A novel light luminaire is proposed and experimentally analyzed, which accurately projects light into a large rectangular area to achieve uniform illumination and a high optical utilization factor at the target. Side-illuminating luminaires for large-scale illuminated area are typically set with an elevated tilt angle to enlarge the illuminated area. However, the light pattern is bent thereby reducing the uniformity and optical utilization factor at the target. In this paper, we propose an efficient and useful approach with a rotationally symmetric projection lens that is trimmed to adjust the bending effect and to form a rectangular illumination light pattern on the ground. The design concept is demonstrated and verified. Several potential applications such as highly uniform illumination with fitting shapes for sport courts are analyzed and discussed.
© 2014 Optical Society of America
Light emitting diodes (LEDs) have been extensively studied for potential applications including indoor and outdoor lighting, and vehicle forward lighting because of competitive features such as compact size, fast response, being free of mercury, wide color range, high efficiency, and long life [1–3]. In comparison with traditional light source in the lighting devices, LED can provide not only high energy efficiency but also high color rendering index so that more human factors can be installed in an LED lamp. Furthermore, a LED-based design with high optical utilization factor and extremely low light pollution has been proposed . Consequently, LEDs have successfully replaced most traditional light sources in general lighting. Large-scale area lighting devices are typically used in many specific applications (e.g., street lighting, large plazas and playgrounds) [4–7]. In general, the illuminating modes for these kinds of applications include two types as shown in Fig. 1: direct-illuminating type, and side-illuminating type. Various optical designs have been proposed for the direct-illuminating type [8–10]. For the side-illuminating type, each luminaire is set at an elevated tilt angle to enlarge the illuminated area, but the pattern will be bent and therefore energy is lost to the illuminated area outside the target, as shown in Fig. 1(b). Therefore, the efficiency and uniformity at the target are reduced. Moreover, the unwanted illumination can cause additional light pollution. Using an asymmetric freeform reflector or lens according to the SMS method has been proposed to solve the problem [11,12]. A collimating system combined with a tilt surface structure diffuser is another solution for side-illuminating type .
In this paper, we propose a new and efficient design to adjust the bending effect. The adjusted pattern shown in Fig. 1(c) can meet the rectangular shape of the target and accurately contain most flux in the target to enhance the optical utilization factor (OUF). The accuracy of the design is discussed through simulated and experimental results in the paper. Finally, the general adoptability of the proposed design in various applications is discussed.
2. A novel design for a side-illuminating luminaire
For the basic design, we consider a rectangular shape area as our target and the apparatus is shown in Fig. 2.The elevated tilt angle is 15° and the height of the lamp is 12 m. The target area is 30 × 40 m2. Single-chip white XM-L LED fabricated by CREE is used, as shown in Fig. 3 . The measured flux is 700 lm with an injection current of 2.4 A. An optical model of the LED is necessary to achieve an accurate design. We follow the mid-field modeling procedure to establish the optical model [15–17] and the light pattern is simulated through a Monte-Carlo ray tracing program such as ASAP . The angular light patterns are verified through simulations and experiments at different distances in the mid-field region, as shown in Fig. 4.Following Fig. 4, we get an accurate model according to the normalized cross correlations (NCC)  higher than 99.5% at each distance.
The proposed optical module consists of a freeform lens array and a reflector, as shown in Fig. 5.There are ten segments in the lens array and each segment is combined with a LED with a freeform lens to provide sufficient flux.
