LCD BACKLIGHT

Jae-Hyeon KoDept. of Physics, Hallym University

Optical Society of KoreaAnnual Meeting 2007. 2. 8-9

전자물리학과

I need a backlight!

~ 2.75 m

CES2007, Sharp

전자물리학과

Contents

I. Introduction – Definition and Structure

II. Light Sources for Backlight

III. Optical Components for Backlight

IV. Advanced Backlight Technologies

전자물리학과

Contents

I. Introduction – Definition and Structure

II. Light Sources for Backlight

III. Optical Components for Backlight

IV. Advanced Backlight Technologies

전자물리학과

Emissive Display vs. Non-emissive Display

NonNon--emissive Displayemissive Display

White Light

Filters(Pixel)

Emissive DisplayEmissive Display

LIGHTSOURCE(=PIXEL)

CRT, PDP, FED, EL…. LCD, DMD…

전자물리학과

Liquid Crystal Display

+ polarizer: controls transmission

controls the color separation

supplies white light

•• LCD belongs to nonLCD belongs to non--emissive displays.emissive displays.•• LCD thus needs an independent lightLCD thus needs an independent light--generating part whichgenerating part which

supplies LCD with bright, uniform white light. supplies LCD with bright, uniform white light. Backlight Unit (BLU) !Backlight Unit (BLU) !

전자물리학과

Basic Structure of LCD Module

LGPlight source

diffuser

prism film

bright, uniform white light

supply white lightsupply white light

polarizercolor filter

polarizer

liquid crystals

alignment layer

ITO electrode

TFT

protection tilm

reflection film

LCD Panel

Backlight

Light Source: Generation of Visible Light

Optical Components:• Spreading/Guiding• Homogenizing• Collimating• Polarization Control

* LGP = Light Guide Plate (도광판)

전자물리학과

Display Unit Terminology (Photometry)

luminous flux (lm) : photometricequivalent of the watt, weightedto match the photopic responseof human eyes

luminous intensity (cd) :luminous flux per unitsolid angle

illuminance (lux): luminousflux per unit area

luminance (cd/m2, nit):luminous flux density perunit solid angle

<photopic response>

* light-generating efficiency: light output (lumen) per electrical power input (lm/W)

* “Light Measurement Handbook” (International Light)downloadable from http://www.intl-light.com

전자물리학과

The Color Gamut of LCD

(xB, yB)

x

y(xR, yR)

(xG, yG)

(xB, yB)

Compared to NTSC standard, • CCFL: ~ 72%• WCG-CCFL: ~ 92%• LED: > 100%

(xR, yR)(xG, yG)

* Figure source: 서울대학교, “디스플레이공학” 강의파일

전자물리학과

Expression of viewing angle characteristics of BLU

Dependence of luminance on viewing angle

contains all information about luminancedistribution on BLU

• Many diffuse surfaces are Lambertian.

• Its brightness looks the sameirrespective of the viewingangle.

luminance (cd/m2)

Lambertian distirbution

0o

Polar plot of angular distribution of luminance

viewing angle (o)

* 3M, BEF catalogue

전자물리학과

How to estimate the luminous flux on each sheet?

-80 -60 -40 -20 0 20 40 60 80

2000

4000

6000

8000

10000

Lum

inan

ce (c

d/m

2 )

Angle (O)

on diffuser plate on prism sheet

LDP

LBEF * *DP DP DPM A L Aφ π= ≅

In case of non-Lambertian distribution,

( )

2 / 2

0 02 / 2

0 0

( , ) ( , )sin

sin cos ,

BEF

BEF

I d d d I

A d d L

π π

π π

φ θ ϕ ϕ θ θ ϕ θ

ϕ θ θ θ θ ϕ

= Ω =

= ⎡ ⎤⎣ ⎦

∫∫ ∫ ∫

∫ ∫

luminous intensity (cd)

emitting area

In case of (quasi-)Lambertian distribution,

Exitance (lm/m2) luminance

전자물리학과

The Energy Flow in LCD

* Edge-lit case

~ Total transmission

•• Main losses occur at the polarizer (55~60%) and the colorMain losses occur at the polarizer (55~60%) and the colorfilter (~70%). filter (~70%).

•• The panel transmission of LCD TV is only 4~5 %. The panel transmission of LCD TV is only 4~5 %.

전자물리학과

Basic requirements on BLU

Bright !

Uniform !

• The luminance of BLU should be larger thanthat of LCD panel by 10 ~ 20 times.

• Lmin/Lmax ≥ 80 % * Gaussian Light Distribution

White ! • The white color of BLU should be adjustedaccording to the specific applications of LCD.

Cheap ! • Currently, the cost portion of BLU amongthe total cost of LCD panel for TV is ~ 1/3.

전자물리학과

Edge-lit BLU vs. Direct-lit BLU for LCD

Type Edge-lit BLU Direct-lit BLU

Structure

Application Mobile, Note PC, Monitor LCD TV, large-size monitor

Luminance 100 ~ 300 cd/m2 400 ~ 500 cd/m2

Typical film configuration

CCFL(side) Reflector LGP diffuser sheet 1 or 2 prism films

protection film

Reflector CCFL(below) diffuser plate diffuser sheet prism film reflective polarization film

Number of CCFL-type

BLU

Note PC 1Monitor 10 ~ 15” 2 ~ 4Monitor 17” ~ 4 ~ 6 (Mobile 1~3 white LED)

20 ~ 22” 6 ~ 1226 ~ 32” ~ 1633 ~ 37” ~ 2040” ~ 22 ~

* CCFL=Cold-cathode fluorescent lamp (냉음극형광램프)

전자물리학과

Basic Roles of BLU

Function Edge-lit BLU Direct-lit BLU

Light generation

Spreading/Guiding

Diffusing

Collimating

LED, CCFL

Reflection sheetLight Guide Plate

Diffusion sheet

Prism sheet

Polarization Control Reflective polarizer

CCFL, EEFL, FFL, LED ...

