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Optical design of a Butterfly lens for a street light based on a double-cluster LED
Microelectronics Reliability 52 (2012) 889–893
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Optical design of a Butterfly lens for a street light based on a double-cluster LED Yi-Chien Lo a,b, Kuan-Teng Huang a,b,c, Xuan-Hao Lee a,b, Ching-Cherng Sun a,b,⇑ a
Department of Optics and Photonics, National Central University, Chung-Li, Taiwan Institute of Lighting and Display Science, National Central University, Chung-Li, Taiwan c Department of Photonics Engineering, Yuan Ze University, Chung-Li, Taiwan b
a r t i c l e
i n f o
Article history: Received 16 January 2011 Received in revised form 20 March 2011 Accepted 3 April 2011 Available online 4 May 2011
a b s t r a c t We construct a precise optical model of the cluster LED and propose a design of a peanut form lens to achieve an optical utilization factor of 45% and uniformity of 1/2.7. Then we propose a new design of so-called Butterfly lens with unequal tilt design aimed to serve a double-cluster LED but with a tightened size. The optical utilization factor of the Butterfly lens is about 43.8%, the uniformity is around 1/2.7, and an averaged illuminance of 14 lx, which could meet almost all types requested by IESNA. The design of the Butterfly lens is proven workable for a double-cluster LED. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
2. Precise optical model of the cluster LED
LEDs have been designed to serve as one of the next-generation light sources in high-brightness illumination owing to the progress in luminous efficacy and improvement of the thermal dissipation [1–3]. In comparison with the traditional light sources in outdoor lighting, LEDs provide not only high energy efficiency but also high color rendering index so that more human factor can be installed in an LED lamp. In regard to street light, the compact size of LED enables the optical design more flexible. A free form lens for LED optical design is effective and useful way to perform desired light pattern [4,5]. Owing to specific request on the illumination pattern on the ground, the optical flux should be directed to a wider angle, various optical designs were proposed to meet the request [6–9]. As the luminous efficacy of LED increases, a large-size LED with cluster package, which generally degrades the luminous efficacy with thermal issue, becomes on of the options in street lighting. The difficulty for installing such a light source in a street light is that not only heat dissipation but also the optical performance, including optical efficiency as well as light pattern, is challenge in the design of the street light. In this paper, we present a study of optical design applied to street light with use of large-scale LEDs with cluster package. Moreover, to meet the illuminance requirement, two cluster LEDs side-by-side as the light source, so that the optical design of a compact lens for high optical utilization factor is again a task. The new design to meet above demand will be presented and the results will be demonstrated.
A reliable optical design for the street light requests a precise optical model of the light source [10–12], which is a cluster with 7 7 die arrays and the size of the array is 21 21 mm2, as shown in Fig. 1. The input power of the cluster LED was around 60 W, and the output flux in steady state was around 4400 lm measured by an integrating sphere, so the luminous efficacy was about 73.3 lm/W. We utilized the mid-field model [10,13] rather than the far field [14] to develop the light source model rather than simply regarded it as a Lambertian one in a general approach. On the surface of the phosphor plate covering the blue die array, we can find that the light emission across the surface is not uniform. Accordingly, a weighting factor is applied to adjust the brightness of the emitting surface so that a precise light source model is achieved [12]. Then more than ten millions of rays are emitted from the surface with use of a Monte Carlo ray tracing software of ASAP [15,16]. The verification in the mid-field region ensures that the optical model is accurate when the normalized cross-correlation (NCC) [12] is larger than 99% at different measured distances. Fig. 2 shows the simulated one-dimensional (1-D) light pattern in comparison with the corresponding measurement at the locations in the mid-field. As shown in Fig. 2, the NCCs between simulations and experimental measurements are always higher than 99.4%. From our experience in designing LED lighting systems for industrial applications, we know that such an LED model is accuracy enough for practical applications.
