Simulations

According to the optimization flow chart in Fig 3-12, the illumination map of the unitary wedge shaped light guide with prismatic micro-bump structures is optimized as shown in Fig. 3-13. The uniformity of the extraction light and the efficiency of the single light guide unit are 93% and 68%, respectively. Furthermore, the directivity of the extraction light is indeed enhanced. The light coupling from light guide has a narrow divergent angle ±15° on the y-z plane, and about ±50° on the x-z plane, which are shown in Fig. 3-14. This directionality is expected to reduce the probability of the light that leak to the region whose state of the LC is still changing.

Fig. 3-13 Luminance map of wedge shared light guide unit.

Fig. 3-14 Rectangular candela distribution plot for divergence of extraction light.

As mentioned above, the tandem light guide will be assembled and connected with each other into a large panel size LCD. Therefore, each connected boundary between light guides is an important issue. The partitions must be not visible from any viewing angle on the panel due to the shade or obviously discontinuous luminance severely affects the display performance. For this reason, except for simulating the single light guide unit, the 3x3 tandem wedged light guide matrix is also simulated as shown in Fig. 3-15. In this situation, we do not observe any obvious discontinuity.

This simulation is in ideal case. However, the defects may occur during fabrication process in real cases. The finally results will be given in the Chapter 4 Experiments.

Fig. 3-15 Illumination map of 3X3 tandem light guide matrix.

Furthermore, in order to estimate the influence of the image color distortion, we introduce an evaluation, light leakage ratio, which defined as

turned on. The corresponding simulation is given in Fig 3-17. The light leakage from the operating block to the neighboring divisions is accounted for 9.2%.

Fig. 3-16 Definition of the light leakage ratio.

Fig. 3-17 Illumination map of light guide with single division lit up.

3.8 Summary

The wedge shaped light guide unit combining with the prismatic micro-bump structures has been optimized. The uniformity of the emitting light can be achieved about 93% from the simulation. Also, the efficiency can be obtained about 68%. In addition, the angular distribution of the extracted light can even be controlled within

±15° in scanning direction with the prismatic micro-bump structures. We also simulate the boundary between each connected light guides. In 3x3 tandem light guide matrix, there is no obvious discontinuity. Furthermore, the light leakage ratio which is used to evaluate the image color distortion is 9.8%.

Chapter 4

Experimental Results and Discussions

4.1 Introduction

In this chapter, the diamond turning process technology, adapted to implement the prismatic micro-bump structures of tandem wedge shaped light guide units, will be briefly introduced in advance. After that, the fabricated micro-bump structures are examined with the optical microscope and the alpha step.

For the backlight system, the integrating sphere is utilized to measure the optical property of the 4-in-1 LED. Moreover, the Conoscope is used to survey the angular distribution of the illuminating light from the whole BLM. Then, the uniformity and efficiency of the light coupling from light guide are also measured by the charge coupled device (CCD). Finally, the light leakage ratio from the operating block to the adjacent blocks will be given.

4.2 Diamond turning process technology

Diamond turning, a kind of micro-mechanism machining, is utilized the copy principle to duplicate the desired structures. Different to the injection molding which is suitable for the mass production, the diamond turning process provides low cost and flexible fabrication. Therefore, the direct carving is the method for implement in this study.

The fabrication steps of the wedge shaped light guides with prismatic micro-bump structures can be roughly illustrated as Fig. 4-1.

(i) At first, the light guide plates are sliced and polished into a wedge shaped.

along the z direction to set an orientation as well as the depth of the structure. Then, the knife is moved horizontally toward the y direction to form a prismatic micro-bump structure.

(iii) After forming a single structure, the diamond knife shifts a specific pitch along the x direction and sets another orientation to repeat the step (ii).

(iv) Continuously repeating step (ii) and (iii), a wedge shaped light guide with prismatic micro-bump structures is fabricated finally.

(i)

(ii)

(iii)

(iv)

Fig. 4-1 The fabrication process of the wedged light guide with micro structure.

