Design of a Bicycle Head Lamp Using an Atypical White Light-Emitting Diode with Separate Dies

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Article Design of a Bicycle Head Lamp Using an Atypical White Light-Emitting Diode with Separate Dies

Hsin-Jung Lin 1, Ching-Cherng Sun 1,2, Chi-Shou Wu 1, Xuan-Hao Lee 1,*, Tsung-Hsun Yang 1 , Shih-Kang Lin 1, Yi-Jou Lin 1 and Yeh-Wei Yu 1

1 Department of Optics and Photonics, National Central University, Chung-Li 320, Taiwan; [email protected] (H.-J.L.); [email protected] (C.-C.S.); [email protected] (C.-S.W.); [email protected] (T.-H.Y.); [email protected] (S.-K.L.); [email protected] (Y.-J.L.); [email protected] (Y.-W.Y.) 2 Department of Electrophysics, National Chiao Tung University, Hsin-Chu 30010, Taiwan * Correspondence: [email protected]; Tel.: +886-3-422-7151 (ext. 65277)

Received: 13 November 2019; Accepted: 3 December 2019; Published: 9 December 2019

Abstract: To the best of our knowledge, this is the first demonstration of a design for a bicycle head lamp with a high-contrast cutoff line using an atypical white light-emitting diode (LED) with two separate dies. The precise optical model was created by setting the weighting factor on the emitting surface. The downward reflector was designed and fabricated to produce a high-contrast cutoff line in both short- and long-axis orientations, but a yellowish outer pattern was observed. A modified two-color optical model was created to describe the yellowish patterns in both orientations and explain the yellowish effect. Such an effect was caused by the larger coverage area of the phosphor than that by the blue dies. To reduce the yellowish effect near the cutoff line, a specific phosphor area was blocked in the experiment. The yellowish effect was greatly reduced, and the contrast across the cutoff line was enhanced. The presented technology is useful for designing a high-contrast light pattern with such an atypical white LED.

Keywords: white LED; k-mark; bicycle head lamp; cutoﬀ line

1. Introduction Vehicle head lamps are crucial for traffic lighting to allow high visibility of the driver, pedestrians, and other people on the road. To prevent glare, a head lamp must have a clear borderline [1,2]. Such a borderline separates the dark and bright zones with minimum contrast, to provide sufficient illumination with less glare. These high-contrast cutoff lines are applicable to cars [3–9], motorcycles [10], and even bicycles [11–14]. In general, most designs apply reflection [3,5,8], projection [6,7,10], and refraction [4,9] to produce a high-contrast cutoff line. By using a combination of various optical principles to design a freeform lens, Chen et al. obtained a highly efficient head lamp to project a high-contrast cutoff line [4]. Wang et al. adopted reflector-lens sets to project a light pattern for a low-beam head lamp [7]. Wu et al. used four reflectors and the parameter-optimization method to obtain a low-beam head lamp with an optical efficiency of 79% [8]. The contrast requirement for the head lamp cutoff line for bicycles is not as strict as that for automotive vehicles, but the head lamp must be lightweight, compact, and inexpensive. Bicycle head lamps have been studied extensively, and most optical designs use a downward reflector or a total internal reflection (TIR) lens [11–15]. The light source is a solid-state light source (i.e., a light-emitting diode (LED)) because of its advantages, such as compact size, vivid color, fast response, environmental benefits, and long life [16–23]. A so-called phosphor-converted white LED (pcW-LED) produces a white color by using a phosphor to cover a blue die [24]. In a pcW-LED, the phosphor layer or volume is a heavy scattering medium with Mie scattering. As blue light passes

