applied sciences

Article A Study of High-Efficiency Headlight Design Using Gradient-Index Lens and Liquid Lens

Yi-Chin Fang * , Yih-Fong Tzeng, Chan-Chuan Wen, Chao-Hsien Chen, Hsiao-Yi Lee, Shun-Hsyung Chang and Yi-Lun Su

Department of Mechatronics Engineering, National Kaohsiung University of Science and Technology, Kaohsiung City 811, Taiwan; [email protected] (Y.-F.T.); [email protected] (C.-C.W.); [email protected] (C.-H.C.); [email protected] (H.-Y.L.); [email protected] (S.-H.C.); [email protected] (Y.-L.S.) * Correspondence: [email protected]; Tel.: +886-7-6011000-32290

 Received: 25 August 2020; Accepted: 12 October 2020; Published: 20 October 2020 

Abstract: In the field of vehicle , due to the diode laser, its small size and high energy conversion efficiency, it can be effectively used as the headlight source of high beam. In recent years, it was adopted by European advanced car manufacturers as a new generation of automotive headlight lighting products. The current mature technology on the market is to extend the laser beam by means of reflection and to use a single high-power laser as the source to meet the needs of surface lighting. In this research, we propose a new integrated optical design for an automotive headlight system with the rod lens, gradient-index lens (GRIN lens) and freeform lens to expand the laser beam. With regard to the diffusion of the beam by reflection and refraction, the liquid lens is used as a switch for the high beam and low beam to meet the needs of vehicle lighting functions and to use low-power diode to synthesize the array light source. Compared with the 24-W LED headlight module available in the current market, the energy saved by this proposed optical design can increase efficiency by an average of 33%. The maximum illuminance is 56.6 lux in the high-beam mode, which is 18% higher than the standard value. Let the laser light meet the lighting requirements of regulatory standard values even beyond.

Keywords: laser automotive headlight; optical design; laser beam expansion; non-image optics; freeform surface; GRIN lens; optimization

1. Introduction A laser headlight, representing the most advanced, cutting-edge and efficient vehicle lighting system, has higher illumination efficiency than halogen and xenon headlights, and its energy efficiency is also no less than that of an LED headlight [1]. With a small size and strong penetration, laser technology has become the mainstream for next wave vehicle lighting. Since its successful development in a laboratory in 1960, laser has been considered an extremely important scientific achievement; however, it is rarely used in lighting applications [2]. Laser automotive headlights, which exhibit the characteristics of single wavelength, high homology, high directivity and high penetration, [3] are considered to be extremely innovative and energy efficient [4,5]. Advanced headlight technology such as adaptive headlighting system are timely research topics. In this paper, a proposed optical design and simulation is our present approach, aimed at not only improving the headlight efficiency with , but also supporting adaptive headlighting technologies. Generally speaking, the advantage of using a laser application in headlights is that the laser diode is only one-tenth or less the size of a conventional halogen lamp. In addition, advanced technology today significantly improves the performance of various laser diodes, which saves space

Appl. Sci. 2020, 10, 7331; doi:10.3390/app10207331 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 7331 2 of 14 and reduces power consumption further. Laser automotive headlights produce 170 lumens per watt, as opposed to the 100 lumens per watt produced by LEDs, indicating that the luminous efficiency efficiency achieved using a laser is 70% higher higher than that that obta obtainedined using using LED LED headlights. headlights. In In terms terms of of penetration, penetration, laser automotive headlights can penetrate as much as 600 m, which is twice twice the the penetration penetration achieved achieved when using traditional halogen, xenon and LED lights lights,, and is considered to be energy efficient efficient in a vehicle-proven environment. Furthermore, the small size of the laser device provides a considerable degree of freedom in the designdesign of the vehicle [[6,7].6,7]. Laser diodes as the light source and their simula simulationtion in this proposed research could be further analyzed from the point of view of the background of the simulationsimulation softwaresoftware andand howhow itit works.works.

1.1. Background Background Automotive illumination technology improved fast over the last decade and a half. As we have seen, incandescent halogen bulbs give way to high intensity intensity discharge (HID) and light emitting emitting diode (LED) and and its its related related technologies. technologies. Even Even Audi Audi and and BM BMWW have have started started to ship to shipvehicles vehicles that use that a laser- use a basedlaser-based illumination illumination system system [8,9] in [8 ,recent9] in recent days. days. Currently, BMW has the most mature technology for the mass production of laser light modules (Figure1 1),), whereinwherein thethe laserlaser sourcesource movesmoves towardtoward thethe mirror,mirror, whichwhich reflectsreflects thethe lightlight onon thethe freeformfreeform reflectorreflector lamp cup. This This then then diffuses diffuses and and projects projects the the laser laser beam beam over over the the area area to to be be illuminated illuminated [10]. [10]. Audi has its design performed by ultra-preciseultra-precise asphericalaspherical reflectionreflection surfacesurface [[11].11].

(a) (b)

Figure 1. ((aa)) BMW BMW i8 i8 laser laser headlight headlight and and ( (b)) its its schematic schematic diagram of light tracks.

