearity of the cut-off line, where the largest angular deviation reaches 0.65o, which exceeds the maximum value of 0.3o in the regulation. In summary, the designed light pattern is too narrow to fit the ECE R113 class A regulation, and the linearity also fails to meet the regulation. To conquer this problem, the reflector should be re-designed. But it is time consuming and not cost effective.
Fig. 3. (a) The regulation table. (b) The check values in the regulation. (c) The G value in the lateral direction. (d) The linearity of the cut-off line.
3. Robust optical design
Figures 2 and 3 shows a fact that a light pattern can meet the regulation of K-mark, but cannot pass ECE R113 class A regulation. Even the designed light pattern is not smooth in the cut-off line, the light pattern is still acceptable in the regulation of K-mark. Besides, practical manufacture could make the condition worse because the plastic reflector could deform due to the process of injection molding. Therefore, to meet the ECE R113 class A regulation, a more robust optical design is demanded.
To apply the new design scheme on an existing reflector, we have to check a factor of optical utilization factor (OUF), which is defined a ratio between the flux on the target and the flux emitted by the emitter [24,25]. Figure 4 shows the energy budget for the entire light pattern by the designed LED headlamp. The OUF at the target and the ground are 28.7% and 38.5%, respectively. It means that the total optical efficiency reaches 71.2%, where the reflectivity of the reflector was set 80%. Such energy distribution satisfies the practical need.
In this case, the OUF is large enough in the original design, a new way to reshape the light pattern without change the original reflector is preferred because any change in the mold cavity could rise the cost. To solve this problem, we propose to use a cylindrical lens array (CLA) shown in Fig. 5 to spread the light pattern along the lateral direction. In design of the CLA, we apply conic section equation
where K is the conic constant and R is the vertex radius of curvature. For a specific R, K determines the bending of lens. Also, the lens width (LW) is a factor which determine how many lens embedded in the CLA plate. The optimization of the CLA is through the three factors including R, K and LW. R decides the optical power of the lens and the divergent angle of the light pattern, K factor decides the thickness of the lens and the fine structure of the light pattern, and LW decides the number of lens, and relates to scattering light.Fig. 5. (a) The structure of the CLA plate. (b) The illustration of the three key factors of each cylindrical lens.
Figure 6 shows a comparison by simulation of the optical efficiency in the cases of CLA embedded in the inner face and outer face. When the CLA is on the outer face and the R is smaller than 2 mm, there are more possibility for total internal reflection. It means that CLA in the inner face is better than that in the outer face. Practically, the outer face of a headlamp should be as smooth as possible to avoid dust attachment. Therefore, the CLA should be embedded in the inner face, which directs to the light source and the reflector. Then in the following optimization, all CLA’s are designed in the inner face. In checking the divergent angle and the light pattern, we tuned R from 1 mm to 10 mm and K from 1 to −2. The results are shown in Fig. 7, where the incident light is collimated with normal incidence. We may find that the effective divergent angle by the CLA is from 4.5o to 56.5o. As indicated in Fig. 6, the optical efficiency in all the cases shown in Fig. 7 is similar when R is larger than 2 mm. It is easy to see that larger divergent angle could have more uniform and wider pattern, but with less intensity. Thus, the divergent angle is the main factor in determining the parameters of the CLA. Based on the analysis in Fig. 6 and the divergent angle requirement to fit ECE R113 class A, we chose R = 3 mm and K = −2 in the simulation shown in Fig. 8 and the experiment will be illustrated in the following.
4. Experimental results
The experimental setup is shown in Fig. 9(a), where a white LED as illustrated in Fig. 2 attached on a multi-segment reflector which was held in a translation stage, and a CLA with the design parameters was attached on the other translation stage. The observation plane was at a distance 10 m from the headlamp. The light pattern with or without the CLA on the observation plane is shown in Fig. 9(c-d). In comparison, the light patterns were similar to those in the design as shown in Fig. 2(c) and Fig. 9. Although the light pattern without the CLA in design met the K-mark regulation, the experimental measurement show that the light pattern failed to pass the regulation owing to distortion of the reflector during manufacture as shown in Fig. 9(b). Thus, some unwanted light condensed at a certain area in the dark zone. Besides, the light pattern was not wide enough to fit the regulation. In contrast, with CLA, the light pattern was lateral extended, and the unwanted light in the dark area was also laterally spread, so the illuminance in the dark area was effectively reduced. In checking the ECE R113 class A regulation, the measurement of the G vales and the flatness of the cut-off line were shown in Fig. 10. In the case of the light pattern without the CLA, the G value was 0.14, which was larger than the minimum value of 0.08, but the angular deviation of the cut-off line reached 0.9o, which was larger than the maximum value of 0.3o. In the case of the light pattern with the CLA, the G value was also 0.14, and the angular deviation of the cut-off line was suppressed to 0.2o. The CLA was shown to increase the flatness of the cut-off line with keeping the G value unchanged or better.
Fig. 9. (a) The headlamp with a CLA in the experiment. (b) The measured illuminance at the specific locations, where the first value is without the CLA, and the second value is with the CLA. The red numbers fail to meet the regulation. The real light pattern in the case (c) without the CLA, and (d) with the CLA.
Fig. 10. G value vs. lateral view angle in the case of (a) without the CLA, (b) with the CLA. Angular deviation of the cut-off line vs. lateral view angle in the case of (c) without the CLA, (d) with the CLA.
5. Summary
In this paper, we have proposed a new robust design scheme for meeting severe different vehicle forward regulations in considering manufacture error factor. First, a general optical design is used to make a cut-off line. Due to design simplicity, the light pattern could pass the one regulation as expected but cannot pass another in higher demand. Also, practical manufactural error could spread unwanted light into the dark zone so that the contrast of the cut-off line could not meet the regulation. As the proposed method, a CLA is applied to reshape the light pattern, including the bright zone and dark zone. The optimization factors of the CLA includes conic constant, the vertex radius of curvature, and the lens width. All the factors relate to the optical efficiency and lateral smoothing effect of the light pattern. The CLA is suggested to attach on the inner face of the cover window of the head lamp to reduce Fresnel loss and to avoid dust attachment. Once the manufacture error distorts the cut-off line or spread unwanted light to a certain location in the dark area, the CLA could reshape the light pattern to meet the regulation without redesigning the reflector.
To show the practical capability, this paper has presented a real-case study. We first design a light pattern to meet the K-mark regulation, but cannot pass the ECE R113 class A regulation. The experimental result shows the similar situation. With use of the CLA, both simulation and experimental measurement show that the light pattern meets the ECE R113 class A regulation as well as the K-mark regulation. The use of the CLA rescues the headlamp from the rough design and the deformed reflector. The new design scheme will be helpful in high-contrast illuminance and in low-precision manufacture for practical applications. It also could raise a new design rule in vehicle headlamp.
Funding
Ministry of Education (MOE) (103G-903-2); Ministry of Science and Technology, Taiwan (MOST) (105-3113-E-008-008-CC2, 106-2221-E-008 -065 -MY3).
Acknowledgments
The author would like to thank the Breault Research Organization for providing the ASAP simulation program.
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