1. Introduction

The conventional light sources used for street lights are incandescent light, fluorescent light and high-pressure sodium (HPS) light, etc. However, these lights emit a great deal of infrared energy or ultraviolet rays, so as to cause light pollution and energy waste. Furthermore, the light sources are omnidirectional, thus it is difficult to control their emitting light to reach the road surface. Compared with these conventional lights, LEDs own several advantages, such as higher optical efficiency, lower heat radiation, lower power consumption, longer lifetime, smaller emitting angle, and free mercury pollution, etc [1,2]. However, LEDs cannot provide the required intensity distribution for all applications by itself. LEDs generally need to be equipped with the secondary lens to tailor their intensity distribution to achieve the targeted illumination distribution [3–8].

A standard street light should be highly efficient, and with low glare to prevent drivers from danger. In these years, there have been several secondary lenses proposed for accomplishing LED street light with those advantages [9–14]. For example, Wang et al. have designed a smooth freeform lens to control the intensity distribution of street light for reducing glare [12]. Lee et al. have equipped a microlens array with street light to achieve uniform illumination, and reduce the street light source luminance for glare decrease [14]. In the study, we propose an asymmetric secondary freeform lens (ASFL) based on total internal reflection (TIR) to accomplish an LED street light with low glare and high efficiency. Instead of inclining the head of street light, the frequently used method of enhancing the road lighting efficiency, the designed ASFL of the new street light guides the light toward the road’s outside area to the road surface by TIR effect. Because the head of the proposed street light is not inclined, but kept horizontal at work, it is not easy for drivers to see the LED street light source directly. And the glare of the proposed street light is thus much dropped. In the study, it will be demonstrated experimentally that the ASFL can carry out a chip on board (COB) LED street light to meet the standard of Chinese National Standards (CNS) and Illuminating Engineering Society of North America (IESNA) road lighting for guaranteeing its glare, optical efficiency and illumination distribution as qualified for drivers on the road [15,16].

2. Principles of designing LED street light with ASFL

The outgoing surface of the ASFL is responsible for refracting its incident light to achieve the design targets. The light refracted by outgoing surface is governed by Snell’s law. The vector equation of Snell’s law can be written as follows:

In order to increase the optical efficiency of the proposed ASFL and its related street light, its reflecting surface is designed to totally and internally reflect the LED emitting light to the outgoing surface. To facilitate prototyping and optical testing, the polymethylmethacrylate (PMMA) is used as the material of the ASFL prototyping sample in the experiments. The refraction index of PMMA is 1.49 and the TIR critical angle is calculated to be 42.15 degree. The proposed street light consists of the ASFL and a set of 140W COB LEDs. The intensity distribution and the illumination of the street light are set as the main target items of the ASFL design merit functions. In the ASFL design process, the total internal reflecting surface and the outgoing surface are free to change for finding the best solutions. The accomplished intensity distribution is required to meet the standard of CNS and IESNA road lighting.

The design process of the LED street light is presented in the flow chart shown in Fig. 1. The center of the LED source is located at the origin of a Cartesian coordinate system, the incident ray I_i (i = 1, 2) from the source center point is aimed at the corresponding point R_i at the refractive surface curve to generate the refracted ray O_i. The R_i separates the refractive surface curve of ASFL as three equal parts. The curve is expressed as f_i(x). The f_i’(x) is the derivation of f_i(x) and can be calculated to get N_i, which is the surface normal vector of f_i(x) at R_i. The end points of the refractive surface are preset as P_i, which are the boundary conditions of the ordinary differential equation, Eq. (1), involving f_i’(x). Thus, the line shape of the curve function f_i(x) can be obtained by solving the ordinary differential equation. We use the non-sequential ray tracing mode of ZEMAX optics design software to decide f_i(x) first. The f_i(x) is free to change, so that N_i can be adjusted freely until O₁ and O₂ are directed to the target angle 45 degrees with respect to y axis for having light cover the road. The initial ASFL design is shown in Figs. 2(a) and 2(b).

 

Fig. 1 The design flow chart of the proposed LED street light.

Download Full Size | PDF

 

Fig. 2 The schematic diagram of optical projection effects of the ASFL. (1) LED light source. (2) The ASFL. (3) Projection light through total internal reflection. (4) Lighting target plane. (5) The refractive surface curve of ASFL. (6) Total internal reflecting surface. (7) Refracting surface. (8) Projection light passing through the ASFL directly.

Download Full Size | PDF

Furthermore, with the intention of having the street light reach the standard of CNS and IESNA specifications for getting high efficiency and low glare, the ASFL is optimized again for searching its final structure, while supervising the energy density on the refracting surface. According to the energy conservation law [5,6], the resulting power of the ASFL on the target surface can be expressed by Eq. (2) as:

3. Experimental results and discussions

For finding the best ASFL solution, we use Solidworks software to bridge the optical design software ZEMAX for optimizing the ASFL. The resulted ASFL design file and a 6000 k, 140 W, 74 mm × 40 mm and 8300 lm white Lambertian COB LED light source are composed as the LED street light module in TracePro optical software to analyze its luminous intensity distribution in space. The street light module with zero inclination emits one million of rays randomly for the simulation, so as to generate its luminous intensity distribution curves (LIDCs), as shown in Fig. 3(a). Correspondingly Fig. 3(a), the simulated 3D luminous intensity map of the new LED street lamp is shown in Fig. 3(b).