One of the major problems is optical design and simulation for newer lighting sources due to their optical characteristics. Furthermore, Furthermore, new new dev developmentselopments in adaptive technologies and laser diode as light source become more more complicat complicateded from from the the point point view view of of optical optical system system design. design. For example, the Audi Pixelated Laser Headlights Li Lightght employed a laser lighting system and a micro mirror device (DMD), which is is very close to to DLP projectors projectors from from the the point point view view of of optical optical design. design. This system can can vary vary its its lighting lighting pattern pattern directionally, directionally, then then have have its its image image projected projected on onthe theroad. road. In thisIn this proposed proposed research, research, we we have have incorporated incorporated an an ad adaptiveaptive lighting lighting capability capability into into our our simulation simulation of automotive Illumination with laser source [[12].12].

1.2. Optical Optical Software: Software: LightTools LightTools In most cases, optical system design plays the rolerole inin opto-mechatronicopto-mechatronic system.system. Optical system design is based on ray tracing so that computer-aid design and optimization isis aa criticalcritical issue.issue. LightTools is a 3D optical engineering and designdesign software product that supports virtual prototyping, simulation, simulation, optimizati optimizationon and and photorealistic photorealistic renderings renderings of ofillumination illumination applications. applications. Its uniqueIts unique design design and and analysis analysis capabilities, capabilities, combined combined with with ease ease of of use, use, support support for for rapid rapid design iterations and automatic system optimization, help help to to ensure the delivery of illumination designs according to specificationsspecifications and schedule [[13].13].

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1.3. Simulation for Laser Diode and System It was not feasible to accurately model the laser diode that can make up a modern headlamp system. A more practical approach was to gather real world data from headlamp measurements and use that as a look-up table in our headlight model. These data employed in this research were provided by LightTools laser diode data base, which provides the luminance value for reference laser diode for simulation purposes [13]. All proposed research work in this paper was executed by LightTools either ray tracking or optimization. Neither protype nor manufacture was done in this experiment. We might have opportunities to proceed with the prototype work in case automotive companies show their interests in further developments. Cost and light efficiency of advanced laser technology will play a role in further improvements. In this study, a diode laser was designed as the headlight source, and a headlight optical model was constructed, together with several new optical devices, using novel optical component design concepts. The designed model could effectively diffuse a laser point source into a laser surface source. Then, light patterns of the high and low beams could be generated by controlling the liquid lens. Section2 introduces the concepts of the headlight optical system and the new optical devices. Section3 details the simulation and analysis of the headlight optical system, and evaluates whether the optical system satisfies the regulatory requirements, based on the simulation results for each device and on the overall simulation results. Section4 also compares the laser headlight system with the current specifications to establish whether it had performed better than anticipated.

2. Concept Design of Laser Car Headlight System This study conceives an optical model that directly diffuses light; it does not use a mode of reflected light and it is different from the optical system used in the BMW i8 laser headlights. The purpose of designing this optical system is to minimize the amount of stray light and, therefore, reduce the loss of light energy. The laser headlight optical system comprises a laser source, an optical fiber, a ROD, a gradient-index lens (GRIN lens), a liquid lens and a freeform lens [14–16]. Figure2a shows the transmission of laser beams in the optical device. The concept of the optical system is one of beam diffusion. A laser outputs a high intensity, polarized, Gaussian distribution beam, which must be diffused into a surface light source to be suitable for use in lighting applications. The laser was transmitted from the laser diode to the headlight system via optical fibers that have the ability to transmit light over long distances and can eliminate the phenomenon of partially polarized light through total reflection [8]. The laser beam was transmitted to the ROD via optical fibers, whereby the ROD rearranged the laser beam and eliminated the polarized light pattern, thus facilitating the subsequent diffusion process. The profile of the ROD’s light output was adjusted using a light shaper, which also removed excess stray light. First, the GRIN lens plays a role in the proposed optical design because the shape of the incident light is modified and optimized simultaneously to meet the specifications. Second, liquid lenses with various focal lengths can converge light from the laser source into the laser surface source to optimize the best performance between the high and low beams of the system specification. In other words, after the optimization work, the liquid lens variations in focal length may provide the best performance for either the low- or high-beam mode. According to the illuminance of the low- and high-beam modes, and the specifications of the illumination area, a freeform lens, with the function of adjusting the light pattern and illumination area, was used to satisfy the requirements of the headlight regulations. The light emitted by the laser diode was first modified by the specially designed ROD through fiber optic cables. Throughout the entire light shape (aperture stop), rays were converged via a GRIN lens, focal length and extremely complex freeform optics to deliver the best high-beam/low-beam model performance. The functions and applications of the optical devices used in this study are described below (see Figure2b,c for the device numbers). The above description corresponds to a single light source module; a collection of many light source modules can form an automobile light system. Appl. Sci. 2020, 10, 7331 4 of 14

(a)

(b)

(c)

FigureFigure 2.2. ((aa)) SchematicSchematic ofof thethe laserlaser automotiveautomotive headlightheadlight system.system. ((b)) SimulationSimulation andand rayray trackingtracking ofof RODROD (orange),(orange), GRINGRIN lenslens (red) andand liquid lens (brown) via LightTools underunder thethe educationeducation licenselicense ofof ORA,ORA, U.S.A;U.S.A; ((cc)) simulationsimulation andand rayray trackingtracking of whole system including fiber, fiber, ROD, GRIN lens (red), liquidliquid lenslens (brown),(brown), freeformfreeform forfor lowlow beambeam (green)(green) andand highhigh beambeam (blue).(blue).

