DESIGN OF 2.2-INCH LED BACKLIGHT UNITS FOR BRIGHTNESS UNIFORMITY AND REDUCTION OF HOT-SPOT EFFECT Jeng-Feng Lin, Chin-Chieh Kang, and Chih-Yang Liu Department of Electro-Optical Engineering, Southern Taiwan University, Tainan, Taiwan 97-EC-17-A-05-S1-114 INTRODUCTION Using LEDs to replace CCFLs is the trend for backlight units of LCDs. However, the LED introduces a serious issue called hot spot, which makes the areas 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, 2, 3]. In this research we tried to develop techniques to obtain a uniform LED backlight unit and reduce the effect of hot spot. A 2.2-inch LED backlight unit has been designed. Brightness uniformity is obtained by microstructures on both sides of the parallel LGP. Reduction of hot-spot effect is obtained by inserting a glass plate with microstructures on both sides. The techniques developed on this small-size backlight unit can be applied to larger backlight units. Figure 1 shows the schematic diagram of the designed backlight unit. There are four LEDs and one 1 mm thick glass plate is inserted between the LED and LGP to reduce the effect of hot spot. The absorbing surface on top of the LGP is only for the purpose of simulation. Figure 1. Schematic of the designed backlight unit. 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, various microstructures are added on top and bottom surfaces of the LGP. In this research, we divide the top surface into 11 regions. The width from region 1 to 9 is 4.4 mm and the width of the last two regions is 2.2 mm. Each region has a periodic v-groove structure with the same period of 144 mm as shown in Fig. 2, but with different heights as shown in Table 1. The direction of the groove is parallel with the front surface of the LGP. In addition, the bottom surface of the LGP is a rough surface, which is equally divided into 10 regions with width of 4.4 mm, but with different rms slopes as shown in Table 2. 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. 144  m Table 1 Height of the v-groove structure on the top surface of the LGP region Height(  m) Figure 2 The v-groove structure on the top surface of the LGP. Abstract--- This paper describes the design of a 2.2-inch LED backlight unit for brightness uniformity and reduction of hot-spot effect. Brightness uniformity is obtained by microstructures on both sides of the parallel light guide plate. Reduction of hot-spot effect is obtained by inserting a glass plate with microstructures on both sides. Table 2 Rms slope of the rough bottom surface of the LGP region Rms slope (rad.) To reduce the effect of hot spotthe incident light to the LGP to diffuse as soon as possible. Therefore, on the front surface we add periodic vertical v-grooves with period of 50 mm and apex angle of 120°. In addition, a 1 mm thick glass plate with microstructures on both sides is inserted between of the LGP and LEDs. The microstructure on the front surface of the glass plate is the same periodic v-groove as that on the front surface of the LGP; the microstructure on the rear surface is a rough surface with rms slope of 0.1 radian. For comparison, in this research we also designed backlight units without glass plate inserted between LEDs and the LGP. For these designs the, we have to let gap between LEDs and the LGP is 1.4 mm wide. RESULTS AND DISCUSSION The designed backlight units are simulated by the optical simulation software ASAP. For the design without the inserted glass plate, Fig. 3 shows its intensity distribution right before and after the front surface of the LGP. The red and green curves represent the intensity in horizontal (xy plane) and vertical (yz plane) directions, respectively. From the red curve of Fig. 3(b), we can see that the v-grooves on the front the surface do enhance the light spread and mix after the light enters the LGP. Fig. 4 shows luminous exitance and intensity from the top surface of the LGP. Figure 4(a) indicates that four hot spots exist in a narrow band close to the front surface of the LGP, but apart from the front surface by more than 3.5 mm, the uniformity of exitance is good. To have good uniformity, parameters of the microstructure in each region have to be tuned. In general, regions farther away from the front surface have larger heights or slopes. Figure 4 shows most of the emergent rays from the top surface leave the surface at zenith angle from 50 ° to 80 ° and maximum intensity occurs at about 70 °. The distribution of zenith angle for emergent rays only change slightly among different regions of the top surface. (a) before (b) after Figure 3 The intensity distribution right before and after the front surface of the LGP. Figure 4 Luminous exitance and intensity from the top surface of the LGP for the design without the inserted glass plate. For the design with the inserted glass plate, Fig. 5 shows luminous exitance and intensity from the top surface of the LGP. Figure 5(a) indicates that hot spots still exist. However, by comparing the exitance curves on the same line in Fig. 4(a) and Fig. 5(a) (2.4 mm away from the front surface of the LGP), the exitance uniformity is improved and the effect of hot spot is reduced. The reason is that the microstructures on the glass plate effectively diffuse the incident light in advance. The cost of this improvement is slight reduction of extraction efficiency, which is defined as the ratio of luminous flux emitted from the four LEDs to luminous flux emergent from the top surface of the LGP. The extraction efficiencies for the designs without and with the inserted glass are 78.4% and 62.4%, respectively. The reason for this decrease of extraction efficiency is the internal reflection at the rear surface of the glass plate. (a) luminous exitance (b) luminous exitance Figure 5 Luminous exitance and intensity from the top surface of the LGP for the design with the inserted glass plate. CONCLUSION AND ACKNOWLEDGEMENT This paper describes the design of a 2.2-inch LED backlight unit for brightness uniformity and reduction of hot-spot effect. Brightness uniformity is obtained by microstructures on both sides of the parallel light guide plate. Reduction of hot-spot effect is obtained by inserting a glass plate with microstructures on both sides. However, the effect of hot spot still exists. This research was sponsored by the Ministry of Economic Affairs under project No. 97-EC-17-A-05-S REFERENCES 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 , CHIH-CHIEH KANG, JENG-FENG LIN, JUN-SHIAN YU, CHO-WEI CHEN, SHI-FU ZENG, “ OPTIMAL DESIGN OF A DOUBLE-SIDE V-GROOVE LIGHT GUIDE PLATE IN A LED BACKLIGHT UNIT, ” ASID ’ 09 (THE 11TH ASIAN SYMPOSIUM ON INFORMATION DISPLAY ), PP , OCT THE 11TH ASIAN SYMPOSIUM ON INFORMATION DISPLAY Chia-Yin Chang, et. al., “ Light guide plate having light diffusing entities on light entering side, ” US Patent No. 7,347,610, Radiant Opto-Electronics, Mar. 25, (a) luminous exitance (b) luminous exitance