3. Optical design for street light ⇑ Corresponding author at: Institute of Lighting & Display Sciences, National Central University, Chung-Li, Taiwan. E-mail address: [email protected] (C.-C. Sun). 0026-2714/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2011.04.007
The optical design is aimed to ensure the largest optical utilization factor on the ground illumination and highest uniformity in the illumination area. According to the regulation by IESNA [17],
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Y.-C. Lo et al. / Microelectronics Reliability 52 (2012) 889–893
Fig. 1. The cluster LED used as the light source of the street light.
we set that the height of the street light is 10 m, and the illumination area on the ground is 10 30 m2, where 30 m is along the traffic flow. Besides, the tilt angle of the street light is about 15°, as shown in Fig. 3. In the basic design, the bottom of the lens is avoided to contact the LED surface because of the thermal issue. Then, the outer surface of the free form lens can be separately designed in the x-axis and y-axis. In the x-axis, the light pattern should be diverged to spread the lights along the traffic flow direction. Therefore, a peanut shape of a free form lens with two tilt lenses along the x-axis is applied. Thus, the central region of the outer surface looks like a
concave form so that illumination of the normal direction is not too large. The lens shape along the y-axis is a typical convex form since the lights should be focused along the y direction, as illustrated in Fig. 4. Fig. 5 shows simulation results of the design. The index is about 1.5 and the dimensions of the lens are 108 85 30 mm3. The optical efficiency is about 81%. The optical utilization factor, defined by the ratio of the flux on the target and the flux from the light source, is about 45%, which is larger than 30–40% of a traditional street light. As for the uniformity, Fig. 6 is an arrangement of the illuminance test matrix according to IESNA. The uniformity is defined by the ratio of the minimum and average illuminance in the test region. The design shows that the uniformity reaches 1/2.7 for a single street light and the value is higher than the most rigorous value of 1/3 suggested by IESNA. When the illuminance request is larger than that afforded by a single LED, two or more light sources with the same optical lens could be installed. The size as well as weight could be too large so that other problems could happen. Therefore, here we propose a new design for a double-cluster LED, where two cluster LEDs are put as close as possible to each other along the y-axis. To enlarge the optical utilization factor and tighten the size of the optical lens, we merge two peanut-form lenses along the y-axis. The new lens is called ‘‘Butterfly lens’’ for its special shape shown in Fig. 7. In the Butterfly lens, the convex lens along y-axis must be tilt to each cluster LED, so the lights along y direction do not convergent as the previous design with a single LED. Thus the optical utilization factor decreases. To solve this problem, in the design, we redirect the lights emitted by the two LEDs to the normal direction with two slanted surfaces on the inner cave. Accordingly, the unequal tilt angles h1 and h2 are induced to the two LEDs, respectively, as shown Fig. 8. The prototype is shown in Fig. 9.
Fig. 2. The comparison of the measured and simulated one-dimensional radiation pattern about the x-axis of the LED: (a) 5 cm, (b) 7 cm, (c) 9 cm and (d) 11 cm.
Y.-C. Lo et al. / Microelectronics Reliability 52 (2012) 889–893
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Fig. 3. (a) The geometry of the street light and (b) the illuminant target on the ground.
Fig. 4. The geometry of the peanut form lens along (a) the x-axis, (b) the y-axis and (c) 3D view.
Fig. 5. The simulated light pattern for a single street light.
The simulation of the angular light pattern of the single street luminaire shows that the flux from the central region of the LEDs is diverged successfully and the intensity peak is on 55° along the x-axis, and along the y-axis the flux is mostly directed to the angular range between 20° and 50°, which coincide with the measurement of the angular pattern with the real lens samples, as shown in Fig. 10. The optical efficiency of the Butterfly lens is 76%, a slightly lower than the single LED for more tilt surfaces, but is still acceptable. Besides, the optical utilization factor achieves 43.8%, which is on a similar level to the design of a peanut form lens. The uniformity is around 1/2.7 and an averaged illuminance of 14 lx is achieved, which could meet almost all types requested by IESNA. The uniformity can even reach 1/1.6 if it is measured with at least three adjacent street lights along the roadway. All the lamps are in the same side. Simultaneously, the distance between each lamp is equal to the length of the target in the direction of the traffic flow. The design of the Butterfly lens is proven workable for a double-cluster LED.