TheThe colorcolor temperaturetemperature of thethe automobileautomobile light source is in thethe rangerange ofof 4000–60004000–6000 KK andand itsits penetrationpenetration powerpower andand lightinglighting abilityability areare excellentexcellent [[17].17]. Therefore, this studystudy usedused 55005500 KK whitewhite laserslasers asas thethe lightlight source.source. No. 1 1 in in Figure Figure 22 isis aa diodediode laser,laser, which which exhibits exhibits the the characteristics characteristics of of homochromatic,homochromatic, highhigh intensity, highhigh penetrationpenetration andand high-beamhigh-beam parallelism,parallelism, andand hashas becomebecome a newnew optionoption inin automobileautomobile lighting.lighting. Due to its small sizesize and high energy conversion eefficiency,fficiency, aa diodediode laser can be effectively used as the headlight source [18,19]. However, the disadvantage of the diode 4

Appl. Sci. 2020, 10, 7331 5 of 14 laser can be effectively used as the headlight source [18,19]. However, the disadvantage of the diode laser is that its small size makes it difficult to dissipate heat, and heat dissipation is a crucial technical consideration [20]. No. 2 is the optical fiber that this study uses to transfer laser light. Optical fibers are flexible and can be bent to direct light to a desired location. The laser light source is small in size, which makes it difficult to dissipate heat; therefore, the laser light source is eliminated from the headlight system and an optical fiber is used to direct the light to the system. A suitable laser source heat dissipation system can, therefore, be effectively established, allowing for the maintenance of the laser light source. This study uses Ø 1 mm single-mode fiber as the optical transmission component. No. 3 is a ROD. A ROD is an all-around surface-polished optical device, with an appearance similar to that of a cylindrical lens. It comprises a highly reflective mirror, and its function is to integrate a light source and produce a uniform light beam with a specific shape created by a ROD [21]. Its purpose is to focus laser then after multiple reflection, finally form a homogenized laser light distribution. Therefore, the laser beam will become more uniform in square shape after entering a light shaper ROD, which is commonly seen in pico-laser projectors. No. 4 is a light shaper. Its main function is to adjust the light type. To produce a clear cut-off line that satisfies the regulations, the exit light should be a square type to produce a cut-off line effect. No. 5 is a GRIN lens. The GRIN lens is a cylindrical lens, which has different refractive indices when viewed along the lens axis, and can be used to focus or diffuse light beams. The use of GRIN lens can reduce the number of lenses required to achieve the effect of light diffusion [22]. No. 6 is a liquid lens. Liquid lenses represent a new technology in optical devices, in which the shape of interface between the two liquids in the lens is changed by controlling the electricity. Therefore, the direction of light projection can be controlled, and the focus can be varied. The use of liquid lenses made from certain special liquids, such as disulfide, enables precise control of the focal length, where the liquid must eliminate the stray light to enable the lens to produce accurate and reliable light. In this study, a liquid lens was used as a change switch, which was capable of switching between the high and low beams by adjusting the liquid lens curvature and optical axis. This pioneering study can effectively satisfy the simplified requirements of the headlight components. No. 7 is a freeform lens. The laser beam processed using the six optical devices described above was diffused in this study. However, the output light pattern failed to satisfy regulatory requirements. Therefore, we adjusted the output light pattern using a freeform lens, which has an aspheric surface with asymmetrical and multi-symmetric axes, and was designed using Equation (1). By controlling the curvature parameters of the X- and Y-axes of the lens, a lens with an asymmetrical pattern can be manufactured, and the projected light pattern can be changed. The shape of the freeform lens is depicted in Figure3. 2 X66 cr m n z = p + Cj x y (1) 1 + 1 (1 + k)c2r2 · − j=2 (m+n)2+m+3n where j = 2 + 1, z denotes the depth from the surface to the mirror, c denotes the central curvature of the mirror, r denotes the perpendicular distance from any point on the mirror to the optical axis, k denotes the conic constant (quadratic curve), Cj denotes the monomial coefficient and m and n denote the powers of x and y terms in the polynomial series. Figures4 and5 denote the European regulations associated with the high- and low-beam distribution range and intensity. The test configurations of the high and low beams are the same. When testing, the light source is 25 m away from the detection screen. The test environment illumination is 0 lux. For the high-beam test, the maximum illumination area needs to be located at the intersection of the h–h line and v–v line, as shown in Figure4. Other locations and specifications are shown in Table1[23]. Appl. Sci. 2020, 10, 7331 6 of 14

FigureFigure 3. Three-dimensional 3. Three-dimensional view view of of the the freeform freeform lens. lens. Plea Pleasese take note thatthat curvaturecurvature of of X-axis X-axis and and Figure 3. Three-dimensional view of the freeform lens. Please take note that curvature of X-axis and Y axisY axis on onthis this surface surface is different is different so sothat that their their optical optical power power and and ray-bending ray-bending varies. varies. This This freeform freeform is Y axis on this surface is different so that their optical power and ray-bending varies. This freeform is simplyis simply a sample, a sample, not the not freeform the freeform employed employed in propos in proposeded optical optical design design in this inpaper. this paper.The 3D Thedrawing 3D simply a sample, not the freeform employed in proposed optical design in this paper. The 3D drawing of freeformdrawing ofof freeform optical ofdesign optical proposed design proposed in this inpa thisper paperis subject is subject to limited to limited drawing drawing capabilities capabilities of of freeform of optical design proposed in this paper is subject to limited drawing capabilities of LightToolsof LightTools for academic. for academic. LightTools for academic.