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Crystals 2018, 8, x FOR PEER REVIEW 2 of 11 through the scattering medium, part of the blue light transforms into yellow light to produce white the blue light transforms into yellow light to produce white light, while mixing the leakage of blue light, while mixing the leakage of blue light [25]. Therefore, the ratio of the yellow and blue light is light [25]. Therefore, the ratio of the yellow and blue light is critical. A pcW-LED used as a light source critical. A pcW-LED used as a light source in a vehicle head lamp should have good heat dissipation, in a vehicle head lamp should have good heat dissipation, high color accuracy, and high flux density high color accuracy, and high flux density for illumination [26–28]. In general, the pcW-LED operates for illumination [26–28]. In general, the pcW-LED operates at 3 W or more, with as small a light source at 3 W or more, with as small a light source as possible to contain the phosphor in a conformal coating, as possible to contain the phosphor in a conformal coating, meaning that the phosphor area is almost meaning that the phosphor area is almost equal to the blue die area [13–15]. This causes higher exitance equal to the blue die area [13–15]. This causes higher exitance for sufficient illuminance, and smaller for sufficient illuminance, and smaller etendue for a high-contrast cutoff line. The light source is etendue for a high-contrast cutoff line. The light source is therefore relatively expensive. By contrast, therefore relatively expensive. By contrast, if an LED dies outside of expected specifications, it can if an LED dies outside of expected specifications, it can be used for pcW-LEDs in low-cost lighting be used for pcW-LEDs in low-cost lighting applications, such as a light bulb. Such a pcW-LED is applications, such as a light bulb. Such a pcW-LED is not used as a light source in head lamps, and not used as a light source in head lamps, and the phosphor area can be larger than that of the blue the phosphor area can be larger than that of the blue dies. This paper presents a design for a K-mark dies. This paper presents a design for a K-mark bicycle head lamp by using a low-cost and atypical bicycle head lamp by using a low-cost and atypical pcW-LED. In the measurement of the fabricated pcW-LED. In the measurement of the fabricated design, a high-contrast cutoff line was observed but design, a high-contrast cutoff line was observed but with a yellowish blur pattern. An advanced with a yellowish blur pattern. An advanced model was used to describe and solve the color problem. model was used to describe and solve the color problem. 2. Optical Modeling 2. Optical Modeling A commercial low-cost pcW-LED with separate dies was used as our light source. The pcW-LED was selectedA commercial on the basislow-cost of its pcW-LED price and with other separate properties, dies including was used total as our flux, light exitance, source. and The dimensions. pcW-LED Thewas totalselected flux ison a keythe factorbasis becauseof its price the illuminationand other properties, distance and including visibility total of the flux, illuminated exitance, target and shoulddimensions. be clear The to total a cyclist. flux Figureis a key1 illustrates factor because the temporal the illumination evolution for distance the luminous and visibility flux, in whichof the theilluminated luminous target flux reachedshould be 150 clear lm atto steady-statea cyclist. Figure operation 1 illustrates under the a current temporal injection evolution of 400 for mA.the Thisluminous light sourceflux, in iswhich more the complicated luminous flux than reached others used150 lm for at bicycles, steady-state because operation two separate under adies current are bondedinjectionunder of 400 amA. circular This light phosphor source area. is more As complicated illustrated in than Figure others2, we used used for abicycles, camera because to capture two theseparate image dies of theare pcW-LEDbonded under when a circular turned on.phosphor To prevent area. overexposure As illustrated ofin theFigure camera, 2, we twoused polarizers a camera wereto capture used the to adjust image the of the incoming pcW-LED light when brightness turned toon. an To appropriate prevent overexposure exposure level of the for camera, the camera. two Thepolarizers images were are presentedused to adjust in Figure the incoming3, wherein light the brightness axis along to the an two appropriate separate diesexposure is called level the for long the axis,camera. and The the images other axis are presented is called the in shortFigure axis. 3, wherei Figuren 3thec illustrates axis along the the lighttwo separate distribution dies contour,is called whichthe long was axis, used and to the define other the axis weighting is called factor the short of each axis. emitting Figure point3c illustrates on the top thephosphor light distribution surface, wherecontour, the which pcW-LED was wasused regarded to define as the a Lambertianweighting factor light sourceof each [ 29emitting]. point on the top phosphor surface, where the pcW-LED was regarded as a Lambertian light source [29].

