Novel optical lens design with a light scattering freeform inner surface for LED down light illumination Raychiy J. Lin, Ming-Shiou Tsai, and Ching-Cherng Sun* Department of Optics and Photonics, National Central University, Chung-Li 320, Taiwan * [email protected]
Abstract: Using the precise mid field angular distribution model of the LED light source and the optical scattering surface property of the Harvey BSDF scattering model, we perform optical simulation to develop and design a novel solid plastic optical lens with freeform inner surface to achieve the optical performance of 89.89% light energy transmission optics for the sake of energy saving reason. The 70 degrees angular light distribution pattern defined at full width half maximum (FWHM) light energy level is performed and the optical utilization factor (OUF) of 51.8% is obtained within −45 degrees and 45 degrees range in the areas of interest with glare reduced for the down light illumination. ©2015 Optical Society of America OCIS codes: (230.3670) Light-emitting diodes; (080.4295) Nonimaging optical systems; (150.2945) Illumination design; (220.0220) Optical design and fabrication; (150.2950) Illumination.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
D. A. Steigerwald, J. C. Bhat, D. Collins, R. M. Fletcher, M. O. Holcomb, M. J. Ludowise, P. S. Martin, and S. L. Rudaz, “Illumination with solid state lighting technology,” IEEE J. Sel. Top. Quantum Electron. 8(2), 310– 320 (2002). A. Zukauskas, M. S. Shur, and R. Caska, Introduction to Solid-State Lighting (John Wiley & Sons, 2002). F. Nguyen, B. Terao, and J. Laski, “Realizing LED illumination lighting applications,” Proc. SPIE 5941, 594105 (2005). J. Jiang, S. To, W. B. Lee, and B. Cheung, “Optical design of a freeform TIR lens for LED streetlight,” Optik (Stuttg.) 121(19), 1761–1765 (2010). Y. Luo, Z. Feng, Y. Han, and H. Li, “Design of compact and smooth free-form optical system with uniform illuminance for LED source,” Opt. Express 18(9), 9055–9063 (2010). K. Wang, D. Wu, Z. Qin, F. Chen, X. Luo, and S. Liu, “New reversing design method for LED uniform illumination,” Opt. Express 19(S4 Suppl 4), A830–A840 (2011). H. Wen and L. B.-S. Lin, “Improvement of illumination uniformity for LED flat panel light by using microsecondary lens array,” Opt. Express 20(S6), A788–A798 (2012). Y. S. Chen, C. Y. Lin, C. M. Yeh, C. T. Kuo, C. W. Hsu, and H. C. Wang, “Anti-glare LED lamps with adjustable illumination light field,” Opt. Express 22(5), 5183–5195 (2014). C. W. Chiang, Y. K. Hsu, and J. W. Pan, “Design and demonstration of high efficiency anti-glare LED luminaires for indoor lighting,” Opt. Express 23(3), A15–A26 (2015). T. Kasahara, D. Aizawa, T. Irikura, T. Moriyama, M. Toda, and M. Iwamoto, “Discomfort glare caused by white LED light sources,” J. Light Visual Environ. 30(2), 95–103 (2006). T. Tashiro, S. Kawanobe, T. K. Minoda, S. Kohko, T. Ishikawa, and M. Ayama, “Discomfort glare for white LED light sources with different spatial arrangements,” Lighting Res. Tech. 0, 1–22 (2014). X. H. Lee, J. L. Tsai, S. H. Ma, and C. C. Sun, “Surface-structured diffuser by iterative down-size molding with glass sintering technology,” Opt. Express 20(6), 6135–6145 (2012). W. R. Ryckaert, K. A. G. Smet, I. A. A. Roelandts, M. Van Gils, and P. Hanselaer, “Linear LED tubes versus fluorescent lamps: an evaluation,” Energy Build. 49, 429–436 (2012). S. Song, Y. Sun, Y. Lin, and B. You, “A facile fabrication of light diffusing film with LDP/polyacrylates composites coating for anti-glare LED application,” Appl. Surf. Sci. 273, 652–660 (2013). C. C. Sun, W. T. Chien, I. Moreno, C. T. Hsieh, M. C. Lin, S. L. Hsiao, and X. H. Lee, “Calculating model of light transmission efficiency of diffusers attached to a lighting cavity,” Opt. Express 18(6), 6137–6148 (2010). X. H. Lee, I. Moreno, and C. C. Sun, “High-performance LED street lighting using microlens arrays,” Opt. Express 21(9), 10612–10621 (2013).
