A Multi-Performance Film for Highly Efficient LCD Backlights 多重性能薄膜應用於高效率的背光模組 Speaker: Jen-Chieh Hsieh (謝仁杰) Advisor: Prof. Han-Ping D. Shieh (謝漢萍教授) Prof. Yi-Pai Huang (黃乙白 助理教授) Dept. of Photonics & Inst. of Electro-Optical Eng.1, Display Institute2, National Chiao Tung University, Hsinchu, Taiwan (國立交通大學) March 6th, 2008 1

Outline Introduction Direct-lit backlight unit Motivation and Objective Multi-Performance Film (MPF) Principle Fabrication method Simulation and Experimental results Discussions Conclusions and Future works 1st I will introduce the background of the direct-lit backlight unit and its issue. After that are motivation and objective. And then I will introduce purposed idea “MPF”, and fabrication method. After that, I will show you sim and exp results. Finally, dis, con and fut work will be given.

Introduction Liquid crystal (LC)-TV tend to develop bigger size! Require higher brightness Brightness outputted from display: > 500 cd/m2 Direct-lit backlight unit for LC-TV Provide highly bright and uniform light source Output brightness: > 10000 cd/m2 (transmittance of LC-panel is ~5%) Uniformity: > 85% As we know, LC-TV are more popular in recent, and it will replace all traditional CRT TV in the future. Now, LC-TV tend to develop bigger size, however, big size display does not like small size display, it require higher brightness. In general, the brightness outputted from display has to higher than 500 nits. Therefore, for this purpose, direct-lit backlight unit is usually utilized in LC-TV. Because, it can provide highly bright and uniform light source. And the general specification of direct-lit B/L, for example, brightness has to higher than ..., and uniformity has to higher ....

Introduction Conventional direct-lit backlight unit Quality of B/L? Top diffuser (adjust light distribution for output) Brightness enhancing film (BEF) (enhance normal brightness) Bottom diffuser (adjust suitable light distribution for input of BEF) Finally, we can get a flat light source. But, how about the quality of a B/L unit? In general, the most important issue is Lamp-Mura defect. Diffusive plate Lamp (CCFLs) (output uniform light distribution) Reflector

How to eliminate Lamp-Mura efficiently? Motivation Issue: Lamp-Mura defects A kind of period pattern caused by lamps with strong brightness Optical films without enough scattering ability General solutions: (M. Schiavoni, et al., SID’07 Digest) Stack more diffusers Increase more lamps Increase distance from lamp to film  high cost, low brightness  high cost  Large volume Objective of this research: Study and develop Multi-performance film (MPF) to replace conventional BEF to eliminate Lamp-mura defects without increasing cost and kept optical performance How to eliminate Lamp-Mura efficiently? What is Lamp-Mura? In the right picture, we can see this a output image from B/L unit, the bright and dark period pattern is Lamp-Mura. It is caused by strongly bright lamps. On the other hand, we can say, because optical films have not enough scattering ability to eliminate Lamp-Mura.

Enhance normal brightness Principle of MPF Multi-performance film (MPF) Concave lens structure on the surface α = 90° n1=1.6000 n2=1.5663 R is variable R n2 α Enhance normal brightness n1 Scattered Passed Blocked Straight Features of MPFs: Integrated functions of BEF and diffuser High light-coupling efficiency Convention BEF MPF

Fabrication method Screen printing process by Emphasis Materials Co. Advantages of screen printing process: Low cost in fabrication Flexible design in different curved-prism structures

Fabrication results Captured results by OM Surface profiles by Alpha-step 0% (BEF) 50% 100% Summaries of MPFs: Integrated functions of brightness enhancement and scattering Simple, low cost and flexible design in fabrication process

Demonstration target 3 cases to do comparison 32” direct-lit backlight unit 12 straight lamps Input power (one lamp): ~6W Output brightness: ~20000 nit 3 cases to do comparison Vertical Horizontal MPF Case Reference Case HEDP: highly efficient diffusive plate Applied on present product Have no Lamp-Mura MPF-Plate Case

Definition of contrast Contrast (or Modulation) Measure 9 points between one pitch by Conoscope Lmax Luminance Position at vertical direction Lmin For example: Contrast = 0  No Lamp-mura Contrast = 1  Worst θ 0 ° one pitch lamp point 9 point 0 films

10% MPF is best with no Lamp-mura Experimental results Compare brightness and contrast at normal direction between Ref. Case and MPF Case in different filling ratios of MPFs 10% MPF is best with no Lamp-mura

Experimental results Comparison of CCD captured results 20 cm Measured area Reference MPF (0%) MPF(10%) MPF-Plate MPF can be efficient to eliminated Lamp-Mura defects with 10% filling ratio

Experimental results Comparison of normal brightness and FWHM luminance viewing angle *FWHM: full width at half maximum MPF can enhance luminance viewing angle at vertical and horizontal direction Non-reduced brightness 10% brightness decreased More than 45% FWHM enhancement (80°116°) ~13% FWHM enhancement (80°90°)

Experimental results Comparison of output lighting efficiency MPF can enhance output lighting efficiency higher than 8% in MPF case and MPF-Plate case ~8% enhancement ~19% enhancement

Discussion (1) Summary of comparison MPF case MPF-Plate case High luminance viewing angle High output lighting efficiency Low cost (without HEDP) MPF-Plate case Low brightness at normal Higher luminance viewing angle Higher output lighting efficiency  suitable to apply to present B/L product Reference Case  suitable for large viewing angle request  increase brightness of light source MPF Case

Discussion (2) Tendence of B/L unit: Slim! Patterned-MPFs When B/L unit become slim, the Lamp-Mura defect will be more serious! MPFs have limitation to eliminate Lamp-Mura in decreasing thickness of B/L unit Patterned-MPFs Optimized filling ratio at corresponding position by screen printing process Smaller amount of lamp and thinner thickness Customization

Conclusions MPF has been designed and realized to eliminated Lamp-Mura defects efficiently Reference MPF (0%) MPF(10%) MPF-Plate(10%)

Conclusions MPF can apply to different B/L structures to improve optical performances MPF case Free Lamp-Mura Non-reduced brightness ~13% enhancement in FWHM ~8% enhancement in output lighting efficiency MPF case Free Lamp-Mura 10% brightness decreased ~45% enhancement in FWHM ~19% enhancement in output lighting efficiency MPF Case MPF-Plate Case

Conclusions MPF can use pattern technology to develop Patterned-MPFs that have some features: Slimmer B/L and smaller amount of lamp in use are possible Customization is possible

Future works Short-term Long-term Finish simulation of MPF and Patterned-MPF Find threshold contrast of free Lamp-Mura by experiment Long-term Realize patterned-MPFs Design 2-D MPF/Patterned-MPF for LED B/L unit

Thank You for Your Kind Attention! rjshie.eo95g@nctu.edu.tw 21

Measurement instrument Conoscope Measure angular luminance distribution BTDF measuring device Measure BTDFs of films by cooperating with Conoscope CCD Capture spatial luminance distribution

Experimental results Compare brightness and contrast between Ref. Case and MPF Case in different filling ratios of MPFs Viewing angle: 0 ° Viewing angle: 60 °