FigureFigureFigure 4. 4.Location 4.Location Location of of ofeach each each test test test point point point for for a a high high beam.beam. beam.

TableTable 1: ECE 1: ECE R-112 R-112 high-beam high-beam test test specification. specification. Illuminance Test Point Position Illuminance Test Point Position Value, Type B (Right side driver) (Units: m) Value, Type B (Right side driver) (Units: m) (Units: lux) (Units: lux) Point: HV (0,0) ≧ 48 Point: HV (0,0) ≧ 48 2.5 R (0,1.125) ≧ 24 2.5 R (0,1.125) ≧ 24 2.5 L (0,−1.125) ≧24 2.5 L (0,−1.125) ≧24 5 L (0,−2.25) ≧6 ≧ 5 L5 R (0,− (0,2.25)2.25) ≧6 6 5 R (0,2.25) ≧ 6 When the low beam is illuminated, it is necessary for it to measure below 20 lux for Zone I, below 0.7When lux forthe Zone low beam III and is above illuminated, 3 lux for it Zone is necessary IV, as sh forown it into Figuremeasure 5. Zonebelow III’ 20s regionallux for Zone illumination I, below 0.7 luxis limitedfor Zone to III 0.7 and lux above to prevent 3 lux glare.for Zone The IV, lower as sh edgeown of in Zone Figure III 5. is Zone the cut-off III’s regional line position, illumination which is limitedmust beto controlled0.7 lux to toprevent prevent glare. glare. The The lower position edge and of specification Zone III is ofthe the cut-off low-beam line detectionposition, pointswhich mustare be shown controlled in Figure to prevent 5. The glare. cut-off The line position of the left-hand and specification car is divided of the into low-beam the v–v line,detection and the points left are shownside is onin Figurethe h–h 5.line. The The cut-off right lineside ofextends the left-hand from the car intersection is divided of intothe h–h the andv–v v–v line, lines and at the 15° left to FigureFigure 5. 5. LocationLocation of of each each test test point point for a lowlow beambeam under under European European law. law. side theis on upper the h–hright line. side. The Detailed right sidespecifications extends from are shown the intersection in Table 2. of the h–h and v–v lines at 15° to the upper right side. Detailed specifications are shown in Table 2. Table 2: ECE R-112 low-beam test specification.

Illuminance Test Point Position Value, Type B (Right Side Driver) (Units: m) (Units: lux) Point: 50 L (−1.5,0.25)6 ≦0.4 75 R (0.5,−0.25) ≧3 75 L (1.5,6 −0.25) ≦12 50 L (−1.5,−0.375) ≦15 50 R (0.75,−0.375) ≧12 50 (0,0.375) ≧6 25 L (−3.96,−0.75) ≧2 25 R (3.96,−0.75) ≧2 Zone I ≦ 20 Zone III ≦ 0.7 Zone IV ≧3 Point 1+2+3 ≧0.3 lux 0.7 ≧ 7 ≧ 0.1 lux 4+5+6 ≧0.6 lux 0.7 ≧ 8 ≧ 0.2 lux

3. Simulation and Analysis of the Headlight Optical System Due to the complexity of the optical system, the output light pattern from the laser source to the freeform lens requires a large number of calculations. Therefore, geometric optical theory was used to design the preliminary light propagation framework. However, the details must be verified using a finite element method. This study used LightTools® optical simulation software to perform the necessary simulations and analysis to verify the accuracy of the optical model results. First, a three- dimensional (3D) digital model of each optical device was developed, with various parameters being assigned to the digital model, according to its optical characteristics. The optical devices were further combined into a 3D digital system based on the optical architecture that was designed in this experiment. There are two lighting modules available in this proposed system: a left and right headlight set. Each light source module was a combination of eight 1 W white diode lasers in a 4 × 2 array. There was a total of 16 laser sources; therefore, the total power of the laser source was 16 W. All laser sources were numbered (see Figure 6). 7

Appl. Sci. 2020, 10, 7331 7 of 14

Table 1. ECE R-112 high-beam test specification.

Illuminance Test Point Position Value, Type B (Right Side Driver) (Units: m) (Units: lux) Point: HV (0,0) =48 2.5 R (0,1.125) =24 2.5 L (0, 1.125) =24 − 5 L (0, 2.25) =6 − 5 R (0,2.25) =6

When the low beam is illuminated, it is necessary for it to measure below 20 lux for Zone I, below 0.7 lux for Zone III and above 3 lux for Zone IV, as shown in Figure5. Zone III’s regional illumination is limited to 0.7 lux to prevent glare. The lower edge of Zone III is the cut-off line position, which must be controlled to prevent glare. The position and specification of the low-beam detection points are shown in Figure5. The cut-o ff line of the left-hand car is divided into the v–v line, and the left side is on the h–h line. The right side extends from the intersection of the h–h and v–v lines at 15◦ to the upper right side. Detailed specifications are shown in Table2.

Table 2. ECE R-112 low-beam test specification.