Reflection sheet

Diffusion plateDiffusion sheet

Prism sheet

Reflective polarizer

Homogeneous, collimated, bright 2D light source!Homogeneous, collimated, bright 2D light source!

전자물리학과

Control of the Light in the Edge-lit BLU

LGP

Reflection film

Reflection

Scattering

Diffusing

Collimating

Guiding

prism films

①

②

③④

Homogeneous, collimated, bright 2D light source!Homogeneous, collimated, bright 2D light source!

①

②

③

④

light source

diffuser film

lamp side

* Figure Source: IDW/AD ’05, pp.1285

전자물리학과

Structure of Edge-lit BLU for LCD Monitor

* Picture Source: ’06, Displaybank column “BLU용 반사기능부품에 대한 소고”

•• Main losses in edgeMain losses in edge--lit BLU occur at the inlet of LGP (~40%) andlit BLU occur at the inlet of LGP (~40%) andin the LGP (~20%). in the LGP (~20%).

•• LGP is the key component of edgeLGP is the key component of edge--lit BLU for better performance.lit BLU for better performance.

전자물리학과

Structure of Direct-lit BLU for LCD TV

(reflective polarizer)(prism sheet)

•• DirectDirect--lit BLU is mainly used forlit BLU is mainly used forlargelarge--size LCD TV. size LCD TV.

•• Adequate homogenization andAdequate homogenization andrecycling of visible light are recycling of visible light are necessary for achieving uniform,necessary for achieving uniform,bright planar illumination. bright planar illumination.

Reflec

tive p

olarize

r

Reflection

Diffusing

Collimating

Reflection sheet CCFL

PolarizationControl

Light Generation

전자물리학과

Technological Trends of LCD TV

* Picture Source: FPD International 2006 Pre-Seminar, “Technology of LCD Backlight for TV Application”

전자물리학과

The Requirements on BLU for LCD TV Applications

Item Present Direction Remarks

DimensionsSize up to 65 inchThickness ~ 25 mm

Larger, thinner, lighter

Luminance ~ 450 cd/m2 higher at the same power

3, 3.4 φ CCFL BLUEEFL/FFL BLUHybrid optical filmsAdaptive dimming

Environment Hg-type lamps dominant

Low Hg-dose lampsHg-free

LED BLUFFL(Xe-type)

Power (BLU)32” ~ 100 W40” ~ 150 W

32” 50 W (’10)40” 70 W (’10)

Higher contrast

> 100% of NTSC

< 4msec (MPRT)

Cost ~ -20% / year ?EEFL BLUFFL BLU

FFL BLULED BLU

Contrast (D-CR) 400 ~ 2700 Dynamic BLU control

Color gamut 72% of NTSCPhosphor tuning of FLLED BLU

Response time 8 ~ 16 msec Blinking/Scanning BLU

전자물리학과

Contents

I. Introduction – Definition and Structure

II. Light Sources for Backlight

III. Optical Components for Backlight

IV. Advanced Backlight Technologies

전자물리학과

Light Sources for LCD Backlight

CCFL (cold cathode fluorescent lamp)냉음극형광램프

EEFL (external electrode fluorescent lamp) 외부전극형광램프

FFL (Flat fluorescent lamp)면광원, 평판형 형광램프

LED (Light Emitting Diode)고체발광다이오드

FEL (Field Emission lamp) 전계방출램프

OLED (Organic Light Emitting Diode) or EL Lamps

전자물리학과

Light Sources for LCD Backlight

Type Characteristics CompanyCCFL Cold-cathode type (Many)EEFL DBD LG-Philips...

FFL DBD or cold-cathode

Field-emission Lamps- Cathodoluminescence

- CNT-based cathode- diode or triode

Nano-pacific, Nikkiso...

CCFL+LEDHybrid-type Enhanced deep-R component Sharp

HCFL Hot-cathode (Arc discharge) Philips

Samsung Corning, Mirae

LEDWhite-LED mobile, note PCRGB LED LCD TV, monitor

Sony, Samsung, LG-Philips, NEC ...

CCFL/EEFL

Cold-cathode or external electrode UshioXe-

typeFFL DBD Osram, CPT...

Solid-state Lighting- e-h recombination- Electroluminescence OLED/EL White OLED ITES, KDT...

Hg-type

Discharge Lamps- Non-equilibrium, low-pressure discharge(Te>>Ti~Tn)- Diffusion-controlled loss mechanism- Photoluminescence

* DBD: Dielectric Barrier Discharge* HCFL=Hot-cathode fluorescent lamp (열음극형광램프)

전자물리학과

Present Status of Light Sources

* Source: Displaybank

•• The portion of new light sources among total BLU market isThe portion of new light sources among total BLU market iscontinuously increasing, but CCFL BLU is still a marketcontinuously increasing, but CCFL BLU is still a market--dominating technology.dominating technology.

전자물리학과

Fluorescent Lamps

“Basically the same physics is applied to all fluorescent lamps.”

