A High Power Light Emitting Diode Module for Projection Display Application Chenhui Peng1, Xiaoning Li1, 2, Jingwei Wang1, Lingling Xiong1, Yanxin Zhang1 and Xingsheng Liu1, 3 State Key Laboratory of Transient Optics and Photonics，Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences No. 17 Xinxi Road, New Industrial Park, Xi'an Hi-Tech Industrial Development Zone, Xi'an, Shaanxi, 710119, P.R. China Tel. 8629-88880786, fax 8629-88887075, [email protected] 2 Key Laboratory for Physical Electronics and Devices of the Ministry of Education & Shaanxi Key Lab of Information Photonic Technique, Xi‟an Jiaotong University, No.28, Xianning West Road, Xi'an, Shaanxi, 710049, P.R. China 3 Xi‟an Focuslight Technologies Co., Ltd. No. 17 Xinxi Road, New Industrial Park, Xi'an Hi-Tech Industrial Development Zone, Xi'an, Shaanxi, 710119, P.R. China 1

Abstract Light emitting diode (LED) has numerous advantages such as long lifetime, large color gamut, small size and the absence of mercury vapor. In recent years, there has been recognition that LED could be an alternative light source for projection display application. In this work, dielectric compound parabolic concentrators (CPCs) were designed and optimized for multiple-LED array packaging structure for projection display application. The performance of rectangular CPC was studied and compared to theoretical simulations. Rectangular CPC was fabricated as the collector and collimator. A high power green LED module combined with rectangular CPC was fabricated and its performance was characterized. More than 90% light emitted by multiple-LED array can be collected by the CPC and transmit within the designed angle. A high power of 2.09W and a high luminous flux of 800 lm were obtained. Introduction Current projection systems typically utilize ultra high performance (UHP) lamp for its high luminous flux. Although UHP lamps have very high luminous output, they have numerous disadvantages including short lifetime, poor color gamut, high voltage operation, high cost and use of mercury that is an environmental hazard. With the improvement in solid state lighting [1-2], high power LED has become an alternative light source for UHP lamps in projection display application [3-6]. In contrast to UHP lamps, LED has longer lifetime, larger color gamut, faster response time, smaller size, etc [7-12]. However, the luminous output of LED is still lower than UHP lamps. Therefore, it is important to improve light utilization efficiency of LED for projection display application [13]. In this paper, a dielectric CPC[14] was designed and optimized for multiple-LED array packaging structure to improve the light utilization efficiency. More than 90% light emitted by multiple-LED array can be collected by the CPC and transmit within the designed angle. A high power LED module was fabricated and its performance was characterized. A high power of 2.09W and a high luminous flux of 800 lm were obtained. Etendue-limited system

Etendue is a very important parameter in projection display. The etendue of a ray bundle will never decrease as the ray bundle passes through an optical system. The definition of etendue E is [15] (1) E  n 2  cos  dA  d where n is the refractive index of the medium, dA is the area of the interest, θ is the angle between the center of the emission cone and the normal to the emitting area and d is the solid angle of the light corresponding to that area. It is difficult to determine the etendue accurately. However, in the case of flat emitting surface normal to the optical axis with a uniform divergence angle 1 2 over the surface, its expression is given by [16] E  n 2 A  An 2 sin 2  12

(2)

where A is the emission area, and 1 2 is the half angle of the emission cone. A projection system is referred to as „etendue-limited‟ when there is an optical element or combination of optical element in the system whose etendue is lower than that of the ray bundle produced by the source/reflector combination [17]. Most practical etendue-limited projection systems are limited by the etendue of microdisplay. According to equation (2), the etendue of the microdisplay ED could be written as

E  A(N.A.)2 D

(3)

where N.A.  n sin  is the numerical aperture of the 1/ 2 projection lens and A is the panel area of the microdisplay. Accordingly, if we want to obtain a full light output through the optical system, the relationship between the etendue of the source Esource and the etendue of the microdisplay ED should be

E E source D

(4)

Optical design and simulation In this work, it was assumed that a 0.73 inch panel with an aspect ratio of 4:3 and a projection lens with a numerical aperture of 0.21 were used. A multiple-LED array with an

area of 6.72 mm2 was packaged. By equation (2), the etendue of source is 21.1 mm2·sr and by equation (3), the etendue of microdisplay is 22.86 mm2·sr. We chose CPC as the collector and collimator. Theoretically, about 96% of the light entering the entrance aperture of a CPC can go through the exit aperture within the designed angle. Thus, the CPC is nearly an ideal concentrator and well suited for LED applications [18]. According to edgeray principle of non-imaging optics, CPC was designed [14, 19]. The maximum angle θmax with which light rays will reach the exit aperture of the CPC is given by [15] (5)  a / a sin  max where a  is the semi-diameter of the entrance aperture and a is the semi-diameter of the exit aperture, as shown in Figure 1.

