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In this study we simulate the illuminance and efficiency of four different types of reflector LED and fluorescent light sources for interior illumination. According to our calculation results of the examination of simulations and real situations, we find that the LEDs do perform better than fluorescent lights. We also consider the problems of glare with LED lights by utilizing a diffuser to protect the eyes. We are assured of the potential advantages of LED lighting in the future.
©2008 Optical Society of America
1. Introduction
A quick examination of traditional light sources shows that, although fluorescent lights are very often used in our daily life, they still have some problems, such as short lifetime, excessive power consumption, and mercury pollution [1]. Consequently, we must try to develop more environmentally friendly light sources. One of the best types for future development are LEDs.
Advancements in semiconductor technology have already made it possible for traditional light sources to be replaced by LED lights. Numerous tiny glowing LED indicators are utilized in modern households for transmitting signals or information from electronic equipment [2]. Increasingly, automobiles are also being equipped with LED indicators and brake lights. Recently LED head lights have been designed for use in concept cars [3]. Modern LEDs are now available in a variety of colors and can be used in a wide range of applications. Rapid improvements in white LEDs and LED Street lights have been made in the multi-billion dollar lighting market. These changes and worldwide development of LED based illumination systems should make it possible for LEDs to be used in interior illumination in the near future [4–7].
2. Theory
In the past several decades, fluorescent lights have come to be commonly used for daily illumination. The luminaires for fluorescent light are fabricated case by case. The most commonly used in our laboratory are compact fluorescent lamps produced from steel plate material with V-type reflectors. The development of LED techniques means that traditional illumination sources are no longer suitable. This is why in this study we develop a new design for a novel LED reflector which is described in this study. We also look at issues of glare [8].
2.1. Fluorescent light
A fluorescent tube is basically a gas-discharge lamp that uses electricity to excite mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that causes the phosphor in the tubes to fluoresce, producing visible light.
In these experiments, Philips TL38W/D fluorescent tubes (with 40W and 2850lm performance) were used as a single light source in our laboratory illumination experiments, as shown in Fig. 1. Fig. 2 shows the compact fluorescent lamp fabricated of steel plate material. Not the V-type reflectors and three fluorescent tubes. The candle power distribution curve is shown in Fig. 3.
Fig. 3. Candle power distribution curve of the compact fluorescent lamp with steel plate material and V-type reflectors.
Although we still use fluorescent lights every day, we still cannot avoid occasional breakage, with the accompanying risk of mercury contamination of the surrounding environment [1]. Clearly, we must try to use more environmentally friendly light sources, one of the best targets being LED lights.
2.2 Light-Emitting Diodes, LEDs
An LED is a semiconductor diode that emits light when an electrical current is applied in the forward direction of the device, as in a simple LED circuit. The effect is a form of electroluminescence where an incoherent and narrow-spectrum light is emitted from the p-n junction.
Here, we simulate the performance of two different LEDs: one is the Lumileds LXHL-PW09; the other is the Lumileds LXML-PWC1-0100, as shown as Fig. 4, with performance of 80 lm/A and 257 lm/A, respectively. Both of these LEDs are of the lambertian type. We obtained the candle power distribution curve (see Fig. 5) from the Philips Lumileds website [9,10]. The reason that the Lumileds LXHL-PW09 was chosen for the simulation is its low price, and the reason that the LXML-PWC1-0100 was chose is its high luminous flux. As a consequence of the time they became available in the lighting market, we first used LXHL-PW09 to design our indoor illumination simulation, and then the LXML-PWC1-0100. Other parameters of these two LEDs are described in Table 1.
Table 1. Parameters of LXHL-PW09 and LXML-PWC1-0100
2.3 Reflector
A reflector is simply a device, like a mirror or a flat metal plane, used to alter the path of the light trace. Thus, reflectors play an important role in a lighting system. Three kinds of reflectors are used in our indoor LED lighting designs, a 45° bevel reflector, a parabolic reflector, and an ellipsoidal reflector.
We calculate the distance between the light source point (focal point) and the reflective surface (as shown in Fig. 6) using Eq. (1) through Eq. (3) [11].
Fig. 6. Distance between the light source point (focal point) and the reflective surface for: (a) is parabolic surface; and (b) an ellipsoidal surface.
2.4 Glare
Glare occurs in two ways [12]. First, there is simply too much light (e.g., sunlight) which can produce a simple photophobic response, causing the observer to squint, blink, or look away. The only solutions to this problem are to reduce the retinal illuminance by obscuring the bright part of the visual field, or to lower the luminance over the whole visual field. Second, glare occurs when the range of luminance in a visual environment is too large. Glare that reduces visual performance is called disability glare and is due to light scattering in the eye, reducing the luminance contrast of the retinal image. The effect of scattered light on the luminance contrast of the target can be mimicked by adding a uniform veil of luminance to the target.
3. Experiments
Figure 7 outlines the experimental procedure followed in our study. We first produced fluorescent tube (Lumileds LXHL-PW09 and LXML-PWC1-0100 LED) light source models with different reflectors [13] using the LightTools software. These three different light source models served as the basis for the laboratory scale models, for simulating the illuminance on the table plane. Finally, we calculated the power produced by these light sources and compared the results with each other.
Figure 8 shows a real fluorescent light source with 40W light tubes and producing 2850 lm which was used as our exemplar. These parameters were adapted to build the fluorescent light source model with the LightTools software. For the other light source, the LED, we chose a Lumileds LXHL-PW09 single LED light, which has an 80 lm/A performance. We designed a 10×10 LED array to take the place of each fluorescent light source. Along with the LED array we also designed four different kinds of light reflectors, which are shown in Figs. 9 (a) to (d).