Illuminance Test Point Position Value, Type B (Right Side Driver) (Units: m) (Units: lux) Point: 50 L ( 1.5,0.25) 50.4 − 75 R (0.5, 0.25) =3 − 75 L (1.5, 0.25) 512 − 50 L ( 1.5, 0.375) 515 − − 50 R (0.75, 0.375) =12 − 50 (0,0.375) =6 25 L ( 3.96, 0.75) =2 − − 25 R (3.96, 0.75) =2 − Zone I 520 Zone III 50.7 Zone IV =3 Point 1 + 2 + 3 = 0.3 lux 0.7 = 7 = 0.1 lux 4 + 5 + 6 = 0.6 lux 0.7 = 8 = 0.2 lux

3. Simulation and Analysis of the Headlight Optical System Due to the complexity of the optical system, the output light pattern from the laser source to the freeform lens requires a large number of calculations. Therefore, geometric optical theory was used to design the preliminary light propagation framework. However, the details must be verified using a finite element method. This study used LightTools® optical simulation software to perform the necessary simulations and analysis to verify the accuracy of the optical model results. First, a three-dimensional (3D) digital model of each optical device was developed, with various parameters being assigned to the digital model, according to its optical characteristics. The optical devices were further combined into a 3D digital system based on the optical architecture that was designed in this experiment. There are two lighting modules available in this proposed system: a left and right headlight set. Each light source module was a combination of eight 1 W white diode lasers in a 4 2 × array. There was a total of 16 laser sources; therefore, the total power of the laser source was 16 W. All laser sources were numbered (see Figure6). Appl. Sci. 2020, 10, 7331 8 of 14

FigureFigure 6. Assembly 6. Assembly schematic schematic of the of 3D the digital 3D digital optical optical headlight headlight system, system, with with laser laser source source numbering. numbering. 2, 3,2, 4, 3, 5, 4, 6, 5, 6,7, 7,8, 8,e, e,f, f,total total nine nine diodes diodes ar aree designed andand optimizedoptimized for for low low beam; beam; compared compared to lowto low beam, beam,eight eight (a, b,(a, c, b, d, e,c, f,d, g, e, h) f, is g, designed h) is designed and optimized and optimized for high beam.for high More beam. laser More diodes laser is employeddiodes is for employedlow beam for islow due beam to light is due path to of light low path beam of su lowffer beam much suffer more much vignetting more e vignettingffect than high effect beam than ones high and beamlow ones beam’s and projectionlow beam’s area projection is slightly area larger is slightly than high larger beam’s. than Inhigh addition, beam’s. this In addition, set of laser this diode set of array laserwas diode designed array forwas further designed upgrade for further of smart upgrade active of laser smart headlight active laser system headlight with digital system signal with processing digital signalso thatprocessing there must so that be morethere roommust forbe mo widere room projection for wide of low projection beam. of low beam.

TheThe ray ray trace trace analysis analysis of the of the optical optical system system is shown is shown in Figure in Figure 7A.7A. Each Each chart chart below below shows shows the the differentdifferent light light source source distribution distribution on on each imageimage planeplane after after simulation simulation to achieveto achieve light light efficiency efficiency [24,25 ]. [24,25].The laserThe laser is a is Gaussian a Gaussian light light source, source, and and the the lightlight intensity at at the the middle middle point point is the is the strongest. strongest. TheThe Gaussian Gaussian distribution distribution of light of light intensity intensity in X in and X and Y directions Y directions is different is different (Figure (Figure 7B).7B). Laser Laser transmissiontransmission through through the the optical fiber significantl significantlyy improved improved laser laser polarization. polarization. Figure Figure 7C7 showsC shows the the lightlight pattern pattern simulation simulation of the of the fiber fiber exit exit side, side, which which indicates indicates the theeffectiveness effectiveness of improving of improving polarized polarized light.light. The The intensity intensity of the of laser the laser light lightoutput output from fromthe optical the optical fiber was fiber still was concentrated; still concentrated; however, however, the polarizationthe polarization of the oflaser the beam laser beamwas eliminated was eliminated after afterit entered it entered the ROD, the ROD, which which performed performed an optical an optical adjustment.adjustment. In addition, In addition, the thelight light intensity intensity was was unif uniformlyormly diffused. diffused. Figure Figure 7D7 showsD shows the the light light pattern pattern on onthe the ROD ROD exit exit side, indicatingindicating the the results results of theof homogenizationthe homogenization of laser of dilaserffusion diffusion and the eliminationand the eliminationof polarization. of polarization. After the After periphery the periphery of the light of the beam light output beam fromoutput the from ROD the was ROD adjusted was adjusted using the usinglight the shaper, light shaper, eliminating eliminating stray light stray and light shaping and shaping the beam, the beam, the light the beamlight beam entered entered the GRIN the GRIN lens for lensdi forffusion. diffusion. Show Show in Figure in Figure7E. 7E. Firstly, in this experiment, we find the right laser diode and its LightTools file from LightTools database. Secondly, we find some fiber files from LightTools database. They are files with all fixed data so these are not able to be optimized. If laser diode is determined like in Figure7B, fiber will be selected in order to achieve best performance according to light efficiency, basically like in Figure7C. Generally speaking, fiber and laser diode both are selected before optimization procedure in order to match of optical components each other’s, which is nothing to do with later optimization. The parameters of optimization with regard to this system are following:

(1) ROD variables: total length; width and height in order to reach best light efficiency in Figure7D. (2) GRIN optics variables: GRIN index from polar coordinates on optical axis and its thickness. In order to reach uniformity in Figure7E. (3) Freeform optics variables: Radius and off-axis X and Y coefficients in order to achieve best results (A) inclusive of light efficiency, uniformity and the most important, match the safety requirement ECE R-112 in Figures8 and9.