Figure 1 Schematic of CPC The focal length of the parabola is (6) f  a (1  sin  ) max The length of the CPC is (7) L  (a   a) tan  max In this work, multiple-LED array was encapsulated in the bottom cavity of the CPC. The cavity is full of silicone with a refractive index of 1.5. If LED dies are encapsulated in the silicone, the source etendue will be increased by a factor of 2.25. But the extraction efficiency of a LED in encapsulation is 2.3 times higher than that in air and encapsulation will result in less of a heat problem that can reduce LED output[20]. Therefore, we adopted the encapsulation solution. The shape of the CPC could be round or rectangular. Since both the microdisplay panel and LED die are rectangular. If a round CPC were used, there would be a mismatch in area and approximately 40% of the light would be wasted, since it would not enter the rectangular aperture[21]. A rectangular CPC has been designed and optimized. The bottom cavity of the rectangular CPC is in the shape of cuboid with a length of 3.4mm, a width of 2.7mm and a height of 0.5mm. LED dies are encapsulated by the silicone in the cavity. From the numerical aperture of the projection lens, the value of θmax is deduced to be 12°. By equations (5) and (7), the length of the rectangular CPC is given as 36.9mm. PMMA is used as the material of the rectangular CPC. The profile of the rectangular CPC is shown in Figure 2.

Figure 2 Profile of the rectangular CPC Simulation results are shown in Figure 3and Figure 4. The rectangular CPC has a colletion efficiency of 90.3%, slightly lower than the theoretical-limit value. From the candela distribution plot, it was found that the output divergence is a little larger than the designed angle. This is due to the refraction in PMMA-air interface on the output surface, which can be accepted in the application[22].

Figure 3 Raytracing results of the rectangular CPC

Figure 4 Candela distribution of light exiting the rectangular CPC The uniformity is also an important parameter in projection display application. The simulated uniformity of the rectangular CPC is shown in Figure 5. It was found that the center position is slightly darker than the edge position.

Figure 5 Simulated uniformity of the rectangular CPC The simulation results show that the collection efficiency of the rectangular CPC is high and its output surface is matched with the microdisplay. Although the uniformity of rectangular CPC is not good, the higher flux toward the edges can be used to advantage by counteracting the fall off in onscreen brightness toward the edges of the field, which is typical on-screen loss due to the cosθ roll-off of typically wide-angle projection optics[21]. Hence, the rectangular CPC was chosen as our light collector and collimator in this work.

Figure 8 Packaged green LED module under lighting condition The performance of green LED module was tested and characterized. The Flux-Current curve and relative candela distribution of the LED module are shown in Figure 9 and Figure 10.

Experimental results According to the simulation results, dielectric rectangular CPC was fabricated as shown in Figure 6.

Figure 6 Fabricated dielectric rectangular CPC The rectangular CPC was combined with multiple-LED array package and the multiple-LED module was fabricated, as shown in Figure 7.

Figure 7 Packaged multiple-LED module Packaged green LED module under lighting condition is shown in Figure 8.

Figure 9 Flux-Current curve of green LED module

Figure 10 Relative candela distribution of green LED module The testing results showed that the output power of green LED module at 6A is 2.09W, and luminous flux is 800 lm. Additionally, the divergence of output light beam is 33°, which is in accordance with the simulation results. Using the packaging structure mentioned above, red multiple-LED module and blue multiple-LED module have also been fabricated. Their lighting condition is shown in Figure 11.

LED module and blue multiple-LED module have also been fabricated. Based on the RGB multiple-LED modules, a prototype mini-projector was demonstrated. Acknowledgments This work is supported by “Open-fund of State Key Laboratory of Transient Optics and Photonics of Chinese Academy of Sciences”, the project of the “Hundred Talents Research Fund of Chinese Academy of Sciences”, the Instrument Developing Project of the Chinese Academy of Sciences, and the National High Technology Research and Development Program of China, Grant No. 2009AA032704.

Figure 11 Lighting condition of red LED module and blue LED module Based on the fabricated RGB multiple-LED modules, a prototype mini-projector was demonstrated, as shown in Figure 12. Its performance will be characterized in further study.

Figure 12 Prototype mini-projector Conclusions In order to improve light utilization efficiency of LED, a rectangular CPC was designed and optimized for multipleLED array packaging structure. More than 90% light emitted by multiple-LED array can be collected by the CPC and transmit within the designed angle. The rectangular CPC was fabricated using PMMA and combined with multiple-LED array. Hence, a high power LED module for projection display application was fabricated. An output power of 2.09W, a high output luminous flux of 800 lm and a divergence of 33°were obtained. Using the same packaging structure, red multiple-

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