Initially, a circular lens based on a single-chip LED is used. According to the feedback modification method [8–10], we design a symmetric freeform lens (SFL) which can provide a uniform pattern shown in Fig. 6(a).The optical efficiency of the SFL is 90%. According to the phenomenon illustrated in Fig. 1, the pattern is bent if we only use an axial SFL. Therefore, we modify the off-axis region of the lens with diamond-shape cutting (so-called L-DSC) shown in Fig. 6(b). In Fig. 6(b), the diamond-like shape of the lens can be observed in x-y plane. The cutting shape is a special design to form a specific light pattern on the ground. The specific light pattern can be set as one of the base patterns for a new pattern to meet a certain requirement. Figure 6(c) shows the tilted lens array module and the corresponding light pattern. To control the backward leakage and enhance the OUF, here the new idea is to make beam shaping with a mirror to form a rectangular illumination pattern on the ground. The tilted lens array module with a planar mirror to reflect the pattern of one side and the corresponding light pattern are shown in Fig. 6(d), where a near rectangular light pattern can be observed.
In the optimized design, we have to calculate the suitable shaping angles and of the L-DSC, as illustrated in Fig. 7(a).In Fig. 7(b), we adjust the elevated tilt angle of the luminaire so that the trapezoid pattern can be projected as a rectangular pattern on the ground. The dimensions of the rectangular target are set . To calculate the shaping angles and in Fig. 7(a), the coordinates and must be calculated first. The trapezoid pattern is switched to the new coordinate axis in Fig. 7(b) because of the elevated tilt angle for the luminaire.
According to the new coordinate axis , and are transferred to and ; and and are transferred to and . The transfer matrix can be writtenFigure 7(c) and 7(d) show the projected area in - and - planes from Fig. 7(b). In Fig. 7(c), and are the projected points of and . In Fig. 7(d), and are the projected points of and . Considering the similar triangles and in Fig. 7(c), we findFig. 7(c), we also findFig. 7(d), we findEqs. (7)-(8). In the simulation, PMMA (n = 1.492) is used as the material of the L-DSC, and the reflectivity of the planar mirror with dimensions of 25 × 25 cm2 is set 80%. The optical efficiency of the L-DSC is 87%, which is near the optical efficiency of 90% without cutting. Because the cutting edge of the L-DSC forms a mirror with total internal reflection, it increases the optical efficiency but causes some bright fringes. If the light hitting the cutting edge of the L-DSC are absorbed by the edge, the simulation shows that the optical efficiency decreases to 74.1%. There are 40.2% of the flux from the lens reflected by the planar mirror and the optical efficiency of the whole luminare is approximately 80%. The total flux is measured approximately 5600 lm when the LEDs are driven at 78W. The final pattern of the single luminaire is shown in Fig. 6(d), in which light leakage can be observed, and is highlighted in red. However, the light leakage is compensated when the illuminating system is composed of several luminaires arranged side by side, as shown in Fig. 8(a). When the number of the luminaires is increased, the loss effect decreases. The final pattern for an array with 59 luminaires on each side is shown in Fig. 8(c). The OUF is approximately 70%, where the OUF is the ratio of the flux at the target and the flux from the LEDs.
3. Experimental verification
According to the simulation result, a prototype fabricated through CNC machining is to verify the accuracy. The prototype is shown in Fig. 9(a).A distance of 1.3m is used for verification. The simulation and experimental results are compared in Figs. 9(b) and 9(c). The patterns for simulation and experiment are nearly identical. Besides, we need to check the illuminance distribution so the sampling points shown in Fig. 10 are set with a spacing of 50 cm in each line. For the results on the vertical axis in Fig. 11 are set with a spacing of 50 cm in each line. For the results on the vertical axis in Fig. 11, the distributions for each line are similar. For the horizontal case in Fig. 12, the difference of the result at H3 is caused by inaccurate alignment, roughness on the surface of each element and manufacturing errors. Based on the comparison, we have confirmed the accuracy of our design.
4. Applications for different requests
In this section, several applications for different playgrounds are discussed based on the DIALUX program . Then, the playgrounds include badminton court, volleyball court, and indoor natatorium in National Central University in Taiwan. According to the illuminance standard in Taiwan, which is called CNS-12112 , the average illuminance for different court is defined and the sampling points are set at a plane with a height of 0.85 m. To meet each request, suitable numbers of the luminaires should be confirmed.
