Design of a free-form lens for LED light with high efficiency and uniform illumination Nguyen Doan Quoc Anh,1 Min-Feng Lai,1 Hsin-Yi Ma,2 and Hsiao-Yi Lee1,* 1
Department of Electrical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 80778, Taiwan 2
Department of Industrial Engineering and Management, Minghsin University of Science and Technology, Hsinchu 30401, Taiwan *Corresponding author: [email protected] Received 1 May 2014; revised 28 June 2014; accepted 25 July 2014; posted 25 July 2014 (Doc. ID 211258); published 27 August 2014
A free-form secondary lens is proposed to optimize an LED light. Based on Snell’s law, energy conservation law, and a Monte Carlo ray-tracing algorithm, the surface contour of the free-form lens can be determined according to the requirements of an LED light. Optical experimental results show that an optical efficiency of 95.69% can be achieved by the lens, of which the illumination uniformity 0.317 is higher than the commercial illumination uniformity 0.259. The lens does not need the help of a white ring holder, so the cost of the LED light can become effective. © 2014 Optical Society of America OCIS codes: (230.3670) Light-emitting diodes; (220.0220) Optical design and fabrication; (220.4298) Nonimaging optics; (350.4600) Optical engineering; (220.2945) Illumination design. http://dx.doi.org/10.1364/AO.53.00H140
1. Introduction
LEDs have many advantages for lighting such as high efficiency, long lifetime, and fast response as well as climate impact resistance [1,2]. Thus, an LED has been considered the most promising general illumination solution for the future. However, compared with conventional light sources, such as an incandescent bulb, an LED usually cannot provide uniform illumination by itself. As a result, LEDs need an additional optical device, called a secondary lens, to produce uniform illumination and proper intensity distribution and to keep high light efficiency [3–6]. A TIR (total internal reflection) lens is usually considered in the secondary lens design because it can control light over a wider range of angles better than traditional reflectors or lenses [7–9]. However, if the lens boundary could not afford total internal reflection, then a TIR lens would not efficiently reflect the 1559-128X/14/29H140-06$15.00/0 © 2014 Optical Society of America H140
APPLIED OPTICS / Vol. 53, No. 29 / 10 October 2014
incident light. Therefore, in order to accomplish a TIR lens with high efficiency, at times, a white holder is used to surround the conventional TIR lens that helps to collect the lost light. In this study, we propose a free-form TIR lens without using white holders, capably bringing down the cost for an LED light. A simple and efficient TIR lens design process is introduced instead of using complicated techniques [10–13]. It is demonstrated experimentally that the proposed TIR lens can accomplish a Lambertian LED as the 30 deg spotlight with uniform illumination and high efficiency. 2. Principles
As shown in Fig. 1, the conventional LED light module consists of a secondary lens, white holder, and LED source package. Here we present a new LED light module without using the white holders, in which the secondary lens is a TIR lens with a freeform surface. The lens is a multisegmented optical collimator (MSOC) with a multistructured optical surface (MSOS). The MSOC can accomplish total internal reflection over a wider range of angles of light
B. MSOS Design
Fig. 1. Conventional LED light module consisting of secondary lens, white holder, and LED source package.
than the conventional secondary lens, so that the proposed free-form lens can achieve high optical efficiency. The MSOS design is for homogenizing the output light and attaining the lens with high light efficiency. A.
MSOC Design
The MSOC design is mainly based on total internal reflection. Through optical software simulations, we understand that, while compared with conic reflecting surfaces, the MSOC reflecting surface needs to be segmented into at least three sections to obtain total internal reflection with a wider range of light angles. The relations among the incoming light direction vector I, the optical surface normal vector N, and the outgoing light direction vector O are precisely governed by reflection law. The normal vector N can be calculated by Eq. (1) [14,15]: N
I−O : jI − Oj
(1)
Assuming that the center of the LED source is located at the origin of a Cartesian coordinate system, one incident ray I i from a source point is aimed at the corresponding point Pi to generate the reflected ray Oi. The surface segment is expressed as f i x (i 1, 2, 3), and N i is the surface normal of f i x at Pi . In the MSOC design process, f i x is free to change, so that N i is varied until Oi is in the normal direction of the MSOC output surface, as shown in Fig. 2.