#241170 (C) 2015 OSA
Received 18 May 2015; revised 10 Jun 2015; accepted 11 Jun 2015; published 16 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016715 | OPTICS EXPRESS 16715
17. Y. C. Lo, J. Y. Cai, M. S. Tasi, Z. Y. Tasi, and C. C. Sun, “Side-illuminating LED luminaires with accurate projection in high uniformity and high optical utilization factor for large-area field illumination,” Opt. Express 22(S2), A365–A375 (2014). 18. C. C. Sun, Y. Y. Chang, T. H. Yang, T. Y. Chung, C. C. Chen, T. X. Lee, D. R. Li, C. Y. Lu, Z. Y. Ting, B. Glorieux, Y. C. Chen, K. Y. Lai, and C. Y. Liu, “Packaging efficiency in phosphor-converted white LEDs and its impact to the limit of luminous efficacy,” J. Solid State Light. 1(19), 1–17 (2014). 19. Prolight Opto Technology Corporation, http://www.prolightopto.com/. 20. C. C. Sun, T. X. Lee, S. H. Ma, Y. L. Lee, and S. M. Huang, “Precise optical modeling for LED lighting verified by cross correlation in the midfield region,” Opt. Lett. 31(14), 2193–2195 (2006). 21. J. W. Goodman, Introduction to Fourier Optics, 2nd ed. (McGraw-Hill, 1996). 22. Breault Research Organization, http://www.breault.com/. 23. K. J. Garcia, “Non-rational and rational parametric descriptions of the geometric propagation of light in an optical system,”Doctoral Dissertation, University of Arizona (1999). 24. M. G. Turner and K. J. Garcia, “Optimization using rational Bézier control points and weighting factors,” Proc. SPIE 7061, 70610H (2008). 25. J. C. Stover, Optical Scattering Measurement and Analysis (McGraw-Hill, 1990). 26. Y. W. Yu, Y. L. Chen, W. H. Chen, H. X. Chen, X. H. Lee, C. C. Lin, and C. C. Sun, “Bidirectional scattering distribution function by screen imaging synthesis,” Opt. Express 20(2), 1268–1280 (2012). 27. J. E. Harvey, Light-scattering characteristics of optical surfaces, Ph.D. Dissertation, University of Arizona (1976). 28. H. Opsira Gmb, http://www.opsira.de/. 29. J. E. Harvey, “Light scattering characteristics of optical surfaces,” Stray Light Instrum. Eng. 107, 41–47 (1977).
1. Introduction The invention of Light Emitting Diode (LED) has been changing the ways of thinking in terms of the general illumination designs and applications for the human beings. It has also brought the great advantage and provided the great solution in terms of both energy saving and environment protection for nowadays and the future [1–3]. Since the LED light emitting characteristic is strongly direction oriented and could be possibly causing the strong glare effect, the optical design of a LED lens is necessary for improving the lighting energy distribution in the areas of interest and reducing the uncomfortable glare effect for the general illumination applications. Nowadays, the method of freeform optical surface design has been extensively adopted for the glass or plastic optical components in the general lighting optical systems. It utilizes optical characteristics of reflection, refraction, and totally internal reflection (TIR) to accurately direct the rays to reach the areas of interest among different media [4–7]. For certain lighting applications with specific light energy distribution requests, it normally requires to allocate certain amount of the light energy to be within certain solid angles or to illuminate on certain areas, and this could possibly cause the strong glare effect due to the high energy level concentration [8–11]. For decreasing the glare effect, typically a white color diffuser cover would be added on the top of the LED light sources to diffuse the concentrated light energy to reduce the strong glare [12–15]. With this type of white color diffuser cover used, a certain part of the light energy could be reflected off the cover, and then trapped and absorbed inside of the diffuser cover. It would actually waste the light energy and decrease the optical efficacy of the LED lighting device if photon recycling is not effective [15]. In this paper, the brand new concept and idea for a transparent plastic optical lens design used for the LED down light illumination application would be introduced. A type of optical transparent plastic lens would be designed by using the freeform surface and scattering surface technologies. This plastic transparent optical lens would be consisting of both a free from and scattering inner surface and a normal spherical shape outer surface. The concept of adopting the transparent optical lens for LED illumination is to get the most LED light energy transmitted through the optical system for the purpose of the energy saving in terms of both the electrical energy consumption saving and the electrical bill costs saving. The idea of developing a free from inner surface of this transparent optical lens is to get the most LED light energy to be illuminated in the areas of interest. For the light energy utilization rating of a lighting device, the optical utilization factor (OUF) would be used for the evaluation [16– 18]. The idea of designing an optical scattering surface on the top of the freeform inner
#241170 (C) 2015 OSA
Received 18 May 2015; revised 10 Jun 2015; accepted 11 Jun 2015; published 16 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016715 | OPTICS EXPRESS 16716
surface of this transparent optical lens is to scatter the strongly direction oriented LED light to be a more evenly distributed light pattern in the areas of interest and reduce the glare caused by the LED light sources while taking into account the goal of the angular distribution pattern to be reached at the same time. 2. Optical modeling In general, a precise LED optical lighting system design would require at least a precise LED light source model with an accurate angular light distribution pattern and optical component models with accurate optical surface and volume properties. First of all, a precise light source model would need to be built up. The 1 watt LED light source with angular light distribution of 130 degrees, model PM2B-1LWE-R8, manufactured by ProLight Opto Technology Corporation [19] was used for this optical system lighting design as shown in Fig. 1. A precise optical light source model was the first step required for resulting a precise optical lighting system performance in the end. Normally, each different LED light source would have its own different specific angular light distribution pattern because of its own unique chip structure, packaging structure, and etc. The transparent optical lens would be installed very close to the top of LED light sources, but this location is neither within the near field region nor within the far field region. The angular light distribution pattern of a LED light source in the mid field region described by a precise optical modeling algorithm [20] was used for the Monte Carlo simulation for the precise optical lighting system design. The mid field region was defined between the near field region and the far field region (Fraunhofer region) [21]. The mid field ranges over the region within the 10 times of the lateral dimension of the LED light source. Figure 2 shows the measured angular light distribution patterns of the LED light source at different locations of 1.5cm, 3cm, 5cm, and 20cm ranging from the mid field region to the far field region.