4.1 Badminton court
The area of the badminton court, as it shown in Fig. 13, is 33 × 39 m2, comprising six racing areas. For normal racing, the range of illuminance is in the range of 300 to 500 lux. To meet the regulation, the number of the luminaires on each side is about 59 pcs. Then, we consider the performance for area A and B because of the symmetry of the apparatus. The analyzed result for the suitable height of the luminaire is shown in Fig. 14.According to the analysis process for the height of the luminaire array, the range for the better choice is in the range of 11 to 13 m. Therefore, we choose the result for the height of 12 m and the simulated scene is shown in Fig. 15. The average illuminace is about 420 lux for section A and 430 lux for section B. The uniformity, defined as the ratio of the minimum and average illuminance, is 78% for section A and 85% for section B.
4.2 Volleyball court
The dimension of the volleyball court shown in Fig. 16 is 32 × 40 m2. Performance is considered only for section A because of the symmetry of the apparatus. To meet the regulation, the number of the luminaires on each side is also about 59 pcs. The analyzed result is shown in Fig. 17. With the same analysis process for the height of the luminaire array, the best choice is near 9.5m. The simulated scene is shown in Fig. 18.The average illuminace is about 400 lux for section A and the uniformity is 75%.
4.3 Indoor natatorium
The dimension of the natatorium shown in Fig. 19 is 29 × 63 m2. The red-line area represents the swimming pool. For entertainment purposes, the range of illuminance is in the range of 75 to 150 lux. To meet the regulation, two arrays should be set in each side and the amount of the luminaire for each array is 59 pcs. The average illuminace is 140 lux and the uniformity is 50%.
We have proposed a novel method to shape the illumination light pattern from a side-illuminating luminaire to a certain shape on the ground by trimming a circular projection lens. The side-illuminating luminaire with cutting-edge lens shape efficiently adjusts the illumination pattern on the ground with high uniformity and high optical utilization factor because the illumination pattern fits well the shape of the target. The luminaire is composed of only two parts: one is a freeform lens array, which has a specific cutting-edge design for each segment; the other is a mirror.
To get an accurate design, the precise LED model based on the mid-field concept has been established and the NCCs are all as high as 99.5% at different distances in the mid-field region. Then, the optical efficiency of the whole luminaire is 80% and the optical utilization factor is higher than 70%. Therefore, it has positive effect on energy saving because the optical flux is efficiently used. For the experimental analysis, the illuminance distribution of the simulation and experiment is similar. Moreover, the virtual reality for real playground has been discussed and analyzed. For the badminton court, 59 pcs luminaires should be set in the array in each side. The optimal height of the array is 12 m. The average illuminance higher than 420 lux and the uniformity higher than 78% in each racing area have been calculated.
The proposed design has many potential applications, such as sport courts. In the volleyball court, the number of luminaire in each array is about 59 pcs and the optimal height is 9.5 m. The average illuminance is higher than 400 lux and the uniformity is higher than 75% in each racing area. For these two playgrounds, the optical performance can meet the normal racing request in the Taiwanese regulation named CNS-12112. For the indoor natatorium, the average illuminance is higher than 140lux so it can meet the amusement request from CNS-12112. The proposed side-illumination design with combination of a freeform lens array and a mirror is proven very useful in lighting a larger-area court with rectangle shape, and benefits energy saving with high optical utilization factor with accurate projection.
This study was supported in part by National Central University’s Plan to Develop First-class Universities and Top-level Research Centers grants 995939 and 100G-903-2, and was also sponsored by the National Science Council of the Republic of China under contracts: 97-2221-E-008-025-MY3, 99-2623-E-008-002-ET and NSC100-3113-E-008-001. The authors would like to thank the Breault Research Organization for the support of simulation with ASAPand Witslight Technology Co. Ltd for the manufacturing support.
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