Fig. 2. Two-dimensional ray-tracing plot for the MSOC.
After finishing the MSOC design, the ray paths through the MSOC can be found through Monte Carlo ray tracing, as shown in Fig. 3(a). Without MSOS, the MSOC output is difficult to be uniform. Through a ray-tracing technique, it can be found that the excessive energy on the output plane is mainly from the green region on the emitting surface of the MSOC, as shown in Fig. 3(b). The marked areas are called the “nonoptimized areas.” In the proposed MSOS, microlenses are built on the emitting surface of the MSOC. In order to accomplish high efficiency and high uniformity, the MSOS needs to be arranged according to the energy density on the emitting surface of the MSOC. Thus, more microlenses are built on the “nonoptimized areas” for diverging light. According to the energy conservation law [16], the resulting power of the MSOC on the target surface can be expressed as ZZ A
Ef ds
Q ZZ X i1
Ci
Ei ds Etotal :
(2)
Here A is the uniform illumination region on the target surface, Ef is the illumination on the target surface, Q is the quantity of the cells on the panel plane, Ci is the region of the cell, Ei is the illumination on the cell Ci , and Etotal is the total flux from the source. It is obvious that the smaller the tailored cells, the more uniform the light through the cell. Based on the mathematical principle in Eq. (2), the quantity of a free-form microlens on the nonoptimized areas should be constructed more than that of the other areas. This can result in a significant increase of the emergent angle at the nonoptimized areas. Through the suitable MSOS design, excessive light can be moved to low illumination areas, so as to obtain uniform illumination. 3. Experiments and Experimental Results
The main experimental work to accomplish the proposed free-form TIR lens includes the following:
Fig. 3. (a) Schematic ray paths through the MSOC. (b) MSOC marked region; the output excessive energy is from there. 10 October 2014 / Vol. 53, No. 29 / APPLIED OPTICS
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1. Building the precise mechanical structures and the optical properties of the LED source. 2. Optimizing the proposed free-form TIR lens design, making the prototype sample, and measuring its performances. 3. Comparing the performances between the proposed free-form TIR lens and the commercial secondary lens in the illumination uniformity and optical efficiency. In the experiments, a NICHIA’s white Lambertian LED: NVSW219AT is used as the light source. We build the LED physical model for the proposed TIR lens design. The physical model of the white LED is shown in Fig. 4. The phosphor layer, with a thickness of 0.05 mm, covers the LED chip. The blue chip with a square base of 1 mm and a height of 0.1 mm is covered by a silicone dome lens. The blue chip is set at 1 W output power and has a peak wavelength of 453 nm. The material of the free-form lens is selected as polymethyl methacrylate (PMMA), and its refractive index is 1.4935. We use TracePro software to trace 10 million light rays randomly through the LED model for optical simulations of the freeform TIR lens design. Referring to the MSOC ray-tracing plot in Fig. 2, the incident light vectors I 1, I 2 , and I 3 are emitted from the LED source and, in turn, meet the reflective surface at three points P1, P2 , and P3 . Based on finding the normal vector N i to have Oi collimated after total internal reflection, the 2D contour functions f 1 x, f 2 x, and f 3 x are adjusted freely to establish 30 deg of intensity distribution under the help of SolidWorks and TracePro software. The MSOC structure can thus be obtained by rotating the 2D contour about the MSOC optical axis. The bottom and top radii of the MSOC are 2.53 and 12 mm, and its height is about 10.73 mm, as shown in Fig. 5. The accomplished MSOC can be used without an additional white ring holder. After considering the molding precision limitation and the yield rate of the MSOS in mass production [17,18], and performing a local searching method based on a damping least-squares algorithm, the dome lens with 1 mm diameter and 0.17 mm height is selected to compose the MSOS. The distance between two consecutive peaks of the dome lenses is chosen as 0.5 mm. The MSOS is formed by rotating two different groups of the dome lenses, called A
Fig. 4. Schematic illustration of the physical model of NICHIA’s white Lambertian LED: NVSW219AT. H142
APPLIED OPTICS / Vol. 53, No. 29 / 10 October 2014
Fig. 5. MSOC design drawing.