* Picture Source: FPD International 2006 Pre-Seminar, “Technology of LCD Backlight for TV Application”

전자물리학과

CCFL (I) Basic Structure

UV emitting element

UV (254nm)

inert gases electrodeelectrons

Visible Light

Phosphor

electrodelead

tube glass

Sodalime or borosilicate glasses

R: Y2O3:EuG: LaPO4:Ce,TbB : BaMg2Al10O18 : Eu

•cold cathode: metal cup(Mo, Ni, W, …)

Ne, Ar, Kr, Mercury type: HgHg-free type: Xe

전자물리학과

CCFL (II) Operating Principle

UV emitting element

UV (254nm)

inert gases electrodeelectrons

Visible Light

Phosphor

electrodelead

tube glass

energy diagram of Hg

Applying high voltage

Acceleration of seed electrons

Formation of glow discharge

UV generation from Hg excitation

Excitation of phosphor by UV

Generation of visible light

excite phosphor

visible lines from Hg plasma

전자물리학과

CCFL (III) Emitting Spectrum

350 400 450 500 550 600 650 700 750

0

20

40

60

80

100

Rel

ativ

e In

tens

ity

Wavelength (nm)

400 500 600 700

0.00

0.01

0.02

0.03

0.04

0.05

rela

tive

inte

nsity

Wavelength (nm)

350 400 450 500 550 600 650 700 750

0.00

0.05

0.10

0.15

0.20

0.25

0.30

577~579 nm63D2 or 1 -> 61P1

546 nm73S1 -> 63P2

405 nm73S1 -> 63P0

436 nm73S1 -> 63P1

Rel

ativ

e In

tens

ity

Wavelength (nm)

emitting spectrum of phosphors emitting spectrum of Hg plasma

emitting spectrum of CCFL

B G R

전자물리학과

CCFL (IV) Phosphor

•• Phosphor plays the role of energy converter (or wavelength convePhosphor plays the role of energy converter (or wavelength converter).rter).•• Phosphor transforms the input energy into visible light. Phosphor transforms the input energy into visible light.

phosphor(host lattice + activators)

• photon absorption - photoluminescence- CCFL, PDP…

• particle absorption- cathodoluminescence- CRT, FED…

• applying electric field- electroluminescence- ELD…

• phonon process

visiblelight

전자물리학과

electric energy kinetic energy UV light visible light lumen

discharge Hg excitation Phosphor eye

response

HCFL: 100 % ~67 % ~25 % ~65 lm/WCCFL: 100 % ~45 % ~15 % ~45 lm/W

HCFL

CCFL (V) Light-generating Efficiency

Luminous efficiency depends on the emitting spectrum and corresponding color coordinates. (The above numbers are with respect to the color coordinates of LCD TV)

* Kumho Electric Ltd., catalogue, 2005

전자물리학과

CCFL (VI): Advantages vs. Disadvantages

• high efficiency• proved, reliable technology• many vendors, stable supply• low cost (~$300@’03 ~$70@’06 for 32”)

• complex structure • individual driving circuit • environmental issue• slow response time • long warm-up time • low color gamut being improved• sensitive to ambient temperature• difficult low-temperature operation

* Source: Harrison-Toshiba, presentation materials

전자물리학과

CCFL (VII) Advanced Technologies

• Green: Remove side peaks and change of the wavelength of the main peak • Red: change of the wavelength of the main peak

Conventional phosphors New phosphors

Red Y2O3: Eu Y(P,V)O4: Eu

Green LaPO4: Ce,Tb BAM: Eu, Mn

Blue BAM: Eu BAM: Eu

•• higher efficiency/lowerhigher efficiency/lowercostcost: CC: CCFL of smallerFL of smallerdiameterdiameter, U, U--shaped CCFLshaped CCFL

•• simpler driving circuit: simpler driving circuit: parallel driving methodparallel driving method

•• higher color gamut higher color gamut (>90%): new phosphors(>90%): new phosphors

•• longer lifetimelonger lifetime withwithsmaller Hg consumptionsmaller Hg consumption

* Samsung Electronics, Maekyeon Seminar note, May 2005

* Sanken catalogue, 2005

전자물리학과

EEFL (I) Basic structure

(cf) CCFL electrode

•• EEFL = External Electrode Fluorescent LampEEFL = External Electrode Fluorescent Lamp•• External electrodes are formed on the outer surface of the tubeExternal electrodes are formed on the outer surface of the tube glass byglass by

taping, capping or dipping methodstaping, capping or dipping methods•• Except electrodes, the structure is almost the same to that of Except electrodes, the structure is almost the same to that of CCFLCCFL•• No internal electrodes, no sputtering No internal electrodes, no sputtering less Hg consumption less Hg consumption long lifetimelong lifetime

전자물리학과

•• Dielectric Barrier Discharge (DBD) Dielectric Barrier Discharge (DBD) positive relationship of lamp voltagepositive relationship of lamp voltageand lamp current and lamp current

multiple multiple EEFLsEEFLs can be operated bycan be operated bya single invertera single inverter

•• Comparable efficiency to that ofComparable efficiency to that ofCCFL as a lampCCFL as a lamp

2 3 4 5 6 7 80.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Lam

p V

olta

ge (k

V)

Lamp Current (mA)

EEFL CCFL

O.D: 2.6 mm Length: 420 mm 60 torrNe:Ar (9:1)

Y. Takeda, Journal of the SID, 11, 4 (2003), pp.667-673

voltage applied to the tube glass

EEFL (II) Electrical properties

* Harrison-Toshiba, Korea Display Conference, June 2004

전자물리학과

EEFL (III) Advantages vs. Disadvantages

Luminance on LCD modules (cd/m2)

Product size

(inch)

Power consumption

(W) CCFL EEFL

26” 67.2 480 510

32” 85.0 450 530

Efficacy comparison (cd/m2/W, %) 100% 106~120 %

* LG-Philips, FPD International 2006 Pre-seminar

• higher efficiency as a BLUlower power consumption

• simpler structure• parallel driving single inverter• lower cost• long lifetime

• possibility of pinhole formation• wide bezel• environmental issue• slow response time• limited in length

* G.-S. Cho, Journal of Physics D: Applied Physics, 37, (2004), pp.2863-2867

전자물리학과

EEFL (IV) Advanced technologies

•• higher efficiency/lower costhigher efficiency/lower cost::smallsmall--diameter diameter EEEEFLFL(but, (but, VVLL increases)increases)

•• narrow bezel: differential narrow bezel: differential diameter EEFL (DEFL)diameter EEFL (DEFL)

•• higher color gamut (>90%): higher color gamut (>90%): new phosphorsnew phosphors

•• Future challenges: Future challenges: longer lamps for > 40longer lamps for > 40”” LCDLCD

(glass optimization under progress)(glass optimization under progress)XeXe--type EEFLtype EEFLnoncircular EEFL

• same electrode area same properties • possibility of getting a higher efficiency???

