Low-Glare Freeform-Surfaced Street Light Luminaire Optimization to Meet Enhanced Road Lighting Standards

Department of Electrical Engineering, National Kaohsiung University of Science and Technology, Kaohsiung 80778, Taiwan Department of Technology, Dong Nai Technology University, Bien Hoa 830000, Vietnam Department of Industrial Engineering and Management, Minghsin University of Science and Technology, Hsinchu 30401, Taiwan Department of Shipping Technology, National Kaohsiung University of Science and Technology, Kaohsiung, Taiwan Department of Mechatronics Engineering, National Kaohsiung University of Science and Technology, Kaohsiung, Taiwan Department of Mechanical Engineering, National Kaohsiung University of Science and Technology, Kaohsiung, Taiwan Department of Microelectronics Engineering, National Kaohsiung University of Science and Technology, Kaohsiung, Taiwan Department of Graduate Institute of Clinical Medicine, Kaohsiung Medical University, Kaohsiung 807, Taiwan


Introduction
Road lighting has a considerable influence on traffic safety and the quality of the human environment [1][2][3] and is thus an indispensable component of pathways [4,5], sidewalks, and road equipment [6][7][8]. On roads, high visibility and facial recognition are imperative components of the interactions between users [9,10], and several studies of road lighting have accordingly demonstrated the benefits of public lighting installations on road safety, crime prevention, and traffic flow [1][2][3]11]. Currently employed street lighting technology is built upon years of experience and research [6-8, 12, 13]. However, it is necessary to improve street lighting quality in terms of efficiency, road surface luminance, illumination uniformity, and glare reduction to meet recent updates to international standards [14,15].
Lighting quality plays an important role in determining the visual performance and comfort of road users and can keep drivers alert to reduce the incidence of car accidents. Indeed, inferior lighting conditions can have negative effects on mobility behavior, subjective perception of public space, and traffic safety [16]. In particular, the subjective experience of safety and security outdoors at night is considerably influenced by street lighting performance [17][18][19]. e luminaire mounting height, street light spacing, luminaire inclination angle, and road surface properties are essential for ensuring the desired street lighting performance, measured in terms of the average road surface brightness, brightness uniformity, longitudinal brightness uniformity, and threshold increment (glare factor) [1][2][3]20]. Furthermore, though the use of lightemitting diode (LED) technology requires less energy consumption and can provide longer-lasting lighting than conventionally used discharge lamps [21][22][23][24], there remain several disadvantages to the use of LED lights, such as their higher cost, unpredictable lifetime, and excess blue/white glare for human eyes [25][26][27].
In this study, a freeform-surfaced luminaire is therefore proposed and demonstrated to meet the requirements of the CIE M3 class standard using a 150 W ceramic metal halide discharge lamp. Based on the experimental results, a road lighting plan for a trial road is then evaluated using the proposed luminaire considering the requirements of the CIE M3 class standard.

Luminaire Design Principles
A freeform-surface street light luminaire should be designed and developed in accordance with relevant lighting standards and specifications in order to ensure that it provides sufficient luminance and uniformity performance. According to the International Commission on Illumination (CIE) standard, the parameters of lighting quality include the average road surface luminance, L avg , brightness uniformity, U o , longitudinal brightness uniformity, U L , and threshold increment (glare factor), TI [1-3, 5-8, 28]. e CIE standards provide different lighting parameter requirements according to level, as shown in Table 1 [6, 9-11, 24-27, 29, 30]. e average luminance, L avg , is the brightness of the road surface as experienced by a driver and must be maintained above a certain level throughout the entire service life of the luminaire. It is related to the light distribution of the luminaire and its installation position as well as the reflective properties of the road surface. e overall uniformity of road surface luminance, U o , is a measure of how evenly lit the road surface is; a low U o value means that there is a significant change in luminance on the road [1-6, 8-10, 28]. It is determined by dividing the minimum value of luminance, L min , by the average luminance, L avg , as given by e longitudinal uniformity of road surface luminance, U L , is related to the comfort of the driver under the subject lighting environment, determined as the ratio of L min to the maximum luminance, L max , on the road, as given by e threshold increment TI is a measurement of the visibility loss caused by the road lighting equipment and is calculated by determining whether the incremental percentage of luminance difference of an object can be clearly identified in the presence of glare. e TI is thus a measure of the loss of contrast due to light shining directly from the luminaire into a driver's eye. is effect is commonly referred to as disability glare. e physiological effects of disability glare increase with driver age, so it is of particular concern in any country with an aging driving population. To calculate TI, if 0.05 cd·m −2 < L avg < 5 cd·m −2 , then and if L avg ＞ 5 cd·m −2 , then where L v is the luminance of the light curtain displayed by n lighting lamps in the field of vision (cd·m 2 ), determined by where E eye,i is the illuminance on the plane perpendicular to the sight line for a viewer's eye height 1.5 m above the road (lux), θ is the angle (in radians) between the line of sight and the center of the luminaire, n is the number of luminaires in the field of view, and k is a constant that varies with the age of the viewer A according to e objective of this study was to design a freeformsurfaced luminaire for a street light that meets the CIE M3 class lighting standards in order to provide a safe and comfortable road lighting environment for drivers. e flow chart of the luminaire design for the new street light is shown in Figure 1. In order to provide a more accurate optical simulation, a physical source model of the Philips Lighting