Fig. 1. (a) The LED light source model PM2B-1LWE-R8. (b) The LED PC board module.
The precise mid field angular light distribution pattern of the above LED light source was established and modeled by the optical simulation program ASAP [22] to coincide with both the mid field and far field angular light distribution patterns measured in the optics laboratory as shown in Fig. 2.
#241170 (C) 2015 OSA
Received 18 May 2015; revised 10 Jun 2015; accepted 11 Jun 2015; published 16 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016715 | OPTICS EXPRESS 16717
Fig. 2. The angular light distribution patterns of LED light source ranging from the mid field region to the far field region.
The inner optical surface of this plastic transparent optical lens was consisting of 36 freeform surfaces which were designed by using rational Bézier mathematics [23,24]. The rational form of the Bézier equation is
P(t)=
Pj w j Bn j ( t )
. w B (t) n
j
(1)
j
where Wj are the weighting factors for each of the control points Pj. Normally, the optical development and design of those optical components with complicated geometry would be performed by using the computer aided design (CAD) to have the CAD geometrical representation for the optical illumination system applications. In this inner optical surface design, each freeform surface was designed by using rational Bézier control points and weighting factors as variables. The non-uniform rational B-splines (NURBs) using the Bézier mathematics have been used in the CAD field extensively. Therefore developing the freeform optical surfaces by using the Bézier representations of optical surface data, we could easily parameterize the freeform optical surfaces with some control points and weighting factors to allow for surface shape manipulations and adjustments to reach the energy distribution goals of the illumination system requirements. Besides the freeform surface developed on the inner surface of this optical transparent lens, the optical scattering surface property was also designed and applied on the top of the optical freeform inner surface to achieve the more evenly light energy distribution in the areas of interest with glare reduced. Light scattering happens at all the optical surfaces. The characteristics of the scattering vary with the different optical surface materials. It would change the directions of the incident ray angles, and the incident rays would be scattered off the optical surface in all directions. The characteristics of the optical scattering surface are defined by BSDF scattering functions [25,26]. In this paper, all optical scattering models are defined in terms of a bidirectional scattering distribution function (BSDF). The bidirectional scattering distribution function (BSDF) consists of both the bidirectional reflection distribution function (BRDF) and the bidirectional transmission distribution function (BTDF). Since the materials of this transparent optical lens are made out of plastic materials like acrylic, PC or PS plastic, the optical scattering property models could be accurately described with Harvey BSDF functions [27]. The BSDF function is equal to L/E where L is the scattered radiance and E is the incident irradiance. The BSDF function is a four dimensional function which depends on the scatter direction (θ, φ) and the specular direction (θ0, φ0). The Harvey BSDF equation is s
sin θ 2 2 BSDF=b0 1+ . l
#241170 (C) 2015 OSA
(2)
Received 18 May 2015; revised 10 Jun 2015; accepted 11 Jun 2015; published 16 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016715 | OPTICS EXPRESS 16718
where b0 is BSDF at θ is zero, s is the slope of BSDF, and l is the shoulder parameter (radians) of BSDF. The optical surface scattering characteristics of the inner surface of this transparent optical lens were precisely measured by using the professional BSDF measurement instrument [28] made by Opsira GmbH located in Germany. The seven curves with the incident angles varying from 0 to 60 degrees almost completely coincided to form a single curve as shown in Fig. 3 and Fig. 4. It indicates that the scattering property of this optical surface is shift invariant, so the scattering properties could be completely characterized by a single set of measurements at a fixed angle of incidence. This shift invariant behavior characterizes the scattering properties of the optical surface completely [29].
Fig. 3. (a) The measured 7 BRDF curves. (b) The 7 BRDF curves coincided to form a single curve.
Fig. 4. (a) The measured 7 BTDF curves. (b) The 7 BTDF curves coincided to form a single curve.
Before building any optical components by using any tooling or molding, the entire three dimensions optical model was simulated thoroughly by using the Monte Carlo simulation executed by ASAP. ASAP is a non-sequential ray tracing program which traces the rays in the three dimensional space and simulates the interaction of light sources with both optical and mechanical components for the entire optical system. 3. Optical design and verification
The task was to develop and design a plastic optical lens to be applied in the LED down light illumination. For designing this plastic optical lens, there were three design goals to be reached under the condition of a total of 4 watts optical power output for the down light with each LED light source to be operated at 3.3V and 350mA level. The three design goals to be reached were the light energy transmission at 80% of the optics for the sake of energy saving reason, the 70 ± 3 degrees angular light distribution pattern defined at full width half maximum (FWHM) light energy level, and the optical utilization factor (OUF) of 50% within the −45 degrees and 45 degrees range with glare reduced. The plastic optical lens was designed to be a transparent optical lens and was consisting of a freeform inner surface and a
#241170 (C) 2015 OSA
Received 18 May 2015; revised 10 Jun 2015; accepted 11 Jun 2015; published 16 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016715 | OPTICS EXPRESS 16719
spherical shape outer surface, and then the freeform inner surface was designed to consist of 36 freeform surfaces. Figure 5 shows the front view and the back view of this transparent optical lens designed and constructed by ASAP.