Figure 1. Temporal-dependent luminous fluxflux of the phosphor-converted whitewhite LEDLED (pcW-LED).(pcW-LED). To verify the validity of the pcW-LED optical model, we compared the simulated light patterns with those measured at different distances in the midfield regime [29–31], where the farthest distance was 10 times the largest lateral size of the emitting area (30 mm in this case). Figure4 presents the comparison between the simulated and measured patterns at three distances for two pcW-LED

Crystals 2019, 9, 659 3 of 10 Crystals 2018, 8, x FOR PEER REVIEW 3 of 11 orientations. The normalized cross correlation (NCC) values were consistently >99.5%, which means that the lightCrystals source 2018, 8 model, x FOR PEER is suREVIEWﬃciently accurate for precise optical design [32]. 3 of 11

Figure 2. Images of the pcW-LED being captured using a camera. Figure 2.FigureImages 2. Images of the of the pcW-LED pcW-LED beingbeing captured captured using using a camera. a camera.

Figure 3. (aFigure) Color 3. ( anda) Color (b )and black-and-white (b) black-and-white images images of of the pcW-LED pcW-LED captured captured usingusing a camera. a camera. (c) Gray (c) Gray Crystals 2018Figure, 8, x FOR 3. ( aPEER) Color REVIEW and (b) black-and-white images of the pcW-LED captured using a camera. (c) Gray 4 of 11 level distributionlevel distribution of (b). L: of long(b). L: axis,long axis, S: short S: short axis. axis. level distribution of (b). L: long axis, S: short axis. To verify the validity of the pcW-LED optical model, we compared the simulated light patterns Towith verify those themeasured validity at differentof the pcW-LED distances inoptical the mi model,dfield regime we compared [29–31], where the simulatedthe farthest lightdistance patterns with thosewas 10 measured times the largestat different lateral distances size of the in emitting the mi dfieldarea (30 regime mm in [29–31], this case). where Figure the 4 presentsfarthest thedistance was 10comparison times the between largest thelateral simulat sizeed of and the measuredemitting areapatterns (30 atmm three in thisdistances case). for Figure two pcW-LED4 presents the orientations. The normalized cross correlation (NCC) values were consistently >99.5%, which means comparison between the simulated and measured patterns at three distances for two pcW-LED that the light source model is sufficiently accurate for precise optical design [32]. orientations. The normalized cross correlation (NCC) values were consistently >99.5%, which means that the light source model is sufficiently accurate for precise optical design [32].

Figure 4. Comparisons between the simulations and measurements at distances of 15, 20, and 30 mm Figure 4. Comparisons between the simulations and measurements at distances of 15, 20, and 30 mm when the pcW-LED was measured along the (a–c) short and (d–f) long axes. when the pcW-LED was measured along the (a–c) short and (d–f) long axes.

3. Optical Design The head lamp was designed to pass the K-mark regulation, which is illustrated in Figure 5, wherein several checkpoints were in the illumination plane 10 m from the head lamp. The first checkpoint is point A, where the illuminance should be ≥20 lux and 1/1.2 of the maximum illuminance of the whole field on the plane. Point A should be located at the same horizontal level as the point of the maximum illuminance. The second checkpoint is located in the dark zone above the cutoff line, where the maximum illuminance should be ≤2 lux. In general, the challenge in optical design is ensuring that the illuminance is ≤2 lux in the dark area. In fact, this is related to the vertical location of the maximum illuminance point. If this location is far lower than the cutoff line, the dark zone would be too close to the cutoff line, and too bright to pass the regulation.

Figure 5. K-mark regulation checkpoints.

An optical design using such a pcW-LED is unusual. The first reason is that the unit has two separate dies. Secondly, the color distribution is not homogenous across the emitting surface. In the design procedure, the optics (i.e., a reflector) were designed on the basis of the white light model. The