* J.-B. Kim, SID ’06 Digest, (2006), pp.1246-1248noncircular EEFL

전자물리학과

FFL (I) Basic Concept

A + e* A* + eA* A + hν (UV)A=Hg or Xe (UV source)

Glow dischargephosphor

Visible light

• ideal for 2-dimensionalillumination

• simpler structure • superior uniformity • low current density

long lifetime

• discharge-space maintenancespacer needed

• delicate control of the plasma isnecessary (plasma contraction)

glassplatesreflector

Inert gases + ‘A’

전자물리학과

FFL (II) Multi-channel-structured FFL

• mainstream of Hg-type FFL technology for commercialization as LCD TV backlights

• glass-forming technology for theformation of multi-channels

• easily scalable to large sizes morethan 40”

• high efficacy • external or internal electrodes • low wall load low luminance drift,low color drift, low Hg consumption

* T. Shiga et al., SID ’04 Digest

전자물리학과

FFL (III) Commercialization example (Samsung Corning)

• First commercialization of FFL backlight• 32” commercialized, 40” will appear this year, 46” planned.

• glass-forming technology for multi-channel formation

• external electrodes one inverter integrated into the power board

• wide-color-gamut FFL, super-slim backlight under development

* C. Kim and S. Lim, FFL, [액정디스플레이 백라이트 (일본어 원저 “液晶ディスプレイバックライト”)] (Science & Technology)

전자물리학과

FFL (IV) Mercury-free FFL

• As a source element of UV light,Xe is used instead of Hg.

• The UV wavelength from Xe iseither 147 nm or 173 nm.

• Efficacy becomes lower owing tothe shorter wavelength of UV light than the that of Hg (254 nm).

• no mercury, environmentally friendly• no temperature dependence of optical properties including efficacy

• easy ignition at low temperatures • no warm-up time• very long lifetime

• low efficacy (normally half of Hg-type FFLs)

• large heat dissipation• pulse or square driving is necessary

inverter efficacy ↓, inverter cost ↑

Pro Con

전자물리학과

Competition between fluorescent lamp-based BLU technologies

CCFL EEFL FFL

Number of lamps /transformers (32”) 16/16 18/2 1/2

BLU efficiency Good Better Good

Lifetime Enough Better Better

Individual driving 2 in one (’06) 4 in one(’07)

Individual soldering socket-type connection

100%

Note PC ~ up to 65”(65”: vertical CCFL arrangement)

Driving technique Parallel Driving Parallel Driving

Assembly of lamps Clip assembly One-body assembly

Cost ~ 93% <90%

Application size 26 ~ 37” 32~40”

전자물리학과

LED (I) Emitting Principle

•• Recombination of the electrons and holes at theRecombination of the electrons and holes at thejunction release energy as photons and heat. junction release energy as photons and heat.

•• The wavelength of the emitted light depends on theThe wavelength of the emitted light depends on theenergy band gap determined by the composition andenergy band gap determined by the composition andstructure of semiconductors. structure of semiconductors.

* [Introduction to Solid-State Lighting] by A. Zukauskas (John Wiley & Sons, 2002)

전자물리학과

LED (II) Generation of White Light from LEDs

Higher color gamut

for mobile, general lighting …

for LCD TV, monitor

* “반도체 조명을 위한 고휘도 LED 기술” 홍 창 희(전북대학교 반도체 물성연구소)* “고출력 광원의 최근 연구동향” (민경익 박사, [물리학과 첨단기술] ’05, 1/2월호)

전자물리학과

LED (III) Electro-optic properties of LED

wavelength binning

intensity binning

* M. Anandan, SID’06 Digest, p.1509 (2006)

전자물리학과

LED (IV) Color gamut

CCFL BLU LED BLU

•• Main peaks of the emitting spectrum of LED is sharper and narrowMain peaks of the emitting spectrum of LED is sharper and narrower than those of er than those of CCFL/EEFL, and thus the color purity of RGB lights on the colorCCFL/EEFL, and thus the color purity of RGB lights on the color filter becomes higher. filter becomes higher. •• If the leakage of emitting spectrum into the overlapped region If the leakage of emitting spectrum into the overlapped region of transmission curves ofof transmission curves of

the color filter can be minimized, color gamut can even becomethe color filter can be minimized, color gamut can even become higher. higher.

전자물리학과

LED (V) BLU: Advantages and Disadvantages

•• Real Color Gamut BacklightReal Color Gamut Backlight•• Tunable CCTTunable CCT•• High Brightness, ContrastHigh Brightness, Contrast•• Rapid Response Time (< Rapid Response Time (< µµs)s)•• Dynamic Backlight Control Dynamic Backlight Control •• No MercuryNo Mercury•• Long Life Time Long Life Time •• Cold Start / Instant StartCold Start / Instant Start

•• Complex optical structureComplex optical structure(point source (point source 2D illumination)2D illumination)

•• Low efficiency ( < 30 lm/W)Low efficiency ( < 30 lm/W)•• Heat dissipation designHeat dissipation design•• High costHigh cost•• Temperature dependenceTemperature dependence•• Property variation Property variation LED sortingLED sorting•• Differential aging

Superior color gamut

LED BLU CCFL BLU

Blink BLU

Improved moving picture quality

Differential aging HOLD BLU

* CCT=Correlated Color Temperature (상관색온도)

전자물리학과

LED (VI) a few prototype examples of edge-lit LED BLU

Lumileds

Nippon Leiz

Sony, Toshiba, Fujitsu…

• commercialized from Sony, Toshiba, Fujitsu

• thinner BLU, lower power consumption

• for high-end products

* source: “Market and Technology in BLU” (Kwangwoon Univ. KDC2006)

전자물리학과

LED (VII) a few prototype examples of Direct-lit LED BLU

Tama Fine Opto: FLATLED

OSRAM Opto: RGGB cluster

Sony: Qualia (side-emitting RGB LED)

전자물리학과

LED (VIII) LED BLU vs. CCFL BLU

* Cree, presentation materials

* M. Zeiler (Osram), SID’06 Digest, p.1524 (2006)

The efficiency of LED BLU is continuously improving.