Fig. 5. (a) Front view of the transparent optical lens, and (b) back view of the transparent optical lens.
Then, these 36 freeform surfaces were to be adjusted, manipulated and designed to form an inner surface combining with a spherical shape outer surface to reach the design goals of the 70 ± 3 degrees angular light distribution pattern defined at full width half maximum (FWHM) light energy level and the 80% of the light energy transmission of optics. At the same time, the optical scattering surface property applied on the freeform inner surface also needed to be taken into account for reaching the above design goals because the optical scattering surface property would generate a diffuser effect to influence upon both the shape of the angular distribution pattern and the LED glare reduction. The more the diffusing power would be enhanced on the freeform inner surface, the more the LED glare would be reduced, but less of the accuracy of the angular light distribution could be controlled. For this plastic optical lens design, the scattering surface property of the inner surface consisted of the Harvey BRDF scattering function with b0 = 1.414, s = −4.91, and l = 0.17 and the Harvey BTDF scattering function with b0 = 63.505, s = −4.96, and l = 0.08. The idea was that with these appropriate optical scattering surface property Harvey BSDF models designed, the accuracy of the angular light distribution could be controlled and the LED glare could be reduced. After the completion of the optical simulation of this plastic optical lens development and design, the prototype of this plastic optical lens was produced by the precision molds and injection molding technologies. Figure 6 shows the prototype of this plastic optical lens. The plastic optical lens of down light was then measured in the optics laboratory for the light energy transmission rate. Without the plastic optical lens installed on the LED PC board module, the optical flux was measured to be 455 lumens, and the optical flux value was measured to be 409 lumens with the plastic optical lens installed. The value of 89.89% transmission rate for this plastic optical lens was calculated. This means that almost 90% transmission rate of this plastic optical lens was achieved. Comparing to the general design of the LED lighting devices that it simply covers the LED PC board module with a white color diffuser cover which basically would block the 25% on an average of the LED light emitting energy to be transmitted through the layer of diffuser cover to reach the areas of interest, the design of this plastic optical lens would be doing a much better job in terms of the energy saving.
#241170 (C) 2015 OSA
Received 18 May 2015; revised 10 Jun 2015; accepted 11 Jun 2015; published 16 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016715 | OPTICS EXPRESS 16720
Fig. 6. (a) Front view of the plastic optical lens, and (b) back view of the plastic optical lens.
The angular light distribution pattern of this plastic optical lens was also measured in the optics laboratory. Figure 7 shows the complete assembly down light with this plastic optical lens installed on the LED PC board module. The angular light distribution pattern of this plastic optical lens was measured, and the 70 degrees angular light distribution pattern defined at full width half maximum (FWHM) light energy level was measured and reached as shown in Fig. 8. The angular light distribution measured and the data calculated also showed to achieve the optical utilization factor (OUF) of 51.8% within −45 degrees and 45 degrees range. It was 10.5% more light energy utilized while comparing to the popular brand LED down light with 41.3% optical utilization factor (OUF) measured and calculated in the same angular distribution range. Figure 8(a) shows the measured polar plot of angular light distribution pattern. Figure 8(b) shows that the curve of the measured intensity plot and the curve of the optical simulation intensity plot are almost coincided completely. In another words, this means that the experimental measurement data and the theoretical design data successfully matched with each other.
Fig. 7. The down light with the plastic optical lens installed on the LED PC board module.
4. Summary
In this paper, a novel transparent optical lens design with an optical scattering freeform inner surface has been presented and demonstrated. The precise mid field angular distribution model of the LED light source and the optical scattering surface property of the Harvey BSDF scattering model were built and measured. With the above precise LED mid field angular distribution model and the Harvey BSDF scattering model, the freeform inner surface and the spherical outer surface of the plastic optical lens were developed and designed, and the performance of the entire optical system of the down light was evaluated by the optical simulation. Then, the prototype of the plastic optical lens was produced by precision molds and injection molding technologies. Finally, the performance of the down light optical system was obtained by the accurately optical measurements. And the mission was completed successfully to reach and surpass the design goals of the light energy transmission at almost
#241170 (C) 2015 OSA
Received 18 May 2015; revised 10 Jun 2015; accepted 11 Jun 2015; published 16 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016715 | OPTICS EXPRESS 16721
90% of the optics, the 70 degrees angular light distribution pattern defined at full width half maximum (FWHM) light energy level, and the optical utilization factor (OUF) of 51.8% within −45 degrees and 45 degrees range with glare reduced. For the light energy utilization rating, it was 10.5% more light energy utilized while comparing to the popular brand LED down light with optical utilization factor (OUF) of 41.3% in the same angular distribution range. Besides the above optics quality reached, at the same time, it also offered the aesthetic quality for this transparent LED optical lens to be crystal looking in both the lit and unlit states.
Fig. 8. (a) The polar plot of angular light distribution pattern. (b) The optical simulation intensity curve almost coincide with the measurement intensity curve.
Acknowledgments
The author would like to thank Yi Jin Photonik Co., Ltd. and Chin-Yi Chen for the support of this research and also thank the sponsor of the Ministry of Science and Technology of Taiwan under the contract MOST 103-2221-E008-063-MY3.