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Crystals 2019, 9, 659 4 of 10 Figure 4. Comparisons between the simulations and measurements at distances of 15, 20, and 30 mm when the pcW-LED was measured along the (a–c) short and (d–f) long axes. 3. Optical Design 3. Optical Design The head lamp was designed to pass the K-mark regulation, which is illustrated in Figure5, whereinThe severalhead lamp checkpoints was designed were into thepass illumination the K-mark planeregulation, 10 m which from theis illustrated head lamp. in Figure The first 5, checkpointwherein several is point checkpoints A, where the were illuminance in the illumina should betion 20plane lux and10 m 1/ 1.2from of thethe maximum head lamp. illuminance The first ≥ ofcheckpoint the whole is fieldpoint on A, thewher plane.e the illuminance Point A should should be locatedbe ≥20 lux at theand same 1/1.2 horizontalof the maximum level asilluminance the point ofof the whole maximum field illuminance.on the plane. ThePoint second A should checkpoint be located is locatedat the same in the horizontal dark zone level above as the the point cuto offf line,the maximum where the illuminance. maximum illuminance The second should checkpoint be 2 is lux. located In general, in the thedark challenge zone above in optical the cutoff design line, is ≤ ensuringwhere the that maximum the illuminance illuminance is 2 luxshould in the be dark ≤2 area.lux. In In general, fact, this the is related challenge to the in vertical optical location design ofis ≤ theensuring maximum that the illuminance illuminance point. is ≤ If2 thislux in location the dark is fararea. lower In fact, than this the is cuto relatedff line, to the the dark vertical zone location would beof toothe closemaximum to the illuminance cutoff line, and point. too If bright this location to pass the is far regulation. lower than the cutoff line, the dark zone would be too close to the cutoff line, and too bright to pass the regulation.

Figure 5. K-mark regulation checkpoints. Figure 5. K-mark regulation checkpoints. An optical design using such a pcW-LED is unusual. The first reason is that the unit has two separateAn optical dies. Secondly, design using the color such distribution a pcW-LED is is not unusua homogenousl. The first across reason the is emitting that the surface. unit has In two the designseparate procedure, dies. Secondly, the optics the color (i.e., distribution a reflector) wereis not designedhomogenous on the across basis the of emitting the white surface. light model. In the Thedesign pcW-LED procedure, was the attached optics to(i.e., the a reflectorreflector) facingwere designed downward. on the The basis reflector of the was white composed light model. of eight The segments, as illustrated in Figure6, and the four bottom segments were used to form the sharp edge of the cutoff line. To form the cutoff line, the light source area should be as small as possible. However, the dimensions of the pcW-LED along the long axis are not equal to those of the short axis. Typically, the short axis of the pcW-LED should be aligned with the vertical direction because the cutoff line spans across the vertical direction. To increase assembly tolerance, the orientation of the pcW-LED was adjusted to align the long axis of the pcW-LED with the vertical direction. Although this was an undesirable case, after the design was successful, we could change the orientation of the pcW-LED to align the short axis with the vertical axis, to increase the contrast of the cutoff line. Therefore, the orientation of the pcW-LED is not a concern in mass production. Figure7 illustrates the simulation results, wherein Figure7a–d display the results of the long- and short-axis orientations, respectively. The simulated illuminance on the checkpoints passed the regulation. However, point A, in the case of the long-axis orientation, was farther below the cutoff line than that of the short-axis orientation. This result was as expected, because the illuminance at a dark checkpoint was higher in the long-axis orientation than that in the short-axis orientation. By contrast, the light pattern in the short-axis orientation was wider than that in the long-axis orientation. Crystals 2018, 8, x FOR PEER REVIEW 5 of 11

pcW-LED was attached to the reflector facing downward. The reflector was composed of eight segments, as illustrated in Figure 6, and the four bottom segments were used to form the sharp edge of the cutoff line. To form the cutoff line, the light source area should be as small as possible. However, the dimensions of the pcW-LED along the long axis are not equal to those of the short axis. Typically, the short axis of the pcW-LED should be aligned with the vertical direction because the cutoff line spans across the vertical direction. To increase assembly tolerance, the orientation of the pcW-LED was adjusted to align the long axis of the pcW-LED with the vertical direction. Although this was an undesirable case, after the design was successful, we could change the orientation of the pcW-LED to align the short axis with the vertical axis, to increase the contrast of the cutoff line. Therefore, the orientation of the pcW-LED is not a concern in mass production. Figure 7 illustrates the simulation results, wherein Figure 7a–d display the results of the long- and short-axis orientations, respectively. The simulated illuminance on the checkpoints passed the regulation. However, point A, in the case of the long-axis orientation, was farther below the cutoff line than that of the short-axis orientation. This result was as expected, because the illuminance at a dark checkpoint wasCrystals higher2019, 9 in, 659 the long-axis orientation than that in the short-axis orientation. By contrast, the 5light of 10 pattern in the short-axis orientation was wider than that in the long-axis orientation.