(silver) and B (green) microlens matrices, as shown in Fig. 6 (top). To construct the MSOS, the A and B matrix microlenses are rotated and then duplicated. Using the global optimization function of optical design software, it is found that the A and B matrix need to spin 9 and 30 deg for each rotation, respectively. After 360 deg rotating, the whole generated dome lenses are grown as the MSOS (Fig. 6, bottom). The illumination map on the target plane at 500 mm away corresponds to both cases of MSOC, with and without MSOS, as shown in Fig. 7 (top) and (bottom). It can be observed that the illumination
Fig. 6. (Top) Top view of A and B microlens matrices. (Bottom) MSOS design drawing.
Fig. 9. (a) New LED light module. (b) Commercial LED light module.
Fig. 7. Simulated illumination map and its distribution curve on the target plane at 500 mm away from the LED source package with the MSOS (top) and without MSOS (bottom).
Fig. 10. Measured intensity distribution curve of the (top) new LED light module and (bottom) commercial LED light module.
Fig. 8. Flow chart of constructing the proposed free-form lens.
Fig. 11. Lighting on the target area using (a) new LED light module and (b) commercial LED light module. 10 October 2014 / Vol. 53, No. 29 / APPLIED OPTICS
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Table 1.
Performance Comparison between New LED Light Module and Commercial LED Light Module
Secondary Lens Commercial lens with white holder Proposed free-form lens without white holder
Target Plane 700 500 700 500
Average Illumination 2
mm mm2 mm2 mm2
270.57 lux 447.44 lux 259.5 lux 432.09 lux
profile becomes much smoother after involving the MSOS. In summary, the design method of the novel TIR lens is presented by the flow chart, as shown in Fig. 8. The method can be divided into two parts: design of MSOC and design of MSOS. Generally, there are three steps in MSOC design: determination of the overall sizes (height, radii, thickness), calculation of the curvatures of MSOC surface, and optimization of the curvatures by adjusting parameters based on simulation luminance distribution results and the three functions f 1 x, f 2 x, and f 3 x. After optimizing MSOC to make the light intensity distribution angle as the specification target, MSOS can be constructed with this MSOC after several iterations as follows: constructing initial A and B matrixes, optimizing the shapes and sizes of A and B microlens by modifying the quantity of dome shape lenses and the distance between two consecutive peaks. Next, based on optical software, a Monte Carlo-based algorithm is applied to determine the angle of rotation of the A and B microlenses. Finally, the optical efficiency and illumination distribution are determined and compared with design requirements. After the TIR lens design, we assemble the NICHIA’s NVSW219AT LED and the prototyped new TIR lens as the new LED light module [Fig. 9(a)] to compare the commercial LED light module [Fig. 9(b)] composed by a NICHIA’s NVSW219AT LED, a 30 deg conventional TIR lens and a white holder, which are made by LedLink Co. Ltd. By means of goniophotometer measurement, the intensity distributions, output lumens, and optical efficiency of these LED light modules can be obtained. Results show that the new LED light module achieves 117.7 lm output and 95.69% optical efficiency with the beam angle of 30 deg, as shown in Fig. 10 (top). The commercial LED light module has 117.9 lm output and 95.85% optical efficiency with the beam angle of 30 deg, as shown in Fig. 10 (bottom). Additionally, at 500 mm away from the LED package, we use a Minolta T-10A photometer to measure the LED light module illumination on the 500 mm × 500 mm and 700 mm × 700 mm areas, respectively, as shown in Fig. 11. In this work, the illumination uniformity is defined as the ratio of the minimum illumination to the average illumination. Referring to the experimental results in Table 1, the illumination uniformity can be enhanced to 0.317 from 0.259, which shows the enhancement of illumination uniformity, since the proposed free-form lens can be accomplished. In addition, the proposed H144