#241170 (C) 2015 OSA
Received 18 May 2015; revised 10 Jun 2015; accepted 11 Jun 2015; published 16 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016715 | OPTICS EXPRESS 16722
Abstract: Using the precise mid field angular distribution model of the LED light source and the optical scattering surface property of the Harvey BSDF scattering model, we perform optical simulation to develop and design a novel solid plastic optical lens with freeform inner surface to achieve the optical performance of 89.89% light energy transmission optics for the sake of energy saving reason. The 70 degrees angular light distribution pattern defined at full width half maximum (FWHM) light energy level is performed and the optical utilization factor (OUF) of 51.8% is obtained within −45 degrees and 45 degrees range in the areas of interest with glare reduced for the down light illumination. ©2015 Optical Society of America OCIS codes: (230.3670) Light-emitting diodes; (080.4295) Nonimaging optical systems; (150.2945) Illumination design; (220.0220) Optical design and fabrication; (150.2950) Illumination.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
D. A. Steigerwald, J. C. Bhat, D. Collins, R. M. Fletcher, M. O. Holcomb, M. J. Ludowise, P. S. Martin, and S. L. Rudaz, “Illumination with solid state lighting technology,” IEEE J. Sel. Top. Quantum Electron. 8(2), 310– 320 (2002). A. Zukauskas, M. S. Shur, and R. Caska, Introduction to Solid-State Lighting (John Wiley & Sons, 2002). F. Nguyen, B. Terao, and J. Laski, “Realizing LED illumination lighting applications,” Proc. SPIE 5941, 594105 (2005). J. Jiang, S. To, W. B. Lee, and B. Cheung, “Optical design of a freeform TIR lens for LED streetlight,” Optik (Stuttg.) 121(19), 1761–1765 (2010). Y. Luo, Z. Feng, Y. Han, and H. Li, “Design of compact and smooth free-form optical system with uniform illuminance for LED source,” Opt. Express 18(9), 9055–9063 (2010). K. Wang, D. Wu, Z. Qin, F. Chen, X. Luo, and S. Liu, “New reversing design method for LED uniform illumination,” Opt. Express 19(S4 Suppl 4), A830–A840 (2011). H. Wen and L. B.-S. Lin, “Improvement of illumination uniformity for LED flat panel light by using microsecondary lens array,” Opt. Express 20(S6), A788–A798 (2012). Y. S. Chen, C. Y. Lin, C. M. Yeh, C. T. Kuo, C. W. Hsu, and H. C. Wang, “Anti-glare LED lamps with adjustable illumination light field,” Opt. Express 22(5), 5183–5195 (2014). C. W. Chiang, Y. K. Hsu, and J. W. Pan, “Design and demonstration of high efficiency anti-glare LED luminaires for indoor lighting,” Opt. Express 23(3), A15–A26 (2015). T. Kasahara, D. Aizawa, T. Irikura, T. Moriyama, M. Toda, and M. Iwamoto, “Discomfort glare caused by white LED light sources,” J. Light Visual Environ. 30(2), 95–103 (2006). T. Tashiro, S. Kawanobe, T. K. Minoda, S. Kohko, T. Ishikawa, and M. Ayama, “Discomfort glare for white LED light sources with different spatial arrangements,” Lighting Res. Tech. 0, 1–22 (2014). X. H. Lee, J. L. Tsai, S. H. Ma, and C. C. Sun, “Surface-structured diffuser by iterative down-size molding with glass sintering technology,” Opt. Express 20(6), 6135–6145 (2012). W. R. Ryckaert, K. A. G. Smet, I. A. A. Roelandts, M. Van Gils, and P. Hanselaer, “Linear LED tubes versus fluorescent lamps: an evaluation,” Energy Build. 49, 429–436 (2012). S. Song, Y. Sun, Y. Lin, and B. You, “A facile fabrication of light diffusing film with LDP/polyacrylates composites coating for anti-glare LED application,” Appl. Surf. Sci. 273, 652–660 (2013). C. C. Sun, W. T. Chien, I. Moreno, C. T. Hsieh, M. C. Lin, S. L. Hsiao, and X. H. Lee, “Calculating model of light transmission efficiency of diffusers attached to a lighting cavity,” Opt. Express 18(6), 6137–6148 (2010). X. H. Lee, I. Moreno, and C. C. Sun, “High-performance LED street lighting using microlens arrays,” Opt. Express 21(9), 10612–10621 (2013).
