Shih-Cheng Yeh, Shao-Tun Chung, Chih-Chieh Kang, and Jeng-Feng Lin

1. LED UNIFORM SURFACE LIGHT SOURCE WITH NO LIGHT GUIDE PLATE Shih-Cheng Yeh, Shao-Tun Chung, Chih-Chieh Kang, and Jeng-Feng Lin Department of Electrooptical Engineering, Southern Taiwan University Tainan, TAIWAN E-mail: kangc@mail.stut.edu.tw Abstract In this paper, we present a novel approach for the design of an edge-type LED backlight unit (BLU) with no light guide plate. The effect of several important design parameters, such as BLU height, LED position, location of bumpy structure as well as its height, on illuminance uniformity is examined. Typical simulation results are presented. They demonstrate the feasibility of our devised approach. Introduction In the rising global tide of energy saving and environmental protection, LED has become the backlighting source of choice for LCD displays. This is due to its advantages of being low power consumption, free of mercury, longer life, continuously improved luminous efficacy, and rapidly decreased price, etc.However, as the size and thickness of a backlight unit (BLU) becomes larger and thinner, the design of the indispensable components for an edge-type LED BLU―light-guide plate (LGP) also becomes more challenging and its manufacturing processes get more complicated and costly. Therefore the concept of edge-type LED BLUs with no LGP[1] exhibits significant competitive advantage over its adversaries: edge-type BLUs with LGP and direct-type BLUs as well in terms of manufacture process and cost. As for its implementation[1-3], in most cases side-emitting LEDs with narrow beam angle, such as ±20° for Luxeon Emitter, are demanded. But the use of side-emitting LEDs implies energy loss due to the multiple reflection of light inside the LED lamp reflection. To rectify this problem, we propose a slightly modified approach―replacing side-emitting LED with shell-type narrow beam LED, e.g., Nichia NSPW500, of which the half angle is within ±10°, illustrated by its radiation pattern[4], shown in Fig. 1. Fig. 2. (a) Schematic and (b) cross-section of proposed edge-type LED BLU with no LGP. Three sections of bumpy structures are constructed on the bottom reflector. Experiment To examine the effectiveness of our proposed approach, An ASAP ray-tracing model for a 20-inch edge-type LED BLU with no LGP, as shown in Fig. 2, has been developed to perform the design work of illuminance uniformity for a LED BLU. With the concern of computation time in simulation, a truncated version of the model is implemented. It consists of 10 LEDs, five on each short side housed inside LED lamp reflectors, a bottom reflection sheet with bumpy structures, and a diffusion plate on-top. The inner surface of lamp reflector is taped with white reflector, whereas the surface of bottom reflector is kept to be of specular reflection. The dimension of the BLU is of 62.6 × 251.5× H mm, where H is the height of the BLU. In order to ensure the accuracy of optical simulation, an accurate BSDF model of ASAP for a white reflector has been constructed according to the measured data. As for the diffusion plate, a developed optical model based on Mie scattering is implemented. To simplify analysis, the LED is assumed to be a monochromatic light source with wavelength of 550 nm and the size of scatter inside the diffusion plate is assumed to be uniform instead of a size distribution function, though the scattering of light through a diffusion plate depends on particle size, volume fraction, the wavelength of incident light, and the relative refractive index between the scatters and the host media, etc[5]. Fig. 3. Simulation results. (a) hB = h/2, lB = lC/2 (b), hB = h/4, lB = lC/2 and , (c) hB = h/2, lB = 3lC/4. Fig. 4. Illumiance uniformity. Simulation results (a) without, and (b) with a diffusion plate. Results and discussions Conclusions By using a shell-type narrow beam LED, we can implement the concept of an edge-type LED BLU without LGP. To achieve illuminance uniformity for a uniform backlight source, the optimal design parameters can be achieved through optical simulations. This demonstrates the feasibility of using our devised approach. However, these parameters can be varied by the size as well as the height of a BLU. And future work will focused on the problem of thinness for this structure. REFERENCES 【1】 Sakai, S., Mori, A., Ishiguchi, K., Kobauashi, K., Kokogawa, T., Sakamoto, T., and Yoneda, T., “A thin LED backlight system with high efficiency for backlighting 22-in. TFT-LCDs”, SID 04 Digest, (2004) pp. 1218- 1221. 【2】Chen, C. H. and Shieh, H. P. D., Inclined LED array for large-sized backlight system, SID 05 Digest, (2005) pp. 558-561. 【3】Hung, C. P., Chen, W. S., Lin, J. H., and Li, W. Y., Novel design for LED lens and backlight system, (2007) IDMC07, pp. 476-479. 【4】NICHIA Corporation, www.nichia.com. 【5】Hsu, Y. C., Kang, C. C., and Lin, J. F., 3D Simulation of a LCD backlight diffuser using Monte Carlo ray- tracing modeling, in Proc. 2006 ISNST, Tainan, (Nov. 2006) pp. 159-168. Fig. 1. Radiation pattern of Nichia NSPW500.