APPLIED OPTICS / Vol. 53, No. 29 / 10 October 2014
Minimum Illumination 26.8 115.9 50.5 137
lux lux lux lux
Uniformity
Lumens
Efficiency
0.099 0.259 0.195 0.317
117.9 lm
95.85%
117.7 lm
95.69%
free-form TIR lens can work without a white ring holder and compete with the commercial lens in terms of optical efficiency, as shown in Table 1. 4. Conclusions
Based on Snell’s law, energy conservation law, and a Monte Carlo ray-tracing method, a free-form TIR lens consisting of the MSOC and MSOS is presented and demonstrated in this study. According to experimental results, the free-form TIR lens can result in the even better performance than the commercial lens. Furthermore, the new LED light module using the proposed TIR lens does not need the assistance of the white ring holder, which the conventional secondary TIR lens is typically equipped with. Consequently, with a TIR lens, LED light becomes more competent in price and performance. This work was supported by the National Science Council of the Republic of China, project 102-2622E-151-021-CC2. References 1. R. G. Camacho, S. F. Phua, Z. X. Lin, Y. C. Lo, C. C. Sun, and C. W. Liang, “Beam-shaping system with high-uniformity illumination using power LEDs,” in Proceedings of Optics & Photonics of Taiwan International Conference (OPTIC) (2013), p. 45. 2. C. Seassal and J. Koshel, “Focus issue introduction: renewable energy and the environment,” Opt. Express 21, A430–A432 (2013). 3. H. C. Chen, J. Y. Lin, and H. Y. Chiu, “Rectangular illumination using a secondary optics with cylindrical lens for LED street light,” Opt. Express 21, 3201–3212 (2013). 4. Y. Ding, X. Liu, Z. R. Zheng, and P. F. Gu, “Free-form LED lens for uniform illumination,” Opt. Express 16, 12958–12966 (2008). 5. F. Chen, K. Wang, Z. Qin, D. Wu, X. B. Luo, and S. Liu, “Design method of high-efficient LED headlamp lens,” Opt. Express 18, 20926–20938 (2010). 6. Y. T. Wang, “Free-form optics and its application in headmounted displays,” in Proceedings of Optics & Photonics of Taiwan International Conference (OPTIC) (2013), p. 43. 7. Z. Zhenrong, H. Xiang, and L. Xu, “Free-form surface lens for LED uniform illumination,” Appl. Opt. 48, 6627–6634 (2009). 8. J. J. Chen and C. T. Lin, “Free-form surface design for a lightemitting diode–based collimating lens,” Proc. SPIE 49, 093001 (2010). 9. J. J. Chen, T. Y. Wang, K. L. Huang, T. S. Liu, M. D. Tsai, and C. T. Lin, “Free-form lens design for LED collimating illumination,” Opt. Express 20, 10984–10995 (2012). 10. A. Bauerle, A. Bruneton, R. Wester, J. Stollenwerk, and P. Loosen, “Algorithm for irradiance tailoring using multiple free-form optical surfaces,” Opt. Express 20, 14477–14485 (2012). 11. Y. C. Fang, C. M. Tsai, and C. L. Chung, “A study of optical design and optimization of zoom optics with liquid lenses through modified genetic algorithm,” Opt. Express 19, 16291–16302 (2011).
12. K. Wang, D. Wu, Z. Qin, F. Chen, X. B. Luo, and S. Liu, “New reversing design method for LED uniform illumination,” Opt. Express 19, A830–A840 (2011). 13. B. F. C. D. Albuquerque, J. Sasian, F. L. D. Sousa, and A. S. Montes, “Method of glass selection for color correction in optical system design,” Opt. Express 20, 13592–13611 (2012). 14. R. Winston, J. C. Miñano, and P. Benítez, eds., with contributions by N. Shatz and J. C. Bortz, eds., Nonimaging Optics (Elsevier, 2005).