Crystals 2018, 8, x FORFigure PEER 6.REVIEW(a) Geometry of the designed reﬂector. (b) Reﬂector with housing. 6 of 11 Figure 6. (a) Geometry of the designed reflector. (b) Reflector with housing.

FigureFigure 7. 7. LightLight patterns patterns in in the the case case of of ( (aa)) long- long- and and ( (bb)) short-axis short-axis alignments alignments with with the the vertical vertical axis. axis. TheThe blue blue point point represents represents the the ma maximumximum illuminance, illuminance, and and the the red red points points represent represent the the checkpoints checkpoints in in thethe regulation. regulation. ( (cc,d,d)) Simulated Simulated values values at at the the checkpoints checkpoints on on a a plane plane at at a a distance distance of of 10 10 m m ((c) ((c) and and (d) (d) correspondcorrespond to to (a) (a) and and (b), (b), respectively). respectively). Green Green nu numbersmbers are are simulated simulated values, values, and and blue blue numbers numbers are are requirementsrequirements at at the the checkpoints. checkpoints.

4.4. Experimental Experimental Verification Verification TheThe fabricated fabricated reflector reflector is is displayed displayed in in Figure Figure 88.. The reflectorreflector was constructedconstructed using computer numericalnumerical control (CNC) machiningmachining and and was was coated coated with with aluminum aluminum film. film. The The reflector reflector was was mounted mounted on ona table a table 1 m 1 above m above the ground the ground and was and located was located 10 m away 10 m from away the illuminationfrom the illumination plane. The plane. checkpoint The checkpointmeasurements measurements for each pcW-LED for each orientationpcW-LED orientatio are presentedn are presented in Figure9 in. The Figure case 9. in The which case thein which short theaxis short was alignedaxis was with aligned the verticalwith the axis vertical produced axis produced a higher-contrast a higher-contrast cutoff line cutoff and wider line and horizontal wider horizontallight pattern, light compared pattern, tocompared the case ofto thethe long-axiscase of the alignment. long-axis Moreover,alignment. the Moreover, illuminance the atilluminance point A in atthe point short-axis A in the case short-axis was larger case than was that larger in the than long-axis that in the case. long-axis case.

Figure 8. (a) Fabricated reflector. (b) Measurement setup.

Crystals 2018, 8, x FOR PEER REVIEW 6 of 11

Figure 7. Light patterns in the case of (a) long- and (b) short-axis alignments with the vertical axis. The blue point represents the maximum illuminance, and the red points represent the checkpoints in the regulation. (c,d) Simulated values at the checkpoints on a plane at a distance of 10 m ((c) and (d) correspond to (a) and (b), respectively). Green numbers are simulated values, and blue numbers are requirements at the checkpoints.

4. Experimental Verification The fabricated reflector is displayed in Figure 8. The reflector was constructed using computer numerical control (CNC) machining and was coated with aluminum film. The reflector was mounted on a table 1 m above the ground and was located 10 m away from the illumination plane. The checkpoint measurements for each pcW-LED orientation are presented in Figure 9. The case in which the short axis was aligned with the vertical axis produced a higher-contrast cutoff line and wider horizontalCrystals 2019, light9, 659 pattern, compared to the case of the long-axis alignment. Moreover, the illuminance6 of 10 at point A in the short-axis case was larger than that in the long-axis case.

Crystals 2018, 8, x FOR PEER REVIEW 7 of 11 Figure 8. ((aa)) Fabricated Fabricated reflector. reﬂector. ( (bb)) Measurement Measurement setup. setup.