#241170 (C) 2015 OSA
Received 18 May 2015; revised 10 Jun 2015; accepted 11 Jun 2015; published 16 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016715 | OPTICS EXPRESS 16715
17. Y. C. Lo, J. Y. Cai, M. S. Tasi, Z. Y. Tasi, and C. C. Sun, “Side-illuminating LED luminaires with accurate projection in high uniformity and high optical utilization factor for large-area field illumination,” Opt. Express 22(S2), A365–A375 (2014). 18. C. C. Sun, Y. Y. Chang, T. H. Yang, T. Y. Chung, C. C. Chen, T. X. Lee, D. R. Li, C. Y. Lu, Z. Y. Ting, B. Glorieux, Y. C. Chen, K. Y. Lai, and C. Y. Liu, “Packaging efficiency in phosphor-converted white LEDs and its impact to the limit of luminous efficacy,” J. Solid State Light. 1(19), 1–17 (2014). 19. Prolight Opto Technology Corporation, http://www.prolightopto.com/. 20. C. C. Sun, T. X. Lee, S. H. Ma, Y. L. Lee, and S. M. Huang, “Precise optical modeling for LED lighting verified by cross correlation in the midfield region,” Opt. Lett. 31(14), 2193–2195 (2006). 21. J. W. Goodman, Introduction to Fourier Optics, 2nd ed. (McGraw-Hill, 1996). 22. Breault Research Organization, http://www.breault.com/. 23. K. J. Garcia, “Non-rational and rational parametric descriptions of the geometric propagation of light in an optical system,”Doctoral Dissertation, University of Arizona (1999). 24. M. G. Turner and K. J. Garcia, “Optimization using rational Bézier control points and weighting factors,” Proc. SPIE 7061, 70610H (2008). 25. J. C. Stover, Optical Scattering Measurement and Analysis (McGraw-Hill, 1990). 26. Y. W. Yu, Y. L. Chen, W. H. Chen, H. X. Chen, X. H. Lee, C. C. Lin, and C. C. Sun, “Bidirectional scattering distribution function by screen imaging synthesis,” Opt. Express 20(2), 1268–1280 (2012). 27. J. E. Harvey, Light-scattering characteristics of optical surfaces, Ph.D. Dissertation, University of Arizona (1976). 28. H. Opsira Gmb, http://www.opsira.de/. 29. J. E. Harvey, “Light scattering characteristics of optical surfaces,” Stray Light Instrum. Eng. 107, 41–47 (1977).
1. Introduction The invention of Light Emitting Diode (LED) has been changing the ways of thinking in terms of the general illumination designs and applications for the human beings. It has also brought the great advantage and provided the great solution in terms of both energy saving and environment protection for nowadays and the future [1–3]. Since the LED light emitting characteristic is strongly direction oriented and could be possibly causing the strong glare effect, the optical design of a LED lens is necessary for improving the lighting energy distribution in the areas of interest and reducing the uncomfortable glare effect for the general illumination applications. Nowadays, the method of freeform optical surface design has been extensively adopted for the glass or plastic optical components in the general lighting optical systems. It utilizes optical characteristics of reflection, refraction, and totally internal reflection (TIR) to accurately direct the rays to reach the areas of interest among different media [4–7]. For certain lighting applications with specific light energy distribution requests, it normally requires to allocate certain amount of the light energy to be within certain solid angles or to illuminate on certain areas, and this could possibly cause the strong glare effect due to the high energy level concentration [8–11]. For decreasing the glare effect, typically a white color diffuser cover would be added on the top of the LED light sources to diffuse the concentrated light energy to reduce the strong glare [12–15]. With this type of white color diffuser cover used, a certain part of the light energy could be reflected off the cover, and then trapped and absorbed inside of the diffuser cover. It would actually waste the light energy and decrease the optical efficacy of the LED lighting device if photon recycling is not effective [15]. In this paper, the brand new concept and idea for a transparent plastic optical lens design used for the LED down light illumination application would be introduced. A type of optical transparent plastic lens would be designed by using the freeform surface and scattering surface technologies. This plastic transparent optical lens would be consisting of both a free from and scattering inner surface and a normal spherical shape outer surface. The concept of adopting the transparent optical lens for LED illumination is to get the most LED light energy transmitted through the optical system for the purpose of the energy saving in terms of both the electrical energy consumption saving and the electrical bill costs saving. The idea of developing a free from inner surface of this transparent optical lens is to get the most LED light energy to be illuminated in the areas of interest. For the light energy utilization rating of a lighting device, the optical utilization factor (OUF) would be used for the evaluation [16– 18]. The idea of designing an optical scattering surface on the top of the freeform inner
#241170 (C) 2015 OSA
Received 18 May 2015; revised 10 Jun 2015; accepted 11 Jun 2015; published 16 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016715 | OPTICS EXPRESS 16716
surface of this transparent optical lens is to scatter the strongly direction oriented LED light to be a more evenly distributed light pattern in the areas of interest and reduce the glare caused by the LED light sources while taking into account the goal of the angular distribution pattern to be reached at the same time. 2. Optical modeling In general, a precise LED optical lighting system design would require at least a precise LED light source model with an accurate angular light distribution pattern and optical component models with accurate optical surface and volume properties. First of all, a precise light source model would need to be built up. The 1 watt LED light source with angular light distribution of 130 degrees, model PM2B-1LWE-R8, manufactured by ProLight Opto Technology Corporation [19] was used for this optical system lighting design as shown in Fig. 1. A precise optical light source model was the first step required for resulting a precise optical lighting system performance in the end. Normally, each different LED light source would have its own different specific angular light distribution pattern because of its own unique chip structure, packaging structure, and etc. The transparent optical lens would be installed very close to the top of LED light sources, but this location is neither within the near field region nor within the far field region. The angular light distribution pattern of a LED light source in the mid field region described by a precise optical modeling algorithm [20] was used for the Monte Carlo simulation for the precise optical lighting system design. The mid field region was defined between the near field region and the far field region (Fraunhofer region) [21]. The mid field ranges over the region within the 10 times of the lateral dimension of the LED light source. Figure 2 shows the measured angular light distribution patterns of the LED light source at different locations of 1.5cm, 3cm, 5cm, and 20cm ranging from the mid field region to the far field region.