15. L. Sun, S. Z. Jin, and S. Y. Cen, “Design of free-form microlens for LED general illumination,” Proc. SPIE 7635, 1–10 (2009). 16. H. W. Lee and B. S. Lin, “Improvement of illumination uniformity for LED flat panel light by using micro-secondary lens array,” Opt. Express 20, A788–A798 (2012). 17. K. Wang, S. Liu, F. Chen, Z. Y. Liu, and X. B. Luo, “Effect of manufacturing defects on optical performance of discontinuous free-form lenses,” Opt. Express 17, 5457–5465 (2009). 18. S. Liu and X. B. Luo, LED Packaging for Lighting Applications: Design, Manufacturing and Testing (Wiley, 2011).
10 October 2014 / Vol. 53, No. 29 / APPLIED OPTICS
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Department of Electrical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 80778, Taiwan 2
Department of Industrial Engineering and Management, Minghsin University of Science and Technology, Hsinchu 30401, Taiwan *Corresponding author: [email protected] Received 1 May 2014; revised 28 June 2014; accepted 25 July 2014; posted 25 July 2014 (Doc. ID 211258); published 27 August 2014
A free-form secondary lens is proposed to optimize an LED light. Based on Snell’s law, energy conservation law, and a Monte Carlo ray-tracing algorithm, the surface contour of the free-form lens can be determined according to the requirements of an LED light. Optical experimental results show that an optical efficiency of 95.69% can be achieved by the lens, of which the illumination uniformity 0.317 is higher than the commercial illumination uniformity 0.259. The lens does not need the help of a white ring holder, so the cost of the LED light can become effective. © 2014 Optical Society of America OCIS codes: (230.3670) Light-emitting diodes; (220.0220) Optical design and fabrication; (220.4298) Nonimaging optics; (350.4600) Optical engineering; (220.2945) Illumination design. http://dx.doi.org/10.1364/AO.53.00H140
1. Introduction
LEDs have many advantages for lighting such as high efficiency, long lifetime, and fast response as well as climate impact resistance [1,2]. Thus, an LED has been considered the most promising general illumination solution for the future. However, compared with conventional light sources, such as an incandescent bulb, an LED usually cannot provide uniform illumination by itself. As a result, LEDs need an additional optical device, called a secondary lens, to produce uniform illumination and proper intensity distribution and to keep high light efficiency [3–6]. A TIR (total internal reflection) lens is usually considered in the secondary lens design because it can control light over a wider range of angles better than traditional reflectors or lenses [7–9]. However, if the lens boundary could not afford total internal reflection, then a TIR lens would not efficiently reflect the 1559-128X/14/29H140-06$15.00/0 © 2014 Optical Society of America H140
APPLIED OPTICS / Vol. 53, No. 29 / 10 October 2014
incident light. Therefore, in order to accomplish a TIR lens with high efficiency, at times, a white holder is used to surround the conventional TIR lens that helps to collect the lost light. In this study, we propose a free-form TIR lens without using white holders, capably bringing down the cost for an LED light. A simple and efficient TIR lens design process is introduced instead of using complicated techniques [10–13]. It is demonstrated experimentally that the proposed TIR lens can accomplish a Lambertian LED as the 30 deg spotlight with uniform illumination and high efficiency. 2. Principles
As shown in Fig. 1, the conventional LED light module consists of a secondary lens, white holder, and LED source package. Here we present a new LED light module without using the white holders, in which the secondary lens is a TIR lens with a freeform surface. The lens is a multisegmented optical collimator (MSOC) with a multistructured optical surface (MSOS). The MSOC can accomplish total internal reflection over a wider range of angles of light
B. MSOS Design
Fig. 1. Conventional LED light module consisting of secondary lens, white holder, and LED source package.
than the conventional secondary lens, so that the proposed free-form lens can achieve high optical efficiency. The MSOS design is for homogenizing the output light and attaining the lens with high light efficiency. A.