전자물리학과

LED (XI) Recent developments (~2006)

Company Size Structure Luminance/efficiency Color gamut Characteristics

Samsung(2005)

32”46”

RGB LED 200 ea (32”) 450 ea (46”) thickness: 40mm

500 nit/185W(250W@46”)CR 10,000:1

NTSC 107%- Xmitter lens: decrease of power consumption by ~ 40% - no cooling system

Samsung(2005, 2006)

32”40”

Small-chip LED2160 ea (40”)

500 nit/120W(40”), 500 nit/82W(32”)CR 2,000:1

NTSC 105%- low power consumption- small LED chip technology- no cooling fan, no heat sink

NEC LCD (2005.10) 21.3” RGB tri-color

LED arrangement250~1000 nitCR 800:1

NTSC 103 %

- 10bit: billion color- QXGA(2048x1536)

AUO(2005) 46” white LED

1000 ea

Adaptive local dimmingpower consumption

50%↓, CR 10000:1

- Divide LED BLU into sub sections and control the luminance of each area

Osram-Opto 24~82”

RGGB arrangement 500 nit / 135 W(32”) NTSC

110% - Golden dragon cluster

CMO(2005) 32” RGB LED

300 ea > NTSC 100% - dynamic control of light output from BLU CR 50000:1

Tama Fine Opto(2005) 32” FLATLED

thickness: 25mmBLU: 11,000 nit (150W)

- FLATLED 32” BLU thickness: 25mm

Mitsubishi(2005) 23” 6-color LED 108

ea (100W) 80 nit sRGB 175% - two groups of RGB LED- WXGA

Sony(2004~2006)

40,46,82”

Tri-color LED(Triluminos) NTSC >105% - xvYCC

- GRBRG arrangement

LG-Philips(CES 2006) 47”

500 nit(average 190W)CR 25000:1

NTSC > 110%(12,000K)

- Full HD, no cooling fan- AFLC(area-focused luminance control)

전자물리학과

•• Lens design of each RGB LED for Lens design of each RGB LED for better color mixing and color better color mixing and color uniformity uniformity

•• How to achieve slim LED BLU, i.e., How to achieve slim LED BLU, i.e., thinner BLU with the same degree thinner BLU with the same degree of luminance uniformity of luminance uniformity

•• How to develop multiHow to develop multi--functional functional optical sheets for LED BLUoptical sheets for LED BLU

•• How to achieve efficient heat How to achieve efficient heat releaserelease

LED (X) Issues of LED BLUFLATLED

* Source: Electronic Journal Technical seminar (Tama Fine Opto) (2006. 1. 26)

* Source: Electronic Journal Technical seminar (Sony) (2006. 1. 26)

전자물리학과

LG필립스LCD, LED 백라이트 액정 패널 출시한다

LG필립스LCD가 발광다이오드(LED) 광원을 이용한 TV용 LCD 패널을 1분기에 출시한다. 삼성전자는 지난해 이미 40인치 LED 패널을 출시한 바 있어 LG필립스LCD의 이번 가세로 LCD에서의LED광원 채택이 본격화될지에 관심이 모아진다. 정인재 LG필립스LCD 최고기술책임자(CTO)는 31일 코엑스에서 개최된 제1회 국제 디스플레이 LED 학술대회(ICDL)의 기조연설에서 “LG필립스LCD는 1분기내에 LED광원을 사용한 47인치 TV용LCD패널과 15.4인치 노트북용 패널을 출시할 계획”이라고 밝혔다. LG필립스LCD는 47인치 TV용 패널을 LG전자에 우선적으로 공급할 예정이다. 이 회사가 출시하는47인치 TV용 LCD 패널은 풀 HD의 해상도를 지원하며 밝기 조절기능(디밍)을 갖춰 이전에 비해 50배개선된 2만5000대 1의 색 대비비를 나타낸다. 색 재현율은 기존 72%에서 110%로 개선했다. LED를 사용한 15.4인치 LCD 패널은 두께와 무게가 기존 제품에 비해 10∼20% 가까이 줄었으며전력사용량은 줄인 반면 휘도는 더 높였다.정인재 CTO는 “앞으로 LED 광원 기술에 부분 밝기 조절 기능(로컬디밍) 등을 적용, 색 재현율을120%로 끌어올리고 색 대비비도 1백만대 1까지 지원할 계획”이라며 “LED 제조업체들과 협력해가격을 적정 수준으로 떨어뜨릴 예정”이라고 밝혔다. 그는 “적정 수준으로 LED가격이 떨어지지 않는다면 기술과 가격에서 빠른 진전을 보이고 있는 냉음극형광램프(CCFL)에 LED가 밀리게 될 것”이라며“LED업계가 혁신적인 성능과 가격을 내놓아야 한다”고 지적했다. 한편 LG필립스LCD는 연내에 7인치, 8인치, 12.1인치, 30인치 등으로 LED광원 사용을 확대해 나갈계획이다.