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Y.-C. Lo et al. / Microelectronics Reliability 52 (2012) 889–893
Fig. 6. The matrix for the measurement of the illuminace.
Fig. 7. The geometry of the proposed Butterfly lens applied for the double-Cluster LEDs: (a) top view and (b) bottom view.
Fig. 8. The unequal tilt design of the Butterfly lens.
Fig. 10. A comparison of the angular patterns between simulation and the corresponding measurement.
4. Conclusion
Fig. 9. The prototype of the Butterfly lens with the double-cluster LED.
In this paper, we first propose an optimized design for a single cluster LED with a peanut form lens started by constructing a precise optical model of the cluster LED. In the design, the optical utilization factor is around 45% and the uniformity for a single street light is 1/2.7. Then we propose a new design of so-called Butterfly lens aimed to serve a double-cluster LED but with a tightened size. In the new design, a double convex lens with unequal tilt angles is introduced to make the light pattern as convergent as possible along the y direction. The intensity peak is at 55° in the x-axis, and in the y-axis, the flux is mostly directed to the angular range between 20° and 50°. The optical utilization factor is about 43.8%, the uniformity is around 1/2.7, and an averaged illuminance of 14 lx is achieved, which could meet almost all types requested by IESNA. The uniformity can even reach 1/1.6 if it is measured
Y.-C. Lo et al. / Microelectronics Reliability 52 (2012) 889–893
with at least three adjacent street lights along the roadway. The design of the Butterfly lens is proven workable for a double-cluster LED. Acknowledgment This study was sponsored by the National Science Council of the Republic of China with the Contracts of nos. 97-2221-E-008-025MY3 and 99-2623-E-008-002-ET. The authors would like to thank Breault Research Organization and Howard Huang for the support of simulation. References [1] Steigerwald DA, Bhat JC, Collins D, Fletcher RM, Holcomb MO, Ludowise MJ, et al. Illumination with solid state lighting technology. IEEE J Select Topics Quantum Electron 2002;8:310. [2] Zukauskas A, Shur MS, Caska R. Introduction to solid-state lighting. New York: John Wiley & Sons; 2002. [3] Nguyen F, Terao B, Laski J. Realizing LED illumination lighting applications. Proc SPIE 2005;5941:31. [4] Winston R, Miñano JC, Benítez P. Nonimaging optics. Elsevier; 2005.
893
[5] Ries H, Muschaweck JA. Tailored freeform optical surfaces. J Opt Soc Am A 2002;19:590–5. [6] Elmer WB. A study in street lighting reflector design. Appl Opt 1966;5:343. [7] Luo Y, Feng Z, Han Y, Li H. Design of compact and smooth free-form optical system with uniform illuminance for LED source. Opt Express 2010;18:9055–63. [8] Ding Y, Liu X, Zheng ZR, Gu PF. Freeform LED lens for uniform illumination. Opt Express 2008;16:12958–66. [9] Bortz J, Shatz N, Pitou D. Optimal design of a nonimaging projection lens for use with an LED source and a rectangular target. Proc SPIE 2000;4092:130–8. [10] Sun CC, Lee TX, Ma SH, Lee YL, Huang SM. Precise optical modeling for LED lighting verified by cross correlation in the midfield region. Opt Lett 2006;31:2193–5. [11] Moreno I, Sun CC. Modeling the radiation pattern of LEDs. Opt Express 2008;16:1808–19. [12] Chien WT, Sun CC, Moreno I. Precise optical model of multi-chip white LEDs. Opt Express 2007;15:7572–7. [13] Sun CC, Chien WT, Moreno I, Hsieh CC, Lo YC. Analysis of the far-field region of LEDs. Opt Express 2009;17:13918–27. [14] Goodman JW. Introduction to fourier optics. 2nd ed. McGraw-Hill; 1996. [15] Lee TX, Lin CY, Ma SH, Sun CC. Analysis of position-dependent light extraction of GaN-based LEDs. Opt Express 2005;13:4175. [16] ASAP.