MSOC Design
The MSOC design is mainly based on total internal reflection. Through optical software simulations, we understand that, while compared with conic reflecting surfaces, the MSOC reflecting surface needs to be segmented into at least three sections to obtain total internal reflection with a wider range of light angles. The relations among the incoming light direction vector I, the optical surface normal vector N, and the outgoing light direction vector O are precisely governed by reflection law. The normal vector N can be calculated by Eq. (1) [14,15]: N
I−O : jI − Oj
(1)
Assuming that the center of the LED source is located at the origin of a Cartesian coordinate system, one incident ray I i from a source point is aimed at the corresponding point Pi to generate the reflected ray Oi. The surface segment is expressed as f i x (i 1, 2, 3), and N i is the surface normal of f i x at Pi . In the MSOC design process, f i x is free to change, so that N i is varied until Oi is in the normal direction of the MSOC output surface, as shown in Fig. 2.
Fig. 2. Two-dimensional ray-tracing plot for the MSOC.
After finishing the MSOC design, the ray paths through the MSOC can be found through Monte Carlo ray tracing, as shown in Fig. 3(a). Without MSOS, the MSOC output is difficult to be uniform. Through a ray-tracing technique, it can be found that the excessive energy on the output plane is mainly from the green region on the emitting surface of the MSOC, as shown in Fig. 3(b). The marked areas are called the “nonoptimized areas.” In the proposed MSOS, microlenses are built on the emitting surface of the MSOC. In order to accomplish high efficiency and high uniformity, the MSOS needs to be arranged according to the energy density on the emitting surface of the MSOC. Thus, more microlenses are built on the “nonoptimized areas” for diverging light. According to the energy conservation law [16], the resulting power of the MSOC on the target surface can be expressed as ZZ A
Ef ds
Q ZZ X i1
Ci
Ei ds Etotal :
(2)
Here A is the uniform illumination region on the target surface, Ef is the illumination on the target surface, Q is the quantity of the cells on the panel plane, Ci is the region of the cell, Ei is the illumination on the cell Ci , and Etotal is the total flux from the source. It is obvious that the smaller the tailored cells, the more uniform the light through the cell. Based on the mathematical principle in Eq. (2), the quantity of a free-form microlens on the nonoptimized areas should be constructed more than that of the other areas. This can result in a significant increase of the emergent angle at the nonoptimized areas. Through the suitable MSOS design, excessive light can be moved to low illumination areas, so as to obtain uniform illumination. 3. Experiments and Experimental Results
The main experimental work to accomplish the proposed free-form TIR lens includes the following:
Fig. 3. (a) Schematic ray paths through the MSOC. (b) MSOC marked region; the output excessive energy is from there. 10 October 2014 / Vol. 53, No. 29 / APPLIED OPTICS
H141
1. Building the precise mechanical structures and the optical properties of the LED source. 2. Optimizing the proposed free-form TIR lens design, making the prototype sample, and measuring its performances. 3. Comparing the performances between the proposed free-form TIR lens and the commercial secondary lens in the illumination uniformity and optical efficiency. In the experiments, a NICHIA’s white Lambertian LED: NVSW219AT is used as the light source. We build the LED physical model for the proposed TIR lens design. The physical model of the white LED is shown in Fig. 4. The phosphor layer, with a thickness of 0.05 mm, covers the LED chip. The blue chip with a square base of 1 mm and a height of 0.1 mm is covered by a silicone dome lens. The blue chip is set at 1 W output power and has a peak wavelength of 453 nm. The material of the free-form lens is selected as polymethyl methacrylate (PMMA), and its refractive index is 1.4935. We use TracePro software to trace 10 million light rays randomly through the LED model for optical simulations of the freeform TIR lens design. Referring to the MSOC ray-tracing plot in Fig. 2, the incident light vectors I 1, I 2 , and I 3 are emitted from the LED source and, in turn, meet the reflective surface at three points P1, P2 , and P3 . Based on finding the normal vector N i to have Oi collimated after total internal reflection, the 2D contour functions f 1 x, f 2 x, and f 3 x are adjusted freely to establish 30 deg of intensity distribution under the help of SolidWorks and TracePro software. The MSOC structure can thus be obtained by rotating the 2D contour about the MSOC optical axis. The bottom and top radii of the MSOC are 2.53 and 12 mm, and its height is about 10.73 mm, as shown in Fig. 5. The accomplished MSOC can be used without an additional white ring holder. After considering the molding precision limitation and the yield rate of the MSOS in mass production [17,18], and performing a local searching method based on a damping least-squares algorithm, the dome lens with 1 mm diameter and 0.17 mm height is selected to compose the MSOS. The distance between two consecutive peaks of the dome lenses is chosen as 0.5 mm. The MSOS is formed by rotating two different groups of the dome lenses, called A
Fig. 4. Schematic illustration of the physical model of NICHIA’s white Lambertian LED: NVSW219AT. H142
APPLIED OPTICS / Vol. 53, No. 29 / 10 October 2014
Fig. 5. MSOC design drawing.