FigureFigure 9. Projected 9. Projected light light pattern pattern in in the the casescases of ( aa)) long- long- and and (b ()b short-axis) short-axis alignments alignments with with the vertical the vertical axis.axis. Blue Blue and and red red points points represent represent maximum maximum illuminance illuminance and and regulation regulation checkpoints,checkpoints, respectively. (c,d) Measured(c,d) Measured illuminance illuminance at the checkpoints at the checkpoints on the plane on the ((c )plane and (((dc)) correspondand (d) correspond to (a) and to (b(a),) and respectively). (b), respectively). Although the experimental measurement verified the validity of the optical design, and the light pattern passedAlthough K-mark the experimental regulation, measurement a new problem verified occurred. the validity Yellowish of the optical light patterns design, and were the obvious light in bothpattern cases of passed short-axis K-mark and regulation, long-axis a alignment. new problem The occurred. short-axis Yellowish case had light a larger patterns yellowish were obvious area above in both cases of short-axis and long-axis alignment. The short-axis case had a larger yellowish area the cutoff line, and this was a crucial concern. The yellowish pattern was caused by the light emitting above the cutoff line, and this was a crucial concern. The yellowish pattern was caused by the light from the LED die area. LED dies can emit light sideward; the down-conversion of yellow light is emitting from the LED die area. LED dies can emit light sideward; the down-conversion of yellow emittedlight isotropically, is emitted isotropically, but the blue but light the blue is emitted light is forward.emitted forward. This was This the was reason the reason that obvious that obvious blue light was observedblue light was on theobserved top of on the the LED top of dies, the butLED yellow dies, but light yellow was light observed was observed across across the whole the whole phosphor area,phosphor as illustrated area, inas Figureillustrated 10. in In Figure the optical 10. In design, the optical the reflectordesign, the was reflector used towas reflect used incoming to reflect light to theincoming target area. light to In the the target design, area. the In lightthe design, source the originated light source from originated a point from source a point at source the center at the of the pcW-LED.center Aof lightthe pcW-LED. source far A fromlight source the center far from area the causes center a area blurring causes of a the blurring light pattern,of the light which pattern, explains the yellowishwhich explains light the originating yellowish light from originating the outer from area the of theouter pcW-LED. area of the To pcW-LED. describe To this describe phenomenon, this phenomenon, we attempted to capture the surface light distribution of the pcW-LED in two colors, as illustrated in Figure 10a,c. The distribution was used to modify the pcW-LED optical model. The simulation results are presented in Figure 11, wherein the difference in light patterns produced by blue and yellow lights are obvious. The yellowish light in the outer area is easily observed.

Crystals 2019, 9, 659 7 of 10 we attempted to capture the surface light distribution of the pcW-LED in two colors, as illustrated in Figure 10a,c. The distribution was used to modify the pcW-LED optical model. The simulation results areCrystals presented 2018, 8,in x FOR Figure PEER 11 REVIEW, wherein the diﬀerence in light patterns produced by blue and yellow lights8 of 11 are obvious. The yellowish light in the outer area is easily observed. Crystals 2018, 8, x FOR PEER REVIEW 8 of 11

Figure 10. (a) Blue light pattern and (b) the corresponding distribution contour. (c) Yellow light Figure 10. (a) Blue light pattern and (b) the corresponding distribution contour. (c) Yellow light pattern pattern and (d) the corresponding distribution contour. andFigure (d) the 10. corresponding (a) Blue light distributionpattern and contour. (b) the corresponding distribution contour. (c) Yellow light pattern and (d) the corresponding distribution contour.

FigureFigure 11. Simulation 11. Simulation of theof the yellow yellow and and blue blue modelsmodels for for the the pcW-LED pcW-LED in inthe the cases cases of (a of,b) ( ashort-,b) short- and and (c,d) long-axis(c,d) long-axis alignments. alignments. Red Red dashed dashed lines lines are are usedused to to set set the the same same cutoff cuto linesﬀ lines level level for the for blue the blueand and yellowyellow patterns. patterns. Figure Figure colors colors only only distinguish distinguish blueblue or yellow. yellow.

5.Figure Discussion 11. Simulation of the yellow and blue models for the pcW-LED in the cases of (a,b) short- and (c,d) long-axis alignments. Red dashed lines are used to set the same cutoff lines level for the blue and yellow patterns. Figure colors only distinguish blue or yellow.