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Optical design of a Butterfly lens for a street light based on a double-cluster LED Yi-Chien Lo a,b, Kuan-Teng Huang a,b,c, Xuan-Hao Lee a,b, Ching-Cherng Sun a,b,⇑ a
Department of Optics and Photonics, National Central University, Chung-Li, Taiwan Institute of Lighting and Display Science, National Central University, Chung-Li, Taiwan c Department of Photonics Engineering, Yuan Ze University, Chung-Li, Taiwan b
a r t i c l e
i n f o
Article history: Received 16 January 2011 Received in revised form 20 March 2011 Accepted 3 April 2011 Available online 4 May 2011
a b s t r a c t We construct a precise optical model of the cluster LED and propose a design of a peanut form lens to achieve an optical utilization factor of 45% and uniformity of 1/2.7. Then we propose a new design of so-called Butterfly lens with unequal tilt design aimed to serve a double-cluster LED but with a tightened size. The optical utilization factor of the Butterfly lens is about 43.8%, the uniformity is around 1/2.7, and an averaged illuminance of 14 lx, which could meet almost all types requested by IESNA. The design of the Butterfly lens is proven workable for a double-cluster LED. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
2. Precise optical model of the cluster LED
LEDs have been designed to serve as one of the next-generation light sources in high-brightness illumination owing to the progress in luminous efficacy and improvement of the thermal dissipation [1–3]. In comparison with the traditional light sources in outdoor lighting, LEDs provide not only high energy efficiency but also high color rendering index so that more human factor can be installed in an LED lamp. In regard to street light, the compact size of LED enables the optical design more flexible. A free form lens for LED optical design is effective and useful way to perform desired light pattern [4,5]. Owing to specific request on the illumination pattern on the ground, the optical flux should be directed to a wider angle, various optical designs were proposed to meet the request [6–9]. As the luminous efficacy of LED increases, a large-size LED with cluster package, which generally degrades the luminous efficacy with thermal issue, becomes on of the options in street lighting. The difficulty for installing such a light source in a street light is that not only heat dissipation but also the optical performance, including optical efficiency as well as light pattern, is challenge in the design of the street light. In this paper, we present a study of optical design applied to street light with use of large-scale LEDs with cluster package. Moreover, to meet the illuminance requirement, two cluster LEDs side-by-side as the light source, so that the optical design of a compact lens for high optical utilization factor is again a task. The new design to meet above demand will be presented and the results will be demonstrated.
A reliable optical design for the street light requests a precise optical model of the light source [10–12], which is a cluster with 7 7 die arrays and the size of the array is 21 21 mm2, as shown in Fig. 1. The input power of the cluster LED was around 60 W, and the output flux in steady state was around 4400 lm measured by an integrating sphere, so the luminous efficacy was about 73.3 lm/W. We utilized the mid-field model [10,13] rather than the far field [14] to develop the light source model rather than simply regarded it as a Lambertian one in a general approach. On the surface of the phosphor plate covering the blue die array, we can find that the light emission across the surface is not uniform. Accordingly, a weighting factor is applied to adjust the brightness of the emitting surface so that a precise light source model is achieved [12]. Then more than ten millions of rays are emitted from the surface with use of a Monte Carlo ray tracing software of ASAP [15,16]. The verification in the mid-field region ensures that the optical model is accurate when the normalized cross-correlation (NCC) [12] is larger than 99% at different measured distances. Fig. 2 shows the simulated one-dimensional (1-D) light pattern in comparison with the corresponding measurement at the locations in the mid-field. As shown in Fig. 2, the NCCs between simulations and experimental measurements are always higher than 99.4%. From our experience in designing LED lighting systems for industrial applications, we know that such an LED model is accuracy enough for practical applications.