(silver) and B (green) microlens matrices, as shown in Fig. 6 (top). To construct the MSOS, the A and B matrix microlenses are rotated and then duplicated. Using the global optimization function of optical design software, it is found that the A and B matrix need to spin 9 and 30 deg for each rotation, respectively. After 360 deg rotating, the whole generated dome lenses are grown as the MSOS (Fig. 6, bottom). The illumination map on the target plane at 500 mm away corresponds to both cases of MSOC, with and without MSOS, as shown in Fig. 7 (top) and (bottom). It can be observed that the illumination
Fig. 6. (Top) Top view of A and B microlens matrices. (Bottom) MSOS design drawing.
Fig. 9. (a) New LED light module. (b) Commercial LED light module.
Fig. 7. Simulated illumination map and its distribution curve on the target plane at 500 mm away from the LED source package with the MSOS (top) and without MSOS (bottom).
Fig. 10. Measured intensity distribution curve of the (top) new LED light module and (bottom) commercial LED light module.
Fig. 8. Flow chart of constructing the proposed free-form lens.
Fig. 11. Lighting on the target area using (a) new LED light module and (b) commercial LED light module. 10 October 2014 / Vol. 53, No. 29 / APPLIED OPTICS
H143
Table 1.
Performance Comparison between New LED Light Module and Commercial LED Light Module
Secondary Lens Commercial lens with white holder Proposed free-form lens without white holder
Target Plane 700 500 700 500
Average Illumination 2
mm mm2 mm2 mm2
270.57 lux 447.44 lux 259.5 lux 432.09 lux
profile becomes much smoother after involving the MSOS. In summary, the design method of the novel TIR lens is presented by the flow chart, as shown in Fig. 8. The method can be divided into two parts: design of MSOC and design of MSOS. Generally, there are three steps in MSOC design: determination of the overall sizes (height, radii, thickness), calculation of the curvatures of MSOC surface, and optimization of the curvatures by adjusting parameters based on simulation luminance distribution results and the three functions f 1 x, f 2 x, and f 3 x. After optimizing MSOC to make the light intensity distribution angle as the specification target, MSOS can be constructed with this MSOC after several iterations as follows: constructing initial A and B matrixes, optimizing the shapes and sizes of A and B microlens by modifying the quantity of dome shape lenses and the distance between two consecutive peaks. Next, based on optical software, a Monte Carlo-based algorithm is applied to determine the angle of rotation of the A and B microlenses. Finally, the optical efficiency and illumination distribution are determined and compared with design requirements. After the TIR lens design, we assemble the NICHIA’s NVSW219AT LED and the prototyped new TIR lens as the new LED light module [Fig. 9(a)] to compare the commercial LED light module [Fig. 9(b)] composed by a NICHIA’s NVSW219AT LED, a 30 deg conventional TIR lens and a white holder, which are made by LedLink Co. Ltd. By means of goniophotometer measurement, the intensity distributions, output lumens, and optical efficiency of these LED light modules can be obtained. Results show that the new LED light module achieves 117.7 lm output and 95.69% optical efficiency with the beam angle of 30 deg, as shown in Fig. 10 (top). The commercial LED light module has 117.9 lm output and 95.85% optical efficiency with the beam angle of 30 deg, as shown in Fig. 10 (bottom). Additionally, at 