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Illumination & Displays

Energy-saving LED light sources

A freeform optical lens system for LED street lamps increases light projection within a targeted road area, reducing waste and improving illumination.
30 March 2011, SPIE Newsroom. DOI: 10.1117/2.1201102.003490

LEDs have many advantages over traditional light sources, such as higher or at least equivalent luminous efficacy, environmental friendliness, long lifetime, application flexibility, and excellent weather resistance. However, LEDs cannot directly replace traditional sources because they suffer from low flux (luminance) and serious glare problems, and are Lambertian emitters, in other words, they radiate light in all directions inappropriately. To provide enough flux, luminaires are designed with an array of multiple traditional packaged LEDs and reflectors to give a desired illumination pattern. Their complex structures, however, reduce their reliability and increase installation difficulty.

One factor that plays an important role in road lighting applications is light utilization, the fraction of the light energy emitted by a source that is projected onto a targeted region of the road. High-pressure sodium (HPS) lamps have been widely employed as street lights because of their efficacy. However, it is impossible to confine all the light rays emitted from a large-size HPS source within the target region, resulting in light pollution and energy waste.

Given the small size of LEDs (∼1mm2), however, they can be combined with specially designed freeform lenses to improve light utilization. Instead of a traditional spherical shape, freeform lenses have complex, multi-curved surfaces without any prescribed symmetry. They precisely control light to achieve a desired energy distribution. Such optical systems have already been designed for road illumination. Each light ray emitted from an LED mounted on a flat installation board is directed by its corresponding lens to an appointed region: see Figure 1(a). In this way, the mapping of light energy between the LEDs and the target plane is established. A street lamp consists of many LED-lens combinations: Figure 1(b).

Figure 1. (a) Combination of an LED and a freeform optical lens. (b) A street light employing multiple LEDs and freeform lenses.

We recently proposed a systematic method for designing a freeform lens for LED street lamps that is optimized to produce a certain luminance distribution on the road.1 The method takes into account the luminance characteristics of the road surface, the energy efficiency of the system, the glare problems of the lamp, and the effects of four adjacent sources shining on the same area.

First we fix geometrical parameters such as mounting height, mounting space, and road width. Then we obtain an optimized illuminance distribution with a polynomial of cosine functions along the road direction. We do this by maximizing the Q value (ratio of the average luminance to the average illuminance) and by satisfying the lighting requirements provided by the International Commission on Illumination. Finally, we construct a smooth freeform lens with the optimized distribution based on the ‘variable separation’ method.2The simulation results may have large deviations from the given illuminance because of the extended light source and freeform surface errors. By employing feedback functions, though, the simulation results of the lens model approach the optimized results3 (see Figure 2). Figure 3 illustrates the final lens model. Table 1 shows the lighting parameters provided by street lights with LEDs and the optimized lenses at different observer positions.

Figure 2. Light distribution curves before and after feedback. γ: Angle between the light ray and the vertical axis of the luminaire. C: Angle between the projection line of the light ray on the road and the axis along the road.

Figure 3. The final lens model coated on the LED.
Table 1. Lighting parameters calculated from the simulation results of an optimized lens model. Lav, Eav: Average luminance, average illuminance. U0, E0: Overall luminance uniformity, illuminance uniformity. UL: Longitudinal luminance uniformity. TI: Glare factor.
Observer positionLav (cd/m2)U0ULTI (%)Eav (lx)E0Q(Lav/Eav)
(−60.000, 1.875, 1.500)1.500.560.7210.0618.980.717.90×10−2
(−60.000, 5.625, 1.500)1.500.550.796.7317.250.718.69×10−2

Our optical system generates light stripes that are parallel to the direction of the road. The stripes are visible upon close inspection. LED street lights with this striped illumination pattern have been set up in more than 20 provinces in mainland China: see Figure 4. Field tests show that these sources consume at least 50% less electrical energy than commercially available HPS street lamps while improving the illumination effect. Since the luminous efficacy of the LED lights (less than 90lm/W) is lower than that of HPS lamps (about 100lm/W), we conclude that high light utilization is the main reason the new luminaires save energy.

Figure 4. Applications of LED street lights with high light utilization and better illumination effect in (a) Guangzhou and (b) Dalian, China.

We are now looking into the application of LEDs for general interior lighting. Here, higher luminous efficacy can save more energy if different sources have identical lighting effects. High-quality interior luminaires need high luminous efficacy, uniform scattering, and soft, low-glare light. Fluorescent lamps provide all these traits. The luminous efficacy of traditional packaged white LEDs surpasses that of fluorescent lamps, but their light emittance causes serious glare problems. Secondary optical systems such as diffusers can reduce glare and produce more uniform scattering. However, diffusers currently involve a trade-off between light transmittance and processing costs. This reduces the luminous efficacy and hence the energy savings of LED sources, and makes them less competitive with traditional indoor lamps. Diffusers with high light transmittance that are low cost and easy to process will be a key technology in the application of LEDs to interior lighting. We plan to pursue the development of this technology.

Yi Luo
Tsinghua University
Beijing, China

1. Z. X. Feng, Y. Luo, Y. J. Han, Design of LED freeform optical system for road lighting with high luminance/illuminance ratio, Opt. Express 18, pp. 22020-22031, 2010.
2. L. Wang, K. Y. Qian, Y. Luo, Discontinuous free-form lens design for prescribed illuminance, Appl. Opt. 46, pp. 3716-3723, 2007.
3. Y. Luo, Z. X. Feng, Y. J. Han, H. T. Li, Design of compact and smooth free-form optical system with uniform illuminance for LED source, Opt. Express 18, pp. 9055-9063, 2010.