4.3 Wedged Light Guide and Micro-structure profiles

In this study, the wedge shaped light guide and prismatic micro-bumps structures are supported by Coretronic and Industrial Technology Research Institute (ITRI), respectively. After the fabrication, the profiles of light guide unit are shown in Fig.

4-2. Fig. 4-3 indicates the micro-bumps profile which is detected by the optical microscope (OM). Because it is difficult to focus in wedge shaped light guide, the fuzzy edge appears. Furthermore, the depth and width of the micro structures are measured by the alpha step as shown in Fig. 4-4. The width of the micro-bump structures is varied along the x direction from 30μ m to hundreds μ m.

Fig. 4-2 The fabricated wedged light guide with micro structure.

Fig. 4-3 The prismatic micro-bumps profiles measured by the optical microscope.

Fig. 4-4 The depth and width of the prismatic micro-bumps profiles measured under the alpha step.

4.4 Light Source Properties

The package of 4 in 1 (RGGB) LEDs is used as light source. The angular distribution directly affects the whole property of the BLM. In the experiment, the performance of LEDs is measured by an integrating sphere as shown in Fig. 4-5.

When current is fixed by 100mA, the voltage on green, blue, and red color states are driven by 3.3v, 3.4v, and 2.2v. The luminous flux is achieved by 8.2lm, 2.2lm, and 5.2lm, respectively.

4.5 Optomechanical Setup

The optomechanical setup which consists of LED light bars, holder mechanism, and light guide units are used to demonstrate the simulation. Each light-bar PCB contains 15 packages of 4-in-1 RGGB LEDs with 3 input / output port. Furthermore, the heat sinks and holder mechanism are implemented due to the thermal and light guide tilt issues as shown in Fig. 4-6. The holders are made by aluminum. The thickness of the entire BLM is about 25mm. Next part we will examine the optical performance of the whole BLM.

(a)

(b)

(a) (b)

Fig. 4-6 (a)Optomechanical setup; (b)Tandem light guides combined with optomechanical setup.

4.6 Optical Performances of BLM

After measuring the light source property, the uniformity of the 2x2 tandem light guide matrix on different color states are captured and analyzed by the CCD camera as shown in Fig. 4-7. The uniformity by 9 points of the LG are 89%, 86%, 84%, and 85% on R, G, B, and the white color states associated with a brightness enhance film and a diffuser. In addition, according to the measurement and calculation, the optical efficiency of BLM is 63%. The lumen per watt in terms of R, G, and B color states are 33 lm/w, 37 lm/w, and 9.1 lm/w, respectively. Furthermore, the discontinuity of the boundary between LGs is undistinguishable.

(a)

(b)

(c)

(d)

Fig. 4-7 Brightness uniformity of 2x2 tandem light guide matrix – (a) Red (b) Green (c) Blue. (d) Single light guide on white state.

Moreover, in order to analyze the light leakage, only single block is lit up as mentioned in simulation. The light leakage from operating block penetrate to the adjacent divisions is well suppressed to below the 9.8%. From the operating block to the third division, the light leakage is repressed near to zero. However, due to the entire light guide and driving program are still in progress, the effect of the light leakage can not be defined very well. Therefore, the influence of the light leakage on the image quality with LC panel will be further evaluated.

Fig. 4-8 Light leakage from operated block to the neighboring blocks

After that, the angular distribution of the wedged type light guide unit is measured by Conoscope. The result is shown in Fig. 4-9. The full width at half maximum (FHWM) of the extraction light on different color state is about 30 degree in y direction (dashed line) and 50 degree in x direction (solid line). Such directionality is expected to reduce the possible color mixing error.

(a)

(b)

(c)

Fig. 4-9 The angular distribution of LG module in (a) Green light (b) Red light (c)

4.7 Summary

The tandem wedge type LGs with prismatic micro-bump structures for scanning FSC BLM are fabricated. The function of the LG unit enables collimating the high divergence of the incident light into ±30° in y direction and ±40° in x direction.