전자신문, 2007년 2월 1일

전자물리학과

New light sources (I) Field-emission lamps for backlight

• transformation of kinetic energy ofelectron beam into visible light viacathodoluminescent phosphors

• need materials (phosphor, field-emitter,…) improvement/development

Pro:• high efficacy• simpler, thinner backlight• environmentally friendly• short response time

Con: • lifetime issue (cathode reliability…)• high vacuum and spacers necessary• need to achieve and maintain high uniformity KIST/Iljin

1~5”, 15,000 nitNanopacific17”, ~60 lm/W

* “탄소나노튜브 전자방출원을 이용한 디스플레이 및 광원기술”(주병권, ICASE Mag. Vol.10, 15 (2004))

전자물리학과

• phosphorescent RGB OLED,

• 18~30 lm/W • x=0.39, y=0.39

* Tung et al., SID ’04 Digest 48-51 (2004)

New light sources (II) OLED white lamps for backlight

Pro:• no Pb, Hg…• thin, simple structure• low-voltage driving, simple driving circuitCon:• long-time reliability• low efficacy (best record ~ 30 lm/W @4000K)• not easily scalable to large size

* 김한기. “OLED 광원 및 BLU 기술”(월간전자부품 2007. 1)

One example of OLED lampsOne example of OLED lamps

전자물리학과

New light sources (III) Inorganic EL lamps for backlight

• Use electroluminescent materials (generation of light by applying the electric field)• thin film-type, fine powder-type / DC-type, AC-type • have been partially applied to the LCD backlights of some calculators, stick controllers of MDP/CDP, keypad of mobile phones…

• Recently, super-bright inorganic blue EL (YAG:Ce) device was developed: - 350,000 cd/m2, 25,000 Hr, a few mA, 3~8 V, 10~15 lm/W (茶谷産業株式会社

株式会社クラレ, Japan)Pro:• cost-effective• thin, simple structure; flexible design is possible• low-voltage driving • no Pb, Hg…Con:• low color gamut• low efficacy• not easily scalable to large size• need materials (binder…) improvement

전자물리학과

Comparison of light sources for backlight unit

> 〇 > > XMercury-type Mercury-freeLight source

CCFL EEFL FFL (Hg) FFL (Xe)

〇〇 〇〇

〇

Color gamut 〇 〇 〇 〇

X

X 〇

〇

〇

〇

X ( 〇?)

〇

〇

〇

X

X

〇

〇

〇

〇〇

〇〇

X

X

〇

〇

〇

〇

LEDLuminance 〇

Efficiency X ( 〇?)

T-dependence 〇

Response time

Environment 〇

Technical maturity

Price XLifetime 〇

BLU Structure X

전자물리학과

Tentative prospects for light sources (I)

•• The most important factor in the adoption of backlight is the The most important factor in the adoption of backlight is the costcost. .

•• CCFL technology will remain as a mainstream of LCD backlight tCCFL technology will remain as a mainstream of LCD backlight technology, echnology, although its portion in the market might decrease to some degalthough its portion in the market might decrease to some degree. ree.

•• The EEFL backlight technology is characterized by low power conThe EEFL backlight technology is characterized by low power consumptionsumption(higher efficacy), longer lifetime, and lower cost compared to(higher efficacy), longer lifetime, and lower cost compared to the CCFL BLU. the CCFL BLU. -- The market of EEFL backlight would not expand rapidly, and the The market of EEFL backlight would not expand rapidly, and the applicationapplicationsize of EEFL B/L would be limited to sizes below 42size of EEFL B/L would be limited to sizes below 42”” for the time being. for the time being.

•• The HgThe Hg--type FFL backlight technology is characterized by higher efficactype FFL backlight technology is characterized by higher efficacy, y, simpler assembly process, longer lifetime, possibility of supsimpler assembly process, longer lifetime, possibility of superer--slim BLU, andslim BLU, andlower cost compared to the CCFL BLU. lower cost compared to the CCFL BLU. -- The HgThe Hg--type FFL has recently been commercialized for 32type FFL has recently been commercialized for 32””, and will expand, and will expand

into 40into 40””. .

전자물리학과

Tentative prospects for light sources (II)

•• The The XeXe--type FFL backlight technology is characterized by environmentalltype FFL backlight technology is characterized by environmentallyy--friendly aspect, superior instant start/cold start properties,friendly aspect, superior instant start/cold start properties, simpler assemblysimpler assemblyprocess, longer lifetime, possibility of superprocess, longer lifetime, possibility of super--slim BLU, and lower cost slim BLU, and lower cost compared to the CCFL BLU. compared to the CCFL BLU. -- XeXe FFL has not come into the LCD TV backlight due to its lower effFFL has not come into the LCD TV backlight due to its lower efficacy icacy than that of Hgthan that of Hg--type lamps. type lamps.

•• LED backlight technology has attracted great attention due to tLED backlight technology has attracted great attention due to the wide color he wide color gamut, easy application of dynamic driving, and scalability togamut, easy application of dynamic driving, and scalability to large size. large size. -- Cost and efficacy issues should be resolved for successful commCost and efficacy issues should be resolved for successful commercialization of ercialization of

LED backlight, in particular, for LCD TV applications. LED backlight, in particular, for LCD TV applications. -- The application field of LED backlight will mainly be mobile phThe application field of LED backlight will mainly be mobile phone, note PCone, note PCand monitor and monitor LCDsLCDs for the time being. for the time being.

•• New light sources and backlight technologies are under developmNew light sources and backlight technologies are under development forent forcostcost--competitiveness, improvement of picture qualities and low power competitiveness, improvement of picture qualities and low power consumption. consumption.

전자물리학과

Contents

I. Introduction – Definition and Structure

II. Light Sources for Backlight

III. Optical Components for Backlight

IV. Advanced Backlight Technologies

전자물리학과

Control of the Light in the Edge-lit BLU

Reflection Scattering

Guiding

Diffusing

Collimating

PolarizationControl

전자물리학과

Reflection: Lamp reflector (Lamp cover)

* Displaybank column, “BLU용 반사기능부품에 대한 소고”* “BLU 부품 및 제조기술 동향” (우영 세미나 자료)* “광확산필름, 프리즘 보호필름, 반사필름” (SKC 세미나 자료)

(note PC) (Monitor)

•• Lamp reflector redirects the generated light intoLamp reflector redirects the generated light intothe light guide panelthe light guide panel

•• Reflector material should be endurable to bothReflector material should be endurable to bothheat and UV. heat and UV.