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Figure 6. Assembly schematic of the 3D digital optical headlight system, with laser source numbering. 2, 3, 4, 5, 6, 7, 8, e, f, total nine diodes are designed and optimized for low beam; compared to low beam, eight (a, b, c, d, e, f, g, h) is designed and optimized for high beam. More laser diodes is employed for low beam is due to light path of low beam suffer much more vignetting effect than high beam ones and low beam’s projection area is slightly larger than high beam’s. In addition, this set of laser diode array was designed for further upgrade of smart active laser headlight system with digital signal processing so that there must be more room for wide projection of low beam.

The ray trace analysis of the optical system is shown in Figure 7A. Each chart below shows the different light source distribution on each image plane after simulation to achieve light efficiency [24,25]. The laser is a Gaussian light source, and the light intensity at the middle point is the strongest. The Gaussian distribution of light intensity in X and Y directions is different (Figure 7B). Laser transmission through the optical fiber significantly improved laser polarization. Figure 7C shows the light pattern simulation of the fiber exit side, which indicates the effectiveness of improving polarized light. The intensity of the laser light output from the optical fiber was still concentrated; however, the polarization of the laser beam was eliminated after it entered the ROD, which performed an optical adjustment. In addition, the light intensity was uniformly diffused. Figure 7D shows the light pattern on the ROD exit side, indicating the results of the homogenization of laser diffusion and the elimination of polarization. After the periphery of the light beam output from the ROD was adjusted Appl.using Sci. 2020the ,light10, 7331 shaper, eliminating stray light and shaping the beam, the light beam entered the GRIN9 of 14 lens for diffusion. Show in Figure 7E.

(A)

8

(B) (C)

(D) (E)

FigureFigure 7. 7.(A ()A Ray) Ray trace trace analysis analysis ofof thethe opticaloptical system. Each Each chart chart indicates indicates the the different different light light source source distributiondistribution at at various various image image planes, planes, (B )( TheB) The output output light light pattern pattern of laser of laser beam, beam (C), the(C) selectedthe selected output lightoutput pattern light from pattern various from opticalvarious fibers. optical If fibers. laser diodeIf laser is diode selected, is selected, fiber must fiber match must match this laser this diode laser to reachdiode maximum to reach maximum light efficiency, light efficiency, (D) The output (D) The light output of ROD, light (ofE) ROD the optimized, (E) the optimized output light output pattern light of GRINpattern lens of (also GRIN check lens Figure(also check2a,b). Figure 2a,b).

Firstly, in this experiment, we find the right laser diode and its LightTools file from LightTools database. Secondly, we find some fiber files from LightTools database. They are files with all fixed data so these are not able to be optimized. If laser diode is determined like in Figure 7B, fiber will be selected in order to achieve best performance according to light efficiency, basically like in Figure 7C. Generally speaking, fiber and laser diode both are selected before optimization procedure in order to match of optical components each other’s, which is nothing to do with later optimization. The parameters of optimization with regard to this system are following:

(1) ROD variables: total length; width and height in order to reach best light efficiency in Figure 7D. (2) GRIN optics variables: GRIN index from polar coordinates on optical axis and its thickness. In order to reach uniformity in Figure 7E. (3) Freeform optics variables: Radius and off-axis X and Y coefficients in order to achieve best results inclusive of light efficiency, uniformity and the most important, match the safety requirement ECE R-112 in Figures 8 and 9.

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Appl. Sci. 2020, 10, 7331 10 of 14

(a) (b)

FigureFigure 8. 8. ((aa,,bb)) Simulation Simulation results results of of the the high-beam high-beam pattern pattern and and experimental experimental results. results. X X and and Y Y axis axis is is thethe meter. meter. Colors indicateindicate thethe lightlight intensity intensity via via Lux. Lux. This This center center of of lighting lighting delivers delivers most most powerful powerful lux. lux.Relatively Relatively high high value value of illuminance of illuminance might might be eliminated be eliminated by control by control of laser of laser output output power. power.

(a) (b)

FigureFigure 9. 9. ((aa,b,b)) Simulation Simulation results results of of low-beam low-beam pattern pattern and and experimental experimental results. results.