5. Discussion

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Crystals 2018, 8, x FOR PEER REVIEW 9 of 11 5. Discussion To design a head lamp, light source selection is as crucial as the optical design. In general, the To design a head lamp, light source selection is as crucial as the optical design. In general, the light light source should have as small an etendue as possible. Therefore, in a pcW-LED, the LED die source should have as small an etendue as possible. Therefore, in a pcW-LED, the LED die should should be aligned as tightly as possible. We used an atypical pcW-LED with two separate dies and be aligned as tightly as possible. We used an atypical pcW-LED with two separate dies and notably, notably, the phosphor layer covered a larger area than the blue dies did. Thus, the pcW-LED was the phosphor layer covered a larger area than the blue dies did. Thus, the pcW-LED was equipped equipped with a larger etendue, which makes a high-contrast cutoff line difficult to produce. In with a larger etendue, which makes a high-contrast cutoff line difficult to produce. In general, such a general, such a pcW-LED is designed for low-cost applications and is not a candidate for a head lamp. pcW-LED is designed for low-cost applications and is not a candidate for a head lamp. By using a By using a design with eight segments, we fabricated a reflector to form the cutoff line. The optical design with eight segments, we fabricated a reflector to form the cutoff line. The optical design is design is typical, but the light source is not. The experimental result demonstrated that the contrast typical, but the light source is not. The experimental result demonstrated that the contrast of the cutoff of the cutoff line can pass the regulation by using an appropriate optical design, but the light pattern line can pass the regulation by using an appropriate optical design, but the light pattern is yellowish. is yellowish. The yellowish pattern is caused by the large phosphor coverage area. Figure 11 The yellowish pattern is caused by the large phosphor coverage area. Figure 11 illustrated that the illustrated that the yellow pattern was larger than the blue pattern. Changing this situation is difficult yellow pattern was larger than the blue pattern. Changing this situation is difficult in any optical in any optical design, except by using a light guide to color mix blue and yellow light. However, color design, except by using a light guide to color mix blue and yellow light. However, color mixing causes mixing causes more blur in the illumination light pattern. Because the optical power emitted from more blur in the illumination light pattern. Because the optical power emitted from the area is not the area is not dominant compared with that on top of the blue die, we could block part of the dominant compared with that on top of the blue die, we could block part of the phosphor area to phosphor area to reduce the yellowish pattern near the cutoff line. In the optical design, the center reduce the yellowish pattern near the cutoff line. In the optical design, the center and upper areas of the and upper areas of the pcW-LED are the most crucial in forming the cutoff line. Thus, we darkened pcW-LED are the most crucial in forming the cutoff line. Thus, we darkened the upper phosphor area the upper phosphor area of the light source as illustrated in Figure 12a. In comparison with Figure of the light source as illustrated in Figure 12a. In comparison with Figure9b, the yellowish e ffect near 9b, the yellowish effect near the cutoff line was greatly reduced. Moreover, the illuminance in the the cutoff line was greatly reduced. Moreover, the illuminance in the dark zone decreased to 0.4 lux, dark zone decreased to 0.4 lux, from 0.71 lux in the original case. As a result, reducing a specific from 0.71 lux in the original case. As a result, reducing a specific phosphor area is useful for forming a phosphor area is useful for forming a high-contrast cutoff line, as well as reducing the yellowish high-contrast cutoff line, as well as reducing the yellowish effect. This technology will be crucial in effect. This technology will be crucial in similar applications when using an atypical pcW-LED. similar applications when using an atypical pcW-LED. Besides, in the case of using this technology, Besides, in the case of using this technology, we examined if the experimental results meet the K- we examined if the experimental results meet the K-mark regulation. Firstly, the illuminance at point mark regulation. Firstly, the illuminance at point A was measured at larger than 20 lux with a cover. A was measured at larger than 20 lux with a cover. Additionally, the illuminance at points CL and CR Additionally, the illuminance at points CL and CR were as large as 60% of the maximum illuminance were as large as 60% of the maximum illuminance in the target region. Furthermore, the illuminance in the target region. Furthermore, the illuminance at the point B was around 20 lux, which is two at the point B was around 20 lux, which is two times the 10 lux requested in the K-mark regulation. times the 10 lux requested in the K-mark regulation. Finally, the illuminance at the points GL, FL, M, Finally, the illuminance at the points GL, FL, M, FR, GR are larger than 2 lux. As a result, all checkpoints FR, GR are larger than 2 lux. As a result, all checkpoints in the measurement pass the K-mark in the measurement pass the K-mark regulation. regulation.