 

Fig. 3 The simulated (a) 2D LIDCs (Unit: cd/Klm). (b) 3D luminous intensity distribution map of the new LED street light (Unit: cd).

Download Full Size | PDF

In Fig. 3(a), it can be observed that the blue asymmetric LIDC on the C0°-180° plane is related to TIR surface of the ASFL and can correlate to the projecting direction. As to the axial symmetric green curve, the LIDC on the C90°-270° plane, which batwing shape indicates the light module can give efficient and low glare illumination along the road length, also matching the target items of the ASFL design.

Based on the correspondences, it is expected that the ASFL can achieve the LED street light to reach the specifications of CNS and IESNA road lighting. The molded ASFL sample mounts on the 6000 k, 140 W, 74 mm × 40 mm and 8300 lm white Lambertian COB LED to achieve the LED street light module for optical testing, shown in Figs. 4(a) and 4(b).

 

Fig. 4 The prototype of the proposed LED street light module. (a) The molded PMMA ASFL test sample (Side view). (b) The proposed LED street light module consisting of the molded ASFL sample and the white Lambertian COB LED (Top view).

Download Full Size | PDF

By using the goniophotometer, type HLED-Gonio 8, as the measuring instrument, the LIDCs of the LED street light sample are generated and shown in Fig. 5. In order to evaluate the deviation between the simulated LED street light model and the actual LED street light sample, their LIDCs are analyzed by means of the normalized cross correlation (NCC). The NCC formula is shown in Eq. (3) as:

 

Fig. 5 The LIDCs of the new LED street light, measured by a goniophotometer.

Download Full Size | PDF

For evaluating the optical glare of the street light, its 3D intensity distribution is simulated by TracePro optics software based on the LED street light model, shown in Fig. 3(b). In addition, the IES light source file can be created at the same time. According to the data in the IES file, the luminous intensity is found to be 5.08 cd/klm at vertical light angle 90°/ horizontal light angle 90°, 11.29 cd/klm at vertical light angle 80°/ horizontal light angle 90°, besides the luminous intensity within vertical light angle 60°/ horizontal light angle 65°~90° are not less than 246.18 cd/klm. Taking CNS and IESNA street light specifications as the measurement standard, the above experimental results show the new LED street light meet the cutoff type specifications, shown in Table 1. The schematic diagram about the definition of the light angle of street light is presented in Fig. 6(a). The θ₁ and θ₂ are used for defining the horizontal light angles and the Φ is defined as the vertical light angle in Fig. 6(a). From Table 1, it can be observed that the lighting performances of the new LED street light are better than the standard specifications. For example, the luminous intensity values is 5.08 cd/klm and 11.29 cd/klm and less than CNS 15233 maximum specifications 10 cd/klm and 30 cd/klm, in vertical angle 90° and 80°, respectively. The enhanced anti-glare efficiency is defined as the ratio of the differential value to the standard value, which means the enhanced anti-glare efficiency of the new street light are 49.2% (∣5.08−10∣/10) and 62.4% (∣11.29-30∣/30) in the vertical light angle 90° and 80°, respectively. Furthermore, within vertical light angle 60°/ horizontal light angle 65°~90°, the luminous intensity of the new street light is not less than 246.18 cd/klm, much higher than 180 cd/klm, which means that the effective light efficiency is enhanced more than 36.8% ((246.18−180)/180) while referring CNS 15233 specification.

Table 1. The performances comparison of the proposed LED street light with standard ones (Unit: cd/Klm).

View Table

 

Fig. 6 (a) The schematic diagram of the definition of the light angle with respect to street light. (b) The simulated light projection scene of the new LED street light under zero inclination of lamp (Side view).

Download Full Size | PDF

The new LED street light module model is set at zero inclination, 1.5 m length of bracket and 8m mounting height shown in Fig. 6(b). In Fig. 6(b), the new street light at zero inclination of the lamp head projects light to the road, thus reducing the light directly to the eyes of drivers, so as to decrease the optical glare from the street light. The street lights are arranged on two different roads for illuminance comparison experiments, as shown in Fig. 7, respectively. One is shown in Fig. 7(a), the road area 32 m × 12 m, the LED street light are located with 32 m spacing and on both sides of a bi-directional four-lane major road in the double cross arrangement. The resulted minimum illuminance is 14 lx, the average illuminance value is 20 lx, and the illumination uniformity (U0 = E_min/E_av) is 0.693. And the other one is shown in Fig. 7(b), the observed road area is 20 m × 8 m, the LED street lights are located with 20 m spacing and on only one side of a bi-directional two-lane major road. The resulted minimum illuminance is 14 lx, the average illuminance value is 20 lx, and the illumination uniformity is 0.708. The street light minimum illuminance is 14 lx for the both kinds of roads, and it is more than 10 lx, the required minimum road lighting illumination normally.

 

Fig. 7 The simulated illuminance distribution map. (a) Due to the double-cross arrangement of the new LED street lights drawn in red color square. (b) Due to the one-side arrangement of the new LED street lights drawn in red color (Unit: lx).

Download Full Size | PDF

4. Conclusions

Based on Snell’s law, energy conservation law, and TIR phenomenon, an ASFL and a new LED street light equipped with the ASFL are presented and demonstrated. We design and optimize the ASFL for creating a high efficient, low glared LED street light working without needing to incline lamps. We demonstrate experimentally that the ASFL prototype can accomplish the new street light module to satisfy specifications of CNS 15233 and IESNA.