•• The shape of the cover may be various, but theThe shape of the cover may be various, but theeffect of absorption of reflected light by the lampeffect of absorption of reflected light by the lampshould be considered. should be considered.

•• The efficient coupling of the visible light into theThe efficient coupling of the visible light into theLGP is very important for the efficiency of edgeLGP is very important for the efficiency of edge--litlitBLU.

* Source: webpage of DSLCD

BLU.

전자물리학과

Reflection: Reflection sheet

•• Reflection sheet redirects theReflection sheet redirects thelight toward the LCD panel bylight toward the LCD panel byreflection. reflection.

•• Reflector material should beReflector material should beendurable to both heat and UV. endurable to both heat and UV.

•• White, opaque PET coated on aWhite, opaque PET coated on asubstrate (SUS, Al...) is mainlysubstrate (SUS, Al...) is mainlyused as diffuse reflectors forused as diffuse reflectors forLCD TV application.

• white PET: includes micro-voids (ex: Toray E60L)* Source: Toray website

LCD TV application. TiO2, BaSO4...

* SKC의 반사 쉬트 구조

<Basic process for making white PET>

* Displaybank column “BLU용 반사기능부품에 대한 소고”* “광확산필름, 프리즘 보호필름, 반사필름” (SKC 세미나 자료)

전자물리학과

Guiding: Light Guiding Plate (I) Structure and Functions

•• LGP is used for LGP is used for the transformation of point or line sources intothe transformation of point or line sources into 22--dimensional planar illuminationdimensional planar illumination in edgein edge--lit lit BLUsBLUs. .

•• The main body of LGP consists of transparent plate in which visThe main body of LGP consists of transparent plate in which visible light isible light isguided via guided via total internal reflectiontotal internal reflection (TIR). (TIR).

•• In order to extract light from LGP, the condition of TIR shouldIn order to extract light from LGP, the condition of TIR should be brokenbe brokenby scattering pattern on the backside of LGP. by scattering pattern on the backside of LGP.

* Source: CVCE 2005, Wooyoung

typical plate: PMMA (Polymethylmethacrylate)transmission: 92% (~ 96%2)refractive index: 1.49 (θc=42.2o)

light source: CCFL or LED

injection(사출)thickness: 0.6~2mm

extrusion(압출)thickness: 2~10mm

전자물리학과

Guiding: Light Guiding Plate (II) Light in the LGP

( )1 omax2 2sin sin 90 /air LGPn nθ −= = 48.2o > θc(=42.1o) for PMMA (n=1.5)

50.9o > θc(=39.1o) for PC (n=1.585)iθ

= 41.8o for PMMA (n=1.5)39.1o for PC (n=1.585)

* source: 김차연 “BLU 기술시장 동향 및 분석” (2005, 산업교육연구소)

max2θ iθLGP

mirror

•• All rays entering the LGP are guided through the All rays entering the LGP are guided through the lightguidelightguide via TIR. via TIR.

satisfy the condition of TIRsatisfy the condition of TIRlight trapped in the LGPlight trapped in the LGPneed to break the condition of TIRneed to break the condition of TIRfor light extraction from LGPfor light extraction from LGP

lamp reflector lamp

전자물리학과

Guiding: Light Guiding Plate (III) Extraction of light from LGP

typetype Scattering typeScattering type Reflection/refraction typeReflection/refraction type

Principle• scatter the guided light from diffuse, white dots or etchedsurfaces on the bottom of LGP

• reflect the guided-light via TIR• control the direction of the reflectedlight and thus the direction of theillumination

Process

• printing dot pattern made of whiteink materials such as TiO2

pigment by silk-printing process• etching the bottom of mold forLGP

• molding 3D structures such asprism(V grooves), microlens, gratingson the bottom and/or top of LGP

Design parameters

• density• size• pattern density in dots• randomization

• density• pitch (grating, groove...)• 3D shape of microlens• randomization

Pro & Con• long history, cost-effective• needs 3~4 additional sheets

• cost-ineffective, • long development time • appropriate for hybrid LGP

전자물리학과

Guiding: Light Guiding Plate (IV) Design of Patterns

uniformity

extraction efficiency

available light power

LGPlamp (example 2) Kalantar, SID ’99 Digest (1999)

* Source: CVCE 2005, Wooyoung(example 1) 사종엽 외, 대한기계학회논문집, 28권, 362 (2004)

(Q) How to optimize the pattern fill factor or gradation function of the pattern in an effective way?

전자물리학과

Guiding: Light Guiding Plate (IV) Design of Patterns (continued)

LGP Examples from Nippon Leiz

* Source: Electronic Journal Technical seminar (Nippon Leiz) (2006. 1. 26)

전자물리학과

Guiding: Light Guiding Plate (V) Recent developments

1st generation 2nd generation 3rd generation 4th generationGeneral Reverse-prism LGP Hybrid LGP Polarizing LGP

Status

• long history, cost-effective• many components necessary

• High on-axis luminance, narrower viewing angle • smaller components

• only 1 (or 2) component• not enough uniformity and on-axis luminance

• polarize the light emitted from LGP• not enough extinction ratio

Structure

Protection film1~2 Prism sheetsDiffuser sheetLGP (bottom-dot-patterned)Reflection sheet

(protection film)Reverse prism sheetPrism LGPReflection sheet

(diffuser sheet)Hybrid LGP(Reflection sheet)

Polarizing LGP(reflection sheet)

components 5~6 3~4 1~3 1~2

Schematicdrawings

전자물리학과

Guiding: Light Guiding Plate (VI) Reverse-prism LGP

•• Higher onHigher on--axis luminance (30% oraxis luminance (30% oreven higher than conventionaleven higher than conventionalstructure) structure)

•• narrower viewing angle (applicablenarrower viewing angle (applicableto note PC) to note PC)

•• low uniformity, limited suppliers low uniformity, limited suppliers

apex angle: 63~68o

important to have a highly-directional light output from LGP, whose emitting angle is matched to the condition of TIR toward vertical direction

For higher on-axis luminance

* http://www.mrc.co.jp/english/pressroom/p03/press2003_1.html

전자물리학과

Guiding: Light Guiding Plate (VII) Hybrid LGP (1) OPF BLU

optical window

•• OpticalOptical--patterned film forms anpatterned film forms anoptical window for extraction ofoptical window for extraction oflight from LGP. light from LGP.