Fig. 1. (a) The LED light source model PM2B-1LWE-R8. (b) The LED PC board module.
The precise mid field angular light distribution pattern of the above LED light source was established and modeled by the optical simulation program ASAP [22] to coincide with both the mid field and far field angular light distribution patterns measured in the optics laboratory as shown in Fig. 2.
#241170 (C) 2015 OSA
Received 18 May 2015; revised 10 Jun 2015; accepted 11 Jun 2015; published 16 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016715 | OPTICS EXPRESS 16717
Fig. 2. The angular light distribution patterns of LED light source ranging from the mid field region to the far field region.
The inner optical surface of this plastic transparent optical lens was consisting of 36 freeform surfaces which were designed by using rational Bézier mathematics [23,24]. The rational form of the Bézier equation is
P(t)=
Pj w j Bn j ( t )
. w B (t) n
j
(1)
j
where Wj are the weighting factors for each of the control points Pj. Normally, the optical development and design of those optical components with complicated geometry would be performed by using the computer aided design (CAD) to have the CAD geometrical representation for the optical illumination system applications. In this inner optical surface design, each freeform surface was designed by using rational Bézier control points and weighting factors as variables. The non-uniform rational B-splines (NURBs) using the Bézier mathematics have been used in the CAD field extensively. Therefore developing the freeform optical surfaces by using the Bézier representations of optical surface data, we could easily parameterize the freeform optical surfaces with some control points and weighting factors to allow for surface shape manipulations and adjustments to reach the energy distribution goals of the illumination system requirements. Besides the freeform surface developed on the inner surface of this optical transparent lens, the optical scattering surface property was also designed and applied on the top of the optical freeform inner surface to achieve the more evenly light energy distribution in the areas of interest with glare reduced. Light scattering happens at all the optical surfaces. The characteristics of the scattering vary with the different optical surface materials. It would change the directions of the incident ray angles, and the incident rays would be scattered off the optical surface in all directions. The characteristics of the optical scattering surface are defined by BSDF scattering functions [25,26]. In this paper, all optical scattering models are defined in terms of a bidirectional scattering distribution function (BSDF). The bidirectional scattering distribution function (BSDF) consists of both the bidirectional reflection distribution function (BRDF) and the bidirectional transmission distribution function (BTDF). Since the materials of this transparent optical lens are made out of plastic materials like acrylic, PC or PS plastic, the optical scattering property models could be accurately described with Harvey BSDF functions [27]. The BSDF function is equal to L/E where L is the scattered radiance and E is the incident irradiance. The BSDF function is a four dimensional function which depends on the scatter direction (θ, φ) and the specular direction (θ0, φ0). The Harvey BSDF equation is s
sin θ 2 2 BSDF=b0 1+ . l
#241170 (C) 2015 OSA
(2)
Received 18 May 2015; revised 10 Jun 2015; accepted 11 Jun 2015; published 16 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016715 | OPTICS EXPRESS 16718
where b0 is BSDF at θ is zero, s is the slope of BSDF, and l is the shoulder parameter (radians) of BSDF. The optical surface scattering characteristics of the inner surface of this transparent optical lens were precisely measured by using the professional BSDF measurement instrument [28] made by Opsira GmbH located in Germany. The seven curves with the incident angles varying from 0 to 60 degrees almost completely coincided to form a single curve as shown in Fig. 3 and Fig. 4. It indicates that the scattering property of this optical surface is shift invariant, so the scattering properties could be completely characterized by a single set of measurements at a fixed angle of incidence. This shift invariant behavior characterizes the scattering properties of the optical surface completely [29].
Fig. 3. (a) The measured 7 BRDF curves. (b) The 7 BRDF curves coincided to form a single curve.
Fig. 4. (a) The measured 7 BTDF curves. (b) The 7 BTDF curves coincided to form a single curve.
Before building any optical components by using any tooling or molding, the entire three dimensions optical model was simulated thoroughly by using the Monte Carlo simulation executed by ASAP. ASAP is a non-sequential ray tracing program which traces the rays in the three dimensional space and simulates the interaction of light sources with both optical and mechanical components for the entire optical system. 3. Optical design and verification
The task was to develop and design a plastic optical lens to be applied in the LED down light illumination. For designing this plastic optical lens, there were three design goals to be reached under the condition of a total of 4 watts optical power output for the down light with each LED light source to be operated at 3.3V and 350mA level. The three design goals to be reached were the light energy transmission at 80% of the optics for the sake of energy saving reason, the 70 ± 3 degrees angular light distribution pattern defined at full width half maximum (FWHM) light energy level, and the optical utilization factor (OUF) of 50% within the −45 degrees and 45 degrees range with glare reduced. The plastic optical lens was designed to be a transparent optical lens and was consisting of a freeform inner surface and a
#241170 (C) 2015 OSA
Received 18 May 2015; revised 10 Jun 2015; accepted 11 Jun 2015; published 16 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016715 | OPTICS EXPRESS 16719
spherical shape outer surface, and then the freeform inner surface was designed to consist of 36 freeform surfaces. Figure 5 shows the front view and the back view of this transparent optical lens designed and constructed by ASAP.
Fig. 5. (a) Front view of the transparent optical lens, and (b) back view of the transparent optical lens.
