1. Reduction of the Hot-Spot Effect on the Light Guide Plate by Inverted Pyramid Arrays Jeng-Feng Lin, Chin-Chieh Kang, Pei-Chiang Kao Department of Electro-Optical Engineering, Southern Taiwan University, Tainan, Taiwan m99l0217@webmail.stut.edu.tw Fig. 1 Schematic of the designed backlight unit. Table 1 Width and rms slope of each region of the rough top surface of the LGP Region1234 Width (mm)3.83.276 Rms slope (rad.) 0.10.050.10.13 Fig. 2 Schematic of the pyramid. Fig. 3 The intensity curves from the top surface of the LGP with the reverse prism sheet. Fig. 4 (a) Luminous exitance considering all rays and (b) luminous exitance considering only emerging rays inside a cone angle of 5  around the normal direction. Fig. 5 (a) Intensity considering all rays and (b) luminous exitance considering only emerging rays inside a cone angle of 5  around the normal direction. (a) (b) Abstract The object of this research is to reduce the effect of hot spots at cross points of the cross pattern, which is observed when luminous exitance considers only emerging rays inside a cone angle of 5  around the normal direction. Simulation results show that the addition of inverted pyramid arrays on the top surface of the light guide plate reduces the effect of hot spots at cross points. Introduction Using LEDs to replace CCFLs is the trend for backlight units of LCDs. However, the LED can introduce a serious issue called hot spot, which makes some spots on light guide plate (LGP) nearby the LEDs are much brighter than other areas on the LGP. Various designs have been proposed to solve this problem [1-3]. In this research we tried to reduce this effect by the application of an inverted pyramid microstructure on the top surface of the LGP. A small-sized LED backlight unit with a parallel LGP has been designed. Both sides of the parallel LGP have microstructures to obtain good brightness uniformity. Design of the backlight unit For parallel LGPs, incident rays can only totally reflect inside the plate and cannot emerge from the top surface of the plate because of total internal reflection. Therefore, The bottom surface has a uniform v-groove structure with period of 28.5μm and apex angle of 115.4°. The direction of the v- groove is perpendicular to the front surface of the LGP. On the top surface the area from the front surface of the LGP to the absorbing surface is divided into four regions. Width and rms slope of each region is shown in Table 1. We can regard the rough surface as composed of many small planes. The slope of each small plane is the sine value of the zenith angle of the surface normal. The rms slope is the rms value of slopes from small planes. On top of the LGP there is a reverse prism sheet with period of 57μm and apex angle of 68°. To reduce the effect of hot spot, three arrays of inverted pyramid are applied in the second region of the top surface, as shown in Fig. 1. For the pyramid, the angle between the tilt surface and the base surface is 54.7°, as shown in Fig. 2. This kind of structure can be obtained by chemical etching on Si substrate. Results and discussion The designed backlight units are simulated by the optical simulation software ASAP. Totally twenty million rays are traced. Figure 3 shows the intensity curves from the top surface of the LGP with the reverse prism sheet. As predicted, emerging rays from the LGP are redirected toward the normal direction to increase the luminance around the normal direction. Usually the luminous exitance for the backlight unit is presented with every ray being included. However, for a viewer only emerging rays inside a small cone angle are collected. In addition, mostly the luminous exitance considering all rays and considering only emerging rays inside a small cone angle around the normal direction are quite different [5]. Figure 4 shows a simulation example with our design. From Fig. 4(b) we can see bright spots right in front of the LEDs and a cross pattern with bright spots at cross points. The former bright spots can be easily blocked by a black tape or plastic frame. However, the latter bright spots is really an issue because we cannot block the whole area covering the cross pattern. For the four middle cross points we put three same arrays of inverted pyramid on dark areas between adjacent cross points, as shown in Fig. 2. Let’s consider emerging rays from the top surface of the LGP with the reverse prism sheet. Figure 5 shows the simulated intensity considering all rays and luminous exitance considering only emerging rays inside a cone angle of 5  around the normal direction. The addition of pyramid arrays hardly changes intensity curves considering all rays; however, for luminous exitance considering only emerging rays inside a cone angle of 5  around the normal direction, it makes luminous exitance around the four middle cross points more uniform. Therefore the addition of pyramid arrays reduces the effect of hot spots at cross points. Through improved designs we believe the uniformity of luminous exitance can be better. Conclusion Simulation results show that the addition of inverted pyramid arrays reduces the effect of hot spots at cross points. Through improved designs we believe the uniformity of luminous exitance can be better. This research was sponsored by the Ministry of Economic Affairs under project No. 97-EC-17- A-05-S1-114. References [1] Seung Ryong Park, Oh Jang Kwon, Dongho Shin, and Seok-Ho Song, “ Grating micro-dot pattern light guide plates for LED backlight, ” Optics Express, vol. 15, no. 6, pp. 2888-2899, 2007. 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