500 mm away from the LED package, we use a Minolta T-10A photometer to measure the LED light module illumination on the 500 mm × 500 mm and 700 mm × 700 mm areas, respectively, as shown in Fig. 11. In this work, the illumination uniformity is defined as the ratio of the minimum illumination to the average illumination. Referring to the experimental results in Table 1, the illumination uniformity can be enhanced to 0.317 from 0.259, which shows the enhancement of illumination uniformity, since the proposed free-form lens can be accomplished. In addition, the proposed H144
APPLIED OPTICS / Vol. 53, No. 29 / 10 October 2014
Minimum Illumination 26.8 115.9 50.5 137
lux lux lux lux
Uniformity
Lumens
Efficiency
0.099 0.259 0.195 0.317
117.9 lm
95.85%
117.7 lm
95.69%
free-form TIR lens can work without a white ring holder and compete with the commercial lens in terms of optical efficiency, as shown in Table 1. 4. Conclusions
Based on Snell’s law, energy conservation law, and a Monte Carlo ray-tracing method, a free-form TIR lens consisting of the MSOC and MSOS is presented and demonstrated in this study. According to experimental results, the free-form TIR lens can result in the even better performance than the commercial lens. Furthermore, the new LED light module using the proposed TIR lens does not need the assistance of the white ring holder, which the conventional secondary TIR lens is typically equipped with. Consequently, with a TIR lens, LED light becomes more competent in price and performance. This work was supported by the National Science Council of the Republic of China, project 102-2622E-151-021-CC2. References 1. R. G. Camacho, S. F. Phua, Z. X. Lin, Y. C. Lo, C. C. Sun, and C. W. Liang, “Beam-shaping system with high-uniformity illumination using power LEDs,” in Proceedings of Optics & Photonics of Taiwan International Conference (OPTIC) (2013), p. 45. 2. C. Seassal and J. Koshel, “Focus issue introduction: renewable energy and the environment,” Opt. Express 21, A430–A432 (2013). 3. H. C. Chen, J. Y. Lin, and H. Y. Chiu, “Rectangular illumination using a secondary optics with cylindrical lens for LED street light,” Opt. Express 21, 3201–3212 (2013). 4. Y. Ding, X. Liu, Z. R. Zheng, and P. F. Gu, “Free-form LED lens for uniform illumination,” Opt. Express 16, 12958–12966 (2008). 5. F. Chen, K. Wang, Z. Qin, D. Wu, X. B. Luo, and S. Liu, “Design method of high-efficient LED headlamp lens,” Opt. Express 18, 20926–20938 (2010). 6. Y. T. Wang, “Free-form optics and its application in headmounted displays,” in Proceedings of Optics & Photonics of Taiwan International Conference (OPTIC) (2013), p. 43. 7. Z. Zhenrong, H. Xiang, and L. Xu, “Free-form surface lens for LED uniform illumination,” Appl. Opt. 48, 6627–6634 (2009). 8. J. J. Chen and C. T. Lin, “Free-form surface design for a lightemitting diode–based collimating lens,” Proc. SPIE 49, 093001 (2010). 9. J. J. Chen, T. Y. Wang, K. L. Huang, T. S. Liu, M. D. Tsai, and C. T. Lin, “Free-form lens design for LED collimating illumination,” Opt. Express 20, 10984–10995 (2012). 10. A. Bauerle, A. Bruneton, R. Wester, J. Stollenwerk, and P. Loosen, “Algorithm for irradiance tailoring using multiple free-form optical surfaces,” Opt. Express 20, 14477–14485 (2012). 11. Y. C. Fang, C. M. Tsai, and C. L. Chung, “A study of optical design and optimization of zoom optics with liquid lenses through modified genetic algorithm,” Opt. Express 19, 16291–16302 (2011).
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