Then, these 36 freeform surfaces were to be adjusted, manipulated and designed to form an inner surface combining with a spherical shape outer surface to reach the design goals of the 70 ± 3 degrees angular light distribution pattern defined at full width half maximum (FWHM) light energy level and the 80% of the light energy transmission of optics. At the same time, the optical scattering surface property applied on the freeform inner surface also needed to be taken into account for reaching the above design goals because the optical scattering surface property would generate a diffuser effect to influence upon both the shape of the angular distribution pattern and the LED glare reduction. The more the diffusing power would be enhanced on the freeform inner surface, the more the LED glare would be reduced, but less of the accuracy of the angular light distribution could be controlled. For this plastic optical lens design, the scattering surface property of the inner surface consisted of the Harvey BRDF scattering function with b0 = 1.414, s = −4.91, and l = 0.17 and the Harvey BTDF scattering function with b0 = 63.505, s = −4.96, and l = 0.08. The idea was that with these appropriate optical scattering surface property Harvey BSDF models designed, the accuracy of the angular light distribution could be controlled and the LED glare could be reduced. After the completion of the optical simulation of this plastic optical lens development and design, the prototype of this plastic optical lens was produced by the precision molds and injection molding technologies. Figure 6 shows the prototype of this plastic optical lens. The plastic optical lens of down light was then measured in the optics laboratory for the light energy transmission rate. Without the plastic optical lens installed on the LED PC board module, the optical flux was measured to be 455 lumens, and the optical flux value was measured to be 409 lumens with the plastic optical lens installed. The value of 89.89% transmission rate for this plastic optical lens was calculated. This means that almost 90% transmission rate of this plastic optical lens was achieved. Comparing to the general design of the LED lighting devices that it simply covers the LED PC board module with a white color diffuser cover which basically would block the 25% on an average of the LED light emitting energy to be transmitted through the layer of diffuser cover to reach the areas of interest, the design of this plastic optical lens would be doing a much better job in terms of the energy saving.
#241170 (C) 2015 OSA
Received 18 May 2015; revised 10 Jun 2015; accepted 11 Jun 2015; published 16 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016715 | OPTICS EXPRESS 16720
Fig. 6. (a) Front view of the plastic optical lens, and (b) back view of the plastic optical lens.
The angular light distribution pattern of this plastic optical lens was also measured in the optics laboratory. Figure 7 shows the complete assembly down light with this plastic optical lens installed on the LED PC board module. The angular light distribution pattern of this plastic optical lens was measured, and the 70 degrees angular light distribution pattern defined at full width half maximum (FWHM) light energy level was measured and reached as shown in Fig. 8. The angular light distribution measured and the data calculated also showed to achieve the optical utilization factor (OUF) of 51.8% within −45 degrees and 45 degrees range. It was 10.5% more light energy utilized while comparing to the popular brand LED down light with 41.3% optical utilization factor (OUF) measured and calculated in the same angular distribution range. Figure 8(a) shows the measured polar plot of angular light distribution pattern. Figure 8(b) shows that the curve of the measured intensity plot and the curve of the optical simulation intensity plot are almost coincided completely. In another words, this means that the experimental measurement data and the theoretical design data successfully matched with each other.
Fig. 7. The down light with the plastic optical lens installed on the LED PC board module.
4. Summary
In this paper, a novel transparent optical lens design with an optical scattering freeform inner surface has been presented and demonstrated. The precise mid field angular distribution model of the LED light source and the optical scattering surface property of the Harvey BSDF scattering model were built and measured. With the above precise LED mid field angular distribution model and the Harvey BSDF scattering model, the freeform inner surface and the spherical outer surface of the plastic optical lens were developed and designed, and the performance of the entire optical system of the down light was evaluated by the optical simulation. Then, the prototype of the plastic optical lens was produced by precision molds and injection molding technologies. Finally, the performance of the down light optical system was obtained by the accurately optical measurements. And the mission was completed successfully to reach and surpass the design goals of the light energy transmission at almost
#241170 (C) 2015 OSA
Received 18 May 2015; revised 10 Jun 2015; accepted 11 Jun 2015; published 16 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016715 | OPTICS EXPRESS 16721
90% of the optics, the 70 degrees angular light distribution pattern defined at full width half maximum (FWHM) light energy level, and the optical utilization factor (OUF) of 51.8% within −45 degrees and 45 degrees range with glare reduced. For the light energy utilization rating, it was 10.5% more light energy utilized while comparing to the popular brand LED down light with optical utilization factor (OUF) of 41.3% in the same angular distribution range. Besides the above optics quality reached, at the same time, it also offered the aesthetic quality for this transparent LED optical lens to be crystal looking in both the lit and unlit states.
Fig. 8. (a) The polar plot of angular light distribution pattern. (b) The optical simulation intensity curve almost coincide with the measurement intensity curve.
Acknowledgments
The author would like to thank Yi Jin Photonik Co., Ltd. and Chin-Yi Chen for the support of this research and also thank the sponsor of the Ministry of Science and Technology of Taiwan under the contract MOST 103-2221-E008-063-MY3.
#241170 (C) 2015 OSA
Received 18 May 2015; revised 10 Jun 2015; accepted 11 Jun 2015; published 16 Jun 2015 29 Jun 2015 | Vol. 23, No. 13 | DOI:10.1364/OE.23.016715 | OPTICS EXPRESS 16722