FigureFigure 12.12. ((aa)) Illumination Illumination pattern pattern with with reduced reduced phosphor phosphor area. area. (b) (bMeasured) Measured illuminance illuminance at the at thecheckpoints. checkpoints. 6. Conclusions 6. Conclusions We presented a new design for a K-mark head lamp based on a low-cost pcW-LED, wherein two We presented a new design for a K-mark head lamp based on a low-cost pcW-LED, wherein two separate dies were bonded under a wide phosphor layer. Therefore, the head lamp had two feature separate dies were bonded under a wide phosphor layer. Therefore, the head lamp had two feature axes: a long axis along the die-separation line and a short axis. The study started by measuring the axes: a long axis along the die-separation line and a short axis. The study started by measuring the steady-state luminous flux to confirm the power level. We then developed a precise optical model steady-state luminous flux to confirm the power level. We then developed a precise optical model for for the pcW-LED with a black-and-white image to determine the weighting factor at the emitting the pcW-LED with a black-and-white image to determine the weighting factor at the emitting surface. surface. The NCCs were all 99.5%, meaning that the optical model was sufficiently accurate for The NCCs were all ≥99.5%, meaning≥ that the optical model was sufficiently accurate for head lamp design. The downward-type reflector was divided into eight segments, of which the four bottom segments were used to form the sharp edge of the cutoff line. After the addition of a housing structure

Crystals 2019, 9, 659 9 of 10 head lamp design. The downward-type reflector was divided into eight segments, of which the four bottom segments were used to form the sharp edge of the cutoff line. After the addition of a housing structure by using CNC machining and aluminum coating, an experimental setup based on K-mark regulation was used to measure the light patterns projected on a plane 10 m away from the light source. Measurements at the checkpoints revealed that the light patterns with long- and short-axis orientations met the K-mark regulation. A sharper cutoff line was observed when the short axis was aligned with the vertical direction. However, yellowish patterns on the outer area of the projected patterns were observed for both orientations. This phenomenon was caused by a much wider spreading area of the phosphor layer, than the area occupied by the blue die. The side-emitting blue light from the LED die causes more yellow light to emit in the phosphor layer on the outer part. To understand this effect, we created an advanced optical model with blue and yellow light. The new model well described the phenomenon of the yellowish projected light pattern. Finally, we demonstrated an atypical pcW-LED at low cost for application in a K-mark bicycle head lamp, and the extraordinary effects were theoretically analyzed. To reduce the yellowish pattern near the cutoff line, we proposed blocking a specific phosphor area by darkening the upper area of the pcW-LED. The experiment had a positive result, in which the yellowish effect was greatly decreased, and a high-contrast cutoff line was observed. The result well satisfies the requirement in the K-mark regulation. Analyzing the yellowish effect and blocking the phosphor area will be useful and helpful when using a large phosphor coverage area in an atypical pcW-LED to design a head lamp.

Author Contributions: Conceptualization and writing work, C.-C.S.; Simulation, H.-J.L., C.-S.W. and T.-H.Y.; Experiment, H.-J.L. and C.-S.W.; discussion, X.-H.L., S.-K.L., T.-H.Y., Y.-J.L. and Y.-W.Y. Funding: This research was funded by the Ministry of Science and Technology of the Republic of China, grant numbers MOST 106-2221-E-008-065-MY3 and 108-2622-E-008-010-CC2. Acknowledgments: The author would like to thank Breault Research Organization (BRO), Inc. for sponsoring ASAP software, and Benex for supporting the fabrication of the optical components. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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