Next,Next, the didiffusedffused laserlaser beambeam entered entered the the liquid liquid lens lens to to enable enable switching switching between between the the high high and and low lowbeams, beams, wherein wherein the liquidthe liquid lens lens curvature curvature and opticaland optical axis wereaxis were changed. changed. The freeform The freeform lens was lens made was madeof Kopp of SharpKopp CutRedSharp CutRed 2424 glass. 2424 Dueglass. to Due effect to of effect complicated of complicated annealing annealing process duringprocess freeform during freeformmanufacture, manufacture, only very only few very optical few glass optical in theglass current in the marketcurrent ismarket feasible is forfeasible this project.for this project. CutRed CutRed2424 glass 2424 is glass always is always strongly strongly recommended recommended for this for kind this of kind project. of project. It has aItcurved has a curved surface surface shape, shape,which waswhich constructed was constructed by Equation by Equation (1). Adjusting (1). Adju thesting parameters the parameters to values thatto values satisfy that the regulationssatisfy the regulationsis time consuming is time consuming and labor intensive. and labor Therefore, intensive. we Therefore, employed we an employed automated an program, automated in whichprogram, the inMonte which Carlo the Monte method Carlo was method used as was the optimizationused as the op enginetimization to iteratively engine to optimize iteratively surface optimize parameters. surface parameters.Figure 10 shows Figure the 10 automated shows the optimization automated optimizati process. First,on process. the initial First, values the initial of the parametersvalues of the in parametersEquation (1) in were Equation set, and (1) the were range set, ofand parameter the rang valuee of parameter intervals wasvalue given. intervals Then, was the given. surface Then, position the surfaceand illuminance position valuesand illuminance detected according values todetected the regulations according were to set the as theregulations optimization were target set as values. the optimizationThe Monte Carlo target method values. employedThe Monte by Carlo LightTools method for employed the basic by theory LightTools of ray for tracking the basic was theory used of to rayconduct tracking an iterative was used operation to conduct using an an automaticiterative operation randomsampling using an technique automatic to random determine sampling optimal techniquesurface parameters, to determine which optimal could su berface obtained parameters, with a which certain could number be ofobtained iterations with [25 a]. certain number of iterationsIn this experiment,[25]. the high-beam system had eight laser sources (a, b, c, d, e, f, g, h) turned on, withIn a totalthis experiment, power of 6.4 the W high-beam for high beam system after had reduction eight laser of systemsources light (a, b, loss c, d, due e, f, to g, noth) turned perfection on, withof light a total effi ciency.power of The 6.4 experimental W for high beam data after of the redu high-beamction of system detection light points loss due are shownto not perfection in Figure8 ofb. lightThe maximumefficiency. The illumination experimental was data 55.2 of lux, the ashigh shown-beam in detection Figure8 pointsa, which are satisfiesshown in the Figure high-beam 8b. The maximumregulation illumina requirements.tion was 55.2 lux, as shown in Figure 8a, which satisfies the high-beam regulation requirements.Liquid optics does not intend to join the full optimization described in Figure8 only because it is a switchLiquid for optics high beam does/ lownot beam.intend Whyto join not the join full full optimization optimization described in order toin achieveFigure 8 best only performance because it isfor a highswitch beam for/low high beam? beam/low It is only beam. because Why liquidnot join optics full is optimization intended to bein withorder aspherical to achieve surface best performancewhich is subject for tohigh gravity. beam/low We are beam? not able It to is find only the because right coe liquidfficient optics to have is itintended optimized. to be with aspherical surface which is subject to gravity. We are not able to find the right coefficient to have it optimized.

10

Appl. Sci. 2020, 10, 7331 11 of 14

Figure 10. FigureOptimization 10. Optimization process process via LightToolsvia LightTools based based on Monte Monte Carlo Carlo merit merit function. function.

The low-beamThe low-beam lights turned lights turned on nine on nine laser laser sources sources (2, (2, 3, 3,4, 5, 4, 6, 5, 7,6, 8, e, 7, f), 8, with e, f), a total with power a total of 4.5 power of W. Zones I, III and IV had an average illuminance of 20, 0.7 and 17.7 lux, respectively. The light 4.5 W. Zonesdistribution I, III and of IV the had low anbeam average is shown illuminance in Figure 9a, ofand 20, detailed 0.7 and experimental 17.7 lux, data respectively. are shown in The light distributionFigure of the 9b. low The detection beam is point shown values in comply Figure with9a, the and specifications, detailed and experimental the cut-off line data is also are within shown in Figure9b. Thethe specified detection range. point Figure values 11a shows comply the illuminati with theon simulation specifications, results, andwhich the indicates cut-o thatff linethe is also within the specifiedcut-off line range.is within Figurethe specified 11a showsrange. Figure the illumination11b shows a bird’s simulation eye view of results, the low whichbeam. To indicates prevent glare, the illumination of Zone III was limited to 0.7 lux, and the height of the cut-off line that the cut-ocouldff line not exceed is within the regulations. the specified The above range. two cond Figureitions 11of bthis shows experiment a bird’s are within eye prescribed view of the low beam. To preventvalues, which glare, can the effectively illumination prevent ofglare Zone and IIIthe was headlight limited system to 0.7used lux, modular and thecontrol. height In of the cut-off line couldaddition not to exceedreducing the the regulations.laser source power, The above the cut-off two line conditions height could of this be adjusted experiment to more are within effectively prevent glare. prescribed values, which can effectively prevent glare and the headlight system used modular control. During the optimization process, as soon as the proposed optical design was set up ready in In additionLightTools, to reducing the simple the laser ray tracking source method power, will the start cut-o first, ffthenline following height LightTool could be’s multiple adjusted ray- to more effectively preventtracking optimization. glare. If the result is not good, cease then repeat the setting work; restart again for next set up and optimization. Simulation result will be determined after optimization. The experimental simulation proves that, for the high-beam mode, the laser beam passing through the liquid lens was the output of the freeform lens, which fine-tuned the light beam. Consequently, the final high-beam patterns were obtained on the detection surface, as shown in Figure 9a, indicating that the high-beam pattern satisfied the regulatory requirements. Figure 9b shows the corresponding experimental values. The value of each detection point is greater than the regulatory value, suggesting that the optical system of the high-beam mode was in line with expectations. The experimental simulation of the low-beam mode is displayed in Figure 10, suggesting that the output light pattern also satisfied regulatory requirements. The experimental values, shown in Figure 10, were also superior to the regulatory standards, thereby experimentally verifying that the optical system devised in this study is feasible, as shown in Figure11. (a)

11

(b)

FigureFigure 11. ( a11.) Illumination (a) Illumination simulation simulation results results from from LightTools, LightTools, (b ()b simulation) simulation results results ofof bird’sbird’s eyeeye view patternview from pattern LightTools. from LightTools.