Design of Freeform-Surfaced Luminaire
In this study, a new street lighting luminaire was designed using a freeform-surfaced optical reflector for a metal halide lamp in order to provide CIE standard street lighting.
Accordingly, a source model of the Philips MASTERColour bulb was constructed and analyzed using the SolidWorks mechanical design software and TracePro optical analysis software, respectively. e resulting source model and its simulated light intensity distribution curve (LIDC) are shown in Figure 4. e optical reflector of the luminaire designed in this study is comprised of multisegmented mirror surfaces and is 251.162 mm long and 173.716 mm wide, as shown in Figure 5. e light source model and the freeform-surfaced luminaire model files from SolidWorks were imported into the TracePro optical simulation software, where the light source parameters and the luminaire surface properties were set in order to obtain the LIDC and the IES far field source file for the new street light luminaire.
In order to conduct a road lighting analysis using the proposed streetlight luminaire on a trial road as per the CIE standard test, the road environment parameters, street light arrangement, and illumination parameters were set in the DIALux lighting design software to calculate L avg , U o , U L , and TI. e street light parameters are shown in Figure 6 and included a luminaire height of 12 m, a distance between lamp pole and luminaire of 2 m, a length of protrusion of 1.5 m, and an arm inclination of 15°. e trial road environment was 14 m wide carrying four lanes and the spacing between the light poles was 50 m along on only one side of the road, as shown in Figure 7.
In order to optimize the design of the proposed luminaire to meet the CIE M3 class standard, the add-on ray tracing simulation tool OptisWorks (Optis SAS, La   International Journal of Optics Farlede, France), embedded in SolidWorks, was used to determine the x i , y j , and z j coordinates of the bulb in the luminaire that provide the optimal lighting performance. ese coordinates are defined in Figure 8, and the optimization process flowchart is shown in Figure 9. During the optimization process, the optimization object function f was established by a genetic algorithm and is given by [22,27] where ϕ i represents the coordinates of each orientation, n j is the value of the measured target, determined by an intensity sensor during each optimization pass when running the program; and t j is the optimization target defined according to the requirements of the CIE M3 class standard, in this   x i · cos x i + y i · cos y i + z i · cos z i , (8) where i is the step number, and x i , y i , and z i are the function coefficients of each coordinate value. For brevity, these coefficients are written as vectors x � (x 1 , x 2 ,..., x i ), y � (y 1 ,  Figure 10. e IES source files associated with these LIDCs were obtained and imported into DIALux to establish the lighting performance simulation according to the road lighting environment settings. e simulation results shown in Table 2 indicate that the optimal bulb position provides improved performance over the basic positions in terms of each evaluation item for the CIE M3 class.

Optical Measurements and Analysis
To confirm the simulation results against actual measurements, the proposed streetlight luminaire was prototyped using a high-precision aluminum mold based on a 3D CAD file of the optimized street light and is shown in Figure 11. A Philips MASTERColour CDM-T Elite 150 W/930 G12 1CT/ 12 metal halide lamp was then fixed in the prototype at the previously obtained optimal position (−0.2, 1.2, 0). An imaging goniophotometer produced by Radiant Imaging Co. Ltd., shown in Figure 12, was then used to obtain the entire light intensity distribution map and LIDC of the prototype, shown in Figure 13. e measured IES light source file for the optimized street light sample was then  International Journal of Optics imported into DIALux to confirm whether or not the road lighting performance conformed to the CIE M3 class standard. e resulting road lighting performance coefficients are detailed in Table 3, which confirms that the proposed luminaire provides trial road lighting that meets the CIE M3 class standard. Based on the simulation results shown in Table 3, the brightness uniformity U o of Lane 1, Lane 2, and Lane 3 is 0.41 and Lane 2 is 0.42, respectively. e longitudinal brightness uniformity U l and the glare factor TI are 0.63 and 8% (Lane 1), 0.69 and 9% (Lane 2), 0.61 and 7% (Lane 3), 0.64 and 5% (Lane 4), respectively. On the other hand, the experimental results indicate a controlling L avg of 1.1 cd/m 2 , U o of 0.41 (compared to a minimum requirement of 0.4), U L of 0.64 (compared to a minimum requirement of 0.6), and TI of 7.6% (compared to a maximum limit of 15%). e prototype street light was also evaluated by the Taiwan Accreditation Foundation (TAF) using a type C mirror goniophotometer in their certification laboratory, with the results shown in Table 4. e data in Table 4 are close to the measurement results obtained by the imaging goniophotometer in Table 3, verifying the accuracy of the optical measurements conducted in our laboratory. A flow chart of the complete optical evaluation of the prototype streetlight is shown in Figure 14.

Discussions and Conclusions
In this study, a freeform-surfaced luminaire was proposed that uses a 150 W Philips CDM-T MASTERColour compact metal halide discharge lamp to provide counter beam lights meeting the requirements of the CIE M3 class street lighting    International Journal of Optics 9 standard. e optimal design of the street light luminaire was achieved using the TracePro and DAILux optical design software packages. In order to demonstrate the practicality of the design, a physical prototype of the proposed luminaire was evaluated in the laboratory using an imaging goniophotometer. e reliability of these test results was confirmed by a mirror goniophotometer test conducted in a laboratory certified by the TAF. e road condition simulation results obtained using the two measurements show only minor deviations between each other and the optimized design, and thus meet the CIE M3 class street lighting standard. e results indicate that the prototype provides an average road surface brightness L avg of 1.1 cd/m 2 , brightness uniformity U o of 0.42 (compared to a minimum requirement of 0.4), longitudinal brightness uniformity U L of 0.75 (compared to a minimum requirement of 0.6), and glare factor TI of 9.5% (compared to a maximum limit of 15%). Moreover, the proposed freeform-surface design was found to enhance the output surface brightness by 5% compared to the conventional design. e findings of this study are   expected to aid in the design of street lights to meet the enhanced requirements of the most recent international standards.

Data Availability
No data were used to support this study.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.