Furthermore, the uniformities on R, G, and B color states exhibit 89%, 86%, and 89%, respectively. The thickness of the whole BLM is about 25 mm. We successfully define the partition down to 9.8% light leakage to the neighboring divisions without any shields or gaps. Such light leakage can avoid the occurrence of color dependence for adjacent portions of the light guide. However, due to the tilt angle of the wedge shaped light guide is more oblique than the common case, the optical efficiency is not good enough. This drawback should be further improved by the proper optimization.

Chapter 5

Conclusions ＆ Future Works

With development of education, communication, and entertainment in human daily life, LCDs become an important display technology. High brightness, resolution, and excellent color rendering are the major concerns. Although several configurations of the hold-type LCDs have been proposed, many issues such as motion blur, optical efficiency, and poor color representation have large space to improve. In this study, the scanning FSC LCD is proposed to overcome these defects.

Scanning FSC LCD has potential to serve as the new approach in terms of offering better image quality. It can efficiently improve the fuzzy edge of the moving picture, provide higher color gamut without color filter less and higher optical efficiency. These advantages drive us to pursue an partitional BLM in such display.

The tandem wedge shaped light guides combining with the prismatic micro-bump structures have been introduced in this thesis. The prototype of the spatial and temporal scanning backlight system have been demonstrated. 4-in-1 red, green, green, and blue LEDs are utilized to generate color fields sequentially during the scanning process. Different with conventional LCDs which are supplied the entire uniform backlight, the scanning FSC LCDs must provide isolated scanning partitions and well light divergence for each division. Otherwise, the light will leak to the adjacent blocks and cause image color distortion. The proposed tandem wedge shaped light guide combining with micro-bump structures can partition the BLM into several isolated blocks without any shields and gaps. The prismatic micro-bump structures are

distribution of the illuminating light as well as the uniformity along the pipe direction.

According to the simulation, the wedge shaped light guide is designed and optimized by modifying the filling factor and incline angle. The uniformity and efficiency of the light guide unit can be obtained 93% and 68%, respectively. The divergence angle in scanning direction is ±15°. In addition, the light leakage ratio from operating block to the neighboring divisions is accounted for 9.2%.

In the experiment, the CCD camera and the Conoscope are utilized to measure the optical performance of BLM. The uniformities of the different color states are 84% to 89%. The divergence angle of scanning direction is ±30°. The light leakage ratio from operating block to the neighboring divisions is suppressed down to 9.8%.

The experimental results in close agreement with the simulation confirm our optical modeling and fabrication precision. The most concerned issue lies in the boundary between light guides is undistinguishable. Finally, the thickness of the whole BLM which consists of holders and optical films are 25 mm without any shields or gaps.

In the future, the BLM will be coupled with FSC scanning program and OCB liquid crystal panel to further evaluate the influence of the light leakage on image quality. In addition, the different driving method for FSC scanning BLM such as the dual scan or two-dimensional scan can be employed to enhance the optical performance.

Reference

[1] J. A. Castellano, Hanbook of display technology, Chapter 1, Academic Press, Inc., San Diego (1992).

[2] T. Yamamoto, Y.Aono and M. Tsumura, “Guiding Principles for High Quality Motion Picture in AMLCDs Applicable to TV Monitors,” SID ‟00 Digest, p.456 (2000).

[3] Taiichiro Kurita “Moving Picture Quility improvement for Hold-type AM-LCDs,” SID ‟01 Digest, p986 (2001).

[4] Beak-woon Lee, Dongsik Sagong, and Gyuha Jeong, “LCDs: How Fast is Enough?” SID ‟01 Digest, p.1106 (2001).

[5] Richard I. McCartney, “A Liquid Crystal Display Response Time Compensation Feature Integrated into an LCD Panel Timing Controller,” SID ‟03 Digest, p.1350 (2003).

[6] Guo-Ping Chen, Masahiko Yamaguti, Naoki Ito, Takayuki Aoki and Atsuo Fukuda, “Target Response Time of Liquid Crystal Displays Estimated by Analyzing the Front and Rear Part Gray Levels of Moving Square Patterns,” Jpn.