4. Conclusions Diode lasers demonstrate outstanding performance in automotive lighting applications because of their excellent characteristics. The optical system application that was devised as an automotive lighting solution was verified in this study using a simulation. This proposed research method is simply a simulation of the system, not actually implemented for reasons related to costs of the experimentation. The following conclusions can be drawn: i. Optical fibers exhibit an excellent performance in terms of laser beam transmission. Along with the capability to transmit light, ROD can adjust the polarization of laser light to achieve precise light pattern control. Using the optical fiber application, the laser sources can be freely configured outside the optical system, which is helpful when dissipating the heat of the laser source. ii. Optical fibers and RODs, which are capable of effectively and accurately adjusting the laser light pattern, can homogenize the laser light through total reflection and eliminate laser light polarization, thereby influencing the light pattern adjustment. iii. The functions of a liquid lens can be used to easily satisfy the demand for switching between the high beam and low beam. If the light pattern is controlled by the array module, it can meet the functional requirements related to the lighting of a specific area. iv. The use of freeform lenses to adjust the light pattern provides the perfect balance between the aesthetic requirements of a vehicle and its functional lighting requirements. In addition, the beam pattern of the diffused laser can satisfy the regulatory requirements for aesthetic and practical purposes after performing adjustments with a freeform lens. The headlight optical system designed in this study satisfied the regulatory requirements, both in terms of uniformity and illumination. The maximum illuminance is 56.6 lux in the high-beam mode, which is 18% higher than the standard value (48 lux). In the low-beam mode, the illuminance meets regulatory standards. Compared with a 24 W LED headlight module, this design saves 33% more energy. This power of this design is 16 W.

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Appl. Sci. 2020, 10, 7331 12 of 14

During the optimization process, as soon as the proposed optical design was set up ready in LightTools, the simple ray tracking method will start first, then following LightTool’s multiple ray-tracking optimization. If the result is not good, cease then repeat the setting work; restart again for next set up and optimization. Simulation result will be determined after optimization. The experimental simulation proves that, for the high-beam mode, the laser beam passing through the liquid lens was the output of the freeform lens, which fine-tuned the light beam. Consequently, the final high-beam patterns were obtained on the detection surface, as shown in Figure9a, indicating that the high-beam pattern satisfied the regulatory requirements. Figure9b shows the corresponding experimental values. The value of each detection point is greater than the regulatory value, suggesting that the optical system of the high-beam mode was in line with expectations. The experimental simulation of the low-beam mode is displayed in Figure 10, suggesting that the output light pattern also satisfied regulatory requirements. The experimental values, shown in Figure 10, were also superior to the regulatory standards, thereby experimentally verifying that the optical system devised in this study is feasible, as shown in Figure 11.

4. Conclusions Diode lasers demonstrate outstanding performance in automotive lighting applications because of their excellent characteristics. The optical system application that was devised as an automotive lighting solution was verified in this study using a simulation. This proposed research method is simply a simulation of the system, not actually implemented for reasons related to costs of the experimentation. The following conclusions can be drawn: i. Optical fibers exhibit an excellent performance in terms of laser beam transmission. Along with the capability to transmit light, ROD can adjust the polarization of laser light to achieve precise light pattern control. Using the optical fiber application, the laser sources can be freely configured outside the optical system, which is helpful when dissipating the heat of the laser source. ii. Optical fibers and RODs, which are capable of effectively and accurately adjusting the laser light pattern, can homogenize the laser light through total reflection and eliminate laser light polarization, thereby influencing the light pattern adjustment. iii. The functions of a liquid lens can be used to easily satisfy the demand for switching between the high beam and low beam. If the light pattern is controlled by the array module, it can meet the functional requirements related to the lighting of a specific area. iv. The use of freeform lenses to adjust the light pattern provides the perfect balance between the aesthetic requirements of a vehicle and its functional lighting requirements. In addition, the beam pattern of the diffused laser can satisfy the regulatory requirements for aesthetic and practical purposes after performing adjustments with a freeform lens.

The headlight optical system designed in this study satisfied the regulatory requirements, both in terms of uniformity and illumination. The maximum illuminance is 56.6 lux in the high-beam mode, which is 18% higher than the standard value (48 lux). In the low-beam mode, the illuminance meets regulatory standards. Compared with a 24 W LED headlight module, this design saves 33% more energy. This power of this design is 16 W. If combined with opto-mechatronic control and integration, this system could increase or decrease the illumination in a specific area, thus providing a feasible solution for headlight optical systems.

Author Contributions: Y.-C.F. and C.-H.C. for concept of optical design. Y.-L.S. and H.-Y.L. for Light Tool optical software work. Y.-F.T. for optimization work. C.-C.W., and S.-H.C. for road safety investigation. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest. Appl. Sci. 2020, 10, 7331 13 of 14

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