Fig. 9. Single LED matrix: (a) design without reflectors; (b) 45° bevel reflectors; (c) parabolic reflectors; and (d) ellipsoid reflector design.
The laboratory space can be seen in Fig. 10(a). The dimensions were taken into consideration during the process of building the scale model. The table plane used in our laboratory scale model is shown in Fig. 10(b). The white rectangular lines indicate the fluorescent light sources and the four kinds of LED matrixes (at seven positions on the laboratory ceiling, as well as the table plane. We simulated and calculated the average illumination and average difference on the table plane.
Fig. 10. (a) Our laboratory; and the (b) scale model. There are seven light sources located on the top of our laboratory. The white line indicates the table plane. The two gray cubes are cabinets.
4. Results and discussion
The results of illumination on the table plane for each light source and reflector design are shown in sections 4.1 to 4.3. The average difference can be computed by:
where the average difference indicates the degree of uniformity on the table plane. The lower the percentage, the better the uniformity.
4.1 Fluorescent light simulation
We took into consideration the dimensions of the compact fluorescent lamp (fabricated of steel plate material, V-type reflectors, and three fluorescent tubes) during the scale model building process. The results of the table plane simulation of laboratory illumination are shown in Fig. 11.
Fig. 11. Illuminance distribution on the table plane illuminated by fluorescent light sources. The average on the table plane is 1052 lux and the average difference is 41%.
4.2 LXHL-PW09 LED light simulation
The other light source, the LED, chosen was a Lumileds LXHL-PW09. A seven block 10×10 LED array was designed to take the place of each fluorescent light source [15,16]. We used four kinds of LED matrix designs. The simulation results of illumination on the table plane are shown in Figs. 12 to 15.
Fig. 12. Illuminance distribution on the table plane illuminated by the LED matrix design with no reflectors. We obtained an average of 1018 lux on the table plane with an average difference of 29.9%.
Fig. 13. Illuminance distribution on the table plane illuminated by the LED matrix bevel reflector design. We obtained an average of 1025 lux on the table plane with an average difference of 30%.
Fig. 14. Illuminance distribution on the table plane illuminated by the LED matrix parabolic reflector design. We obtained an average of 1655 lux on the table plane with an average difference of 63%.
Fig. 15. Illuminance distribution on the table plane illuminated by the LED matrix ellipsoidal reflector design. We obtained an average of 1740 lux on the table plane and an average difference of 136%.
The two somewhat rectangular-like blue areas in Fig.12 through Fig.14 indicate the positions of our two cabinets. We can determine the uniformity by looking at the average difference. The results show that the ellipsoidal reflector design has the best focusing ability. We thus used this format to develop a new lighting design, spreading 363 LEDs over the whole ceiling. We obtain the table plane illuminance distribution shown in Fig. 16 from this simulation.
Fig. 16. Illuminance distribution on the table plane illuminated by 340 LEDs spread over the whole ceiling with an ellipsoidal reflector design. We obtained an average of 987 lux on the table plane and an average difference of 26%.
Then, in order to decrease the number of LEDs used, we replaced the original LEDs with Lumiled LXML-PWC1-0100 LEDs, which have a higher luminous flux. We used 168 of these LEDs and removed the two cabinets from the simulation so as to better see the complete distribution on the table plane. Furthermore, the glare problem was also a serious illumination problem which we dealt with by using a diffuser in the simulation to protect our eyes from the damage glare damage [17–18].
4.3 LXML-PWC1-0100 LED light spacing
Considering both iluminance performance and glare discomfort, we were not only able to improve the illuminance, but also decrease the glare. As described above, we developed a new lighting design with 168 LEDs spread over the whole ceiling at the same interval and distance. We then installed an LG VEGACHEM Gr1(2t) diffuser (as described in Table 2) below the LEDs to reduce the glare. However, from the beginning of the experiments, we found that most of the light that passed through the diffuser was guided to the central part of the table plane, as shown in Fig.17. In order to solve this unequal distribution problem, we adjusted the interval distance between the 168 LEDs. The new LED arrangement, shown in Fig.18, was similar to the pattern of the Light Guiding Panel used in the TFT-LCD. After adjustment, the interval distances between each LED were no longer the same. We can see that to get the more uniform distribution on the table plane shown in Fig. 19, the LEDs had to be more crowded around the perimeter and less so in the center of the ceiling.
Table 2. Specification of LG VEGACHEM Gr1(2t) diffuser
Fig. 17. Light diffusion on the table plane with the same LED interval distance design. We obtained an average of 538 lux on the table plane with an average difference of 13%.
Fig. 19. Light diffusion and illumination on the table plane after adjusting the interval between each LED. We obtained an average of 511 lux on the table plane with an average difference of 10%.
Finally, we calculated the power consumption for 100,000 hours. We found the cost of the LED design to be lower than that of fluorescent light. If we simulated higher efficiency LEDs, we could reasonably expect even lower costs, and greatly reduced expense.
5. Conclusions
In this study we analyzed an LED reflector design. The reflector is an important feature in controlling the illumination. We found the most advantageous LED reflector design and the best way to avoid glare. We used a diffuser to decay the glare and to simultaneously make the illuminance more uniform. In addition, the LightTools software and the Matlab program allowed us to easily simulate the illuminance and calculate the average difference and cost. Finally, we point out the possibility of using LED designs for indoor illumination in the near future.
Acknowledgment
This study was sponsored by the National Science Council under contract number NSC 96-2221-E-008-112.
References and links
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