J. Appl. Phys. Vol.38 pp. L646-648 (1999).

[7] T. Nose, M. Suzuki, D. Sasaki, M. Imai, and H. Hayama, “A Black Stripe Driving Scheme for Displaying Motion Pictures on LCDs,” SID ‟01 Digest, p.994 (2001).

Picture Quality in LCDs by using of “Motion Picture Response Time” and Subjective Evaluation,” SID ‟03 Digest, p.1044 (2003).

[9] Tsutomu Furuhashi and Kazuyoshi Kawabe, “High Quality TFT-LCD System for Moving Picture,” SID ‟02 Digest, p.1284 (2002).

[10] D. S. Park, J. M. Han, K. W. Bae, S. Y. Kim, Y. H. Kim, and Y. J. Lim, ”The improvement of moving picture quality in TFT-LCD with Blinking Backlight Unit and Overdriving,” IDW ‟03, p.1535 (2003).

[11] Sunkwang Hong, Jac-Ho Oh, Po-Yun Park, Tae-Sung Kim, and S. S. Kim,

“Enhancement of Motion Image Quality in LCDs,” SID ‟04 Digest, p.1353 (2004).

[12] T. Fukuzawa, T. Toyooka, Y. Sakaguchi, K. Takeda, and F.Yamada,

“Rapid-Response Fluorescent Lamps for Field-sequential Full-color LCDs,”

SID ‟98 Digest, p.247 (1998).

[13] N. Ogawa, T. Miyashita, and T. Uchida, “Field-Sequential-Color LCD Using Switched Organic EL Backlight,” SID ‟99 Digest, p.1098 (1999).

[14] Norio Koma, Tetsuya Miyashita, Tatsuo Uchida, and Nobuhiro Mitani,”Color Field Sequential LCD Using an OCB-TFT-LCD,” SID ‟00 Digest, p.632 (2000).

[15] T. Yoshihara, T.Makino, and H. Inoue, “A 254-ppi Full-color Video Rate TFT-LCD Based on Field Sequential Color and FLC Display,” SID ‟00 Digest, p.1176 (2000).

[16] Fumiaki Yamada, Hajime Nakamura, Yoshitami Sakaguchi, and Yoichi Taira,

“Sequential-color LCD based on OCB with an LED Backlight,” Journal of the

SID, vol.10,no.1, pp.81-85(2002).

[17] Fang Jin Yoo, Jong Hoon Woo, Hyun Ho Shin and Chang Ryong Seo, “Side Light Type Field Sequential Color LCD Using Divided Light Guide Plates,”

IDRC 03, p.180 (2003).

[18] Kälil Käläntär, Tadashi Kishimoto, Kazuo Sekiya, Tesuya Miyashita, Tatsuo Uchida, “Spatio-temporal scanning backlight mode for field-sequential-color optically-compensated-bend liquid-crystal display”, Journal of the SID, vol. 14, p.151-159 (2006).

[19] Ming-Chien, et al., “Led Light Lit for Field-Sequental-Color Backlight System,” SID ‟07 Digest, p.441-444 (2007).

[20] Kälil Käläntär, Tadashi Kishimoto, Kazuo Sekiya, Tesuya Miyashita, Tatsuo Uchida, “Spatio-temporal scanning backlight for Color-Field-Sequential optically Compensated Bend Liquid Crystal Display”, SID ‟05 Digest, p.1316-1319 (2005).

[21] K. Sekiya, T. Kishimoto, K. Wako, S. Nakano, H.Ishigami, “Spatio-Temporal Scanning Backlight for Llarge Size Field Sequential Color LCD”, IDW‟05, p.1261-1264 (2005).

[22] Ｋ.Ｈ.Chen , et al., “Spatial-temporal Division in Field Sequential Color Technique for Color Filterless LCD ,” SID „07 Digest, p.1806-1809 (2007).

[23] Tastsuo Uchida and Takahiro Ishinabe, et al., “Color Imaging and Display System with Field Sequential OCB LCD,” SID „06 Digest, p.166-169 (2006).