3. Optical design for street light ⇑ Corresponding author at: Institute of Lighting & Display Sciences, National Central University, Chung-Li, Taiwan. E-mail address: [email protected] (C.-C. Sun). 0026-2714/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2011.04.007
The optical design is aimed to ensure the largest optical utilization factor on the ground illumination and highest uniformity in the illumination area. According to the regulation by IESNA [17],
890
Y.-C. Lo et al. / Microelectronics Reliability 52 (2012) 889–893
Fig. 1. The cluster LED used as the light source of the street light.
we set that the height of the street light is 10 m, and the illumination area on the ground is 10 30 m2, where 30 m is along the traffic flow. Besides, the tilt angle of the street light is about 15°, as shown in Fig. 3. In the basic design, the bottom of the lens is avoided to contact the LED surface because of the thermal issue. Then, the outer surface of the free form lens can be separately designed in the x-axis and y-axis. In the x-axis, the light pattern should be diverged to spread the lights along the traffic flow direction. Therefore, a peanut shape of a free form lens with two tilt lenses along the x-axis is applied. Thus, the central region of the outer surface looks like a
concave form so that illumination of the normal direction is not too large. The lens shape along the y-axis is a typical convex form since the lights should be focused along the y direction, as illustrated in Fig. 4. Fig. 5 shows simulation results of the design. The index is about 1.5 and the dimensions of the lens are 108 85 30 mm3. The optical efficiency is about 81%. The optical utilization factor, defined by the ratio of the flux on the target and the flux from the light source, is about 45%, which is larger than 30–40% of a traditional street light. As for the uniformity, Fig. 6 is an arrangement of the illuminance test matrix according to IESNA. The uniformity is defined by the ratio of the minimum and average illuminance in the test region. The design shows that the uniformity reaches 1/2.7 for a single street light and the value is higher than the most rigorous value of 1/3 suggested by IESNA. When the illuminance request is larger than that afforded by a single LED, two or more light sources with the same optical lens could be installed. The size as well as weight could be too large so that other problems could happen. Therefore, here we propose a new design for a double-cluster LED, where two cluster LEDs are put as close as possible to each other along the y-axis. To enlarge the optical utilization factor and tighten the size of the optical lens, we merge two peanut-form lenses along the y-axis. The new lens is called ‘‘Butterfly lens’’ for its special shape shown in Fig. 7. In the Butterfly lens, the convex lens along y-axis must be tilt to each cluster LED, so the lights along y direction do not convergent as the previous design with a single LED. Thus the optical utilization factor decreases. To solve this problem, in the design, we redirect the lights emitted by the two LEDs to the normal direction with two slanted surfaces on the inner cave. Accordingly, the unequal tilt angles h1 and h2 are induced to the two LEDs, respectively, as shown Fig. 8. The prototype is shown in Fig. 9.
Fig. 2. The comparison of the measured and simulated one-dimensional radiation pattern about the x-axis of the LED: (a) 5 cm, (b) 7 cm, (c) 9 cm and (d) 11 cm.
Y.-C. Lo et al. / Microelectronics Reliability 52 (2012) 889–893
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Fig. 3. (a) The geometry of the street light and (b) the illuminant target on the ground.
Fig. 4. The geometry of the peanut form lens along (a) the x-axis, (b) the y-axis and (c) 3D view.
Fig. 5. The simulated light pattern for a single street light.
The simulation of the angular light pattern of the single street luminaire shows that the flux from the central region of the LEDs is diverged successfully and the intensity peak is on 55° along the x-axis, and along the y-axis the flux is mostly directed to the angular range between 20° and 50°, which coincide with the measurement of the angular pattern with the real lens samples, as shown in Fig. 10. The optical efficiency of the Butterfly lens is 76%, a slightly lower than the single LED for more tilt surfaces, but is still acceptable. Besides, the optical utilization factor achieves 43.8%, which is on a similar level to the design of a peanut form lens. The uniformity is around 1/2.7 and an averaged illuminance of 14 lx is achieved, which could meet almost all types requested by IESNA. The uniformity can even reach 1/1.6 if it is measured with at least three adjacent street lights along the roadway. All the lamps are in the same side. Simultaneously, the distance between each lamp is equal to the length of the target in the direction of the traffic flow. The design of the Butterfly lens is proven workable for a double-cluster LED.
