Transcript 下載/瀏覽Download

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
[email protected]
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.
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
Region
1
2
3
4
Width (mm)
3.8
3.2
7
6
Rms slope
(rad.)
0.1
0.05
0.1
0.13
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 vgroove 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.
Fig. 2 Schematic of the pyramid.
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.
Fig. 3 The intensity curves from the top surface of the LGP with the reverse prism sheet.
(a)
(b)
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.
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-17A-05-S1-114.
References
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.
[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.
[2] 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,” in ASID’09 (the
11th Asian Symposium on Information Display ), pp. 274-277, Oct. 2009.
[3] 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, 2008.
[4] Man Soo Kim , “Light guide plate having visual angle adjusting member and liquid crystal display apparatus having the same,” US Patent No. 6,836,303, Samsung Electronics, Dec.
28, 2004.
[5] Chih-Chieh Kang, Jeng-Feng Lin, Cho-Wei Chen, Yu-Chang Wu, “Virtual mechanism for displaying viewing angle related mura of a backlight unit in simulation,” in iMiD09 (The
9th International Meeting on Information Display), Oct. 2009.