892
Y.-C. Lo et al. / Microelectronics Reliability 52 (2012) 889–893
Fig. 6. The matrix for the measurement of the illuminace.
Fig. 7. The geometry of the proposed Butterfly lens applied for the double-Cluster LEDs: (a) top view and (b) bottom view.
Fig. 8. The unequal tilt design of the Butterfly lens.
Fig. 10. A comparison of the angular patterns between simulation and the corresponding measurement.
4. Conclusion
Fig. 9. The prototype of the Butterfly lens with the double-cluster LED.
In this paper, we first propose an optimized design for a single cluster LED with a peanut form lens started by constructing a precise optical model of the cluster LED. In the design, the optical utilization factor is around 45% and the uniformity for a single street light is 1/2.7. Then we propose a new design of so-called Butterfly lens aimed to serve a double-cluster LED but with a tightened size. In the new design, a double convex lens with unequal tilt angles is introduced to make the light pattern as convergent as possible along the y direction. The intensity peak is at 55° in the x-axis, and in the y-axis, the flux is mostly directed to the angular range between 20° and 50°. The optical utilization factor is about 43.8%, the uniformity is around 1/2.7, and an averaged illuminance of 14 lx is achieved, which could meet almost all types requested by IESNA. The uniformity can even reach 1/1.6 if it is measured
Y.-C. Lo et al. / Microelectronics Reliability 52 (2012) 889–893
with at least three adjacent street lights along the roadway. The design of the Butterfly lens is proven workable for a double-cluster LED. Acknowledgment This study was sponsored by the National Science Council of the Republic of China with the Contracts of nos. 97-2221-E-008-025MY3 and 99-2623-E-008-002-ET. The authors would like to thank Breault Research Organization and Howard Huang for the support of simulation. References [1] Steigerwald DA, Bhat JC, Collins D, Fletcher RM, Holcomb MO, Ludowise MJ, et al. Illumination with solid state lighting technology. IEEE J Select Topics Quantum Electron 2002;8:310. [2] Zukauskas A, Shur MS, Caska R. Introduction to solid-state lighting. New York: John Wiley & Sons; 2002. [3] Nguyen F, Terao B, Laski J. Realizing LED illumination lighting applications. Proc SPIE 2005;5941:31. [4] Winston R, Miñano JC, Benítez P. Nonimaging optics. Elsevier; 2005.
893
[5] Ries H, Muschaweck JA. Tailored freeform optical surfaces. J Opt Soc Am A 2002;19:590–5. [6] Elmer WB. A study in street lighting reflector design. Appl Opt 1966;5:343. [7] Luo Y, Feng Z, Han Y, Li H. Design of compact and smooth free-form optical system with uniform illuminance for LED source. Opt Express 2010;18:9055–63. [8] Ding Y, Liu X, Zheng ZR, Gu PF. Freeform LED lens for uniform illumination. Opt Express 2008;16:12958–66. [9] Bortz J, Shatz N, Pitou D. Optimal design of a nonimaging projection lens for use with an LED source and a rectangular target. Proc SPIE 2000;4092:130–8. [10] Sun CC, Lee TX, Ma SH, Lee YL, Huang SM. Precise optical modeling for LED lighting verified by cross correlation in the midfield region. Opt Lett 2006;31:2193–5. [11] Moreno I, Sun CC. Modeling the radiation pattern of LEDs. Opt Express 2008;16:1808–19. [12] Chien WT, Sun CC, Moreno I. Precise optical model of multi-chip white LEDs. Opt Express 2007;15:7572–7. [13] Sun CC, Chien WT, Moreno I, Hsieh CC, Lo YC. Analysis of the far-field region of LEDs. Opt Express 2009;17:13918–27. [14] Goodman JW. Introduction to fourier optics. 2nd ed. McGraw-Hill; 1996. [15] Lee TX, Lin CY, Ma SH, Sun CC. Analysis of position-dependent light extraction of GaN-based LEDs. Opt Express 2005;13:4175. [16] ASAP.