•• The The thethe shape and the width of theshape and the width of themicrolensmicrolens controls collimation andcontrols collimation andviewing angle characteristic of theviewing angle characteristic of theOPF BLU. OPF BLU.

•• Inside microInside micro--particles are used toparticles are used todiffuse the light and improve thediffuse the light and improve thecolor characteristics. color characteristics.

•• Efficiency and uniformity shouldEfficiency and uniformity shouldbe improved for commercialization. be improved for commercialization.

(PMMA or PC)

(PMMA)

* IDW/AD ’05, Digest, p.1285 (2005)* K. Fujisawa, Jpn. J. Appl. Phys. 46, 194 (2007)* http://www.kuraray.co.jp/en/release/2005/050104.html

전자물리학과

Guiding: Light Guiding Plate (VIII) Hybrid LGP (2) HSOT LGP

HSOT LGP (I) HSOT LGP (II)pitch ~ 120 µm, depth: 6~18 µm

on LGP on prism sheet •• Uniform illumination without Uniform illumination without MoireMoireformation can easily be achieved by multipleformation can easily be achieved by multiple

(forward) scattering effect in LGP. (forward) scattering effect in LGP. •• Color dispersion can be avoided by adjustingColor dispersion can be avoided by adjusting

the refractive index and size of the refractive index and size of scatterersscatterers. . •• Optimization of microOptimization of micro--prisms on the back ofprisms on the back of

LGP realized LGP without any optical sheets. LGP realized LGP without any optical sheets.

Highly scattering optical transmission polymer Highly scattering optical transmission polymer are included in LGP. (are included in LGP. (MieMie scattering effect)scattering effect)

* www.enplas.co.jp* A. Tagaya, Appl. Opt. Vol.40, 6274 (2001)* T. Okumura, Appl. Phys. Lett. Vol.83, 2515 (2003)

전자물리학과

Guiding: Light Guiding Plate (XI) Hybrid LGP (3) OMRON BLU

•• The density can be adjusted for The density can be adjusted for uniform illumination. uniform illumination.

•• The randomization of prisms canThe randomization of prisms canremove color dispersion. remove color dispersion.

•• The hybrid prism coupler extracts andThe hybrid prism coupler extracts andcollimates the light, and may eliminatecollimates the light, and may eliminatediffuser sheet and prism sheet.diffuser sheet and prism sheet.

•• The grating pattern in front of microThe grating pattern in front of micro--prisms improve the viewing angleprisms improve the viewing anglecharacteristic along noncharacteristic along non--radial angularradial angulardirection. direction.

hybrid prism couplerhybrid prism coupler

grating patterngrating pattern

* http://www.omron.co.jp/ecb/products/bklight/english/genri/index.html* A. Funamoto, IDW’05 Digest, 1277 (2005)* S. Aoyama, Appl. Opt. Vol.45, 7273 (2006)* G. Kurata, IDW’06 Digest, 1727 (2006)

전자물리학과

Guiding: Light Guiding Plate (XI) Diffractive, holographic LGP

•• Output coupling is based on the diffraction characteristics of tOutput coupling is based on the diffraction characteristics of the surfacehe surface--relief holographic patternrelief holographic patternimprinted on one side of the LGP. Holographic patterns should imprinted on one side of the LGP. Holographic patterns should be optimized to increase the lightbe optimized to increase the light--extraction efficiency as well as uniformity and avoid color diextraction efficiency as well as uniformity and avoid color dispersion. spersion.

•• The diffraction efficiency of the light is sensitive to the incThe diffraction efficiency of the light is sensitive to the incident angle. ident angle. •• Holographic, diffractive LGP aims at Holographic, diffractive LGP aims at sheetlesssheetless BLU. BLU.

Diffuser

Source

P

RG B

θ i

θ t

2 22sin sini ti t

n nmP

π ππθ θλ λ

+ =

* S. M. Lee, SID’03 Digest

(m: diffraction order)* http://www.hitachi-chem.co.jp/japanese/products/do/008.html

LED

Hologramdiffuser

V-grooves

small diffusing areahigh on-axis luminance

* http://www.modilis.com

Outcoupling of light is based on modulated diffractive opticstructure having variable grating units for high uniformity.

전자물리학과

Guiding: Light Guiding Plate (XI) Diffractive, holographic LGP(Continued)

outcoupling grating(non-conical) couple the from LGP

For better color uniformityFor better color uniformity• use different pitches for R, G, B • use beat patterns

For better luminance uniformityFor better luminance uniformity

P - α P + αP

Sinusoidal grating pattern with three kinds of pitch

R

R

GGB

BB

G

R

LED

White Color balance

New H-LGP modelNew H-LGP model

P - α P + αP

Sinusoidal grating pattern with three kinds of pitch

R

R

GGB

BB

G

R

LED

White Color balance

New H-LGP modelNew H-LGP model

P - α P + αP

Sinusoidal grating pattern with three kinds of pitch

R

R

GGB

BB

G

R

LED

White Color balanceWhite Color balance

New H-LGP modelNew H-LGP model

* S. M. Lee, SID’03 Digest

* H.-S. Lee, IDW’05Digest, p.1281

* M. Parikka, Appl. Opt. Vol.40, 2239 (2001)

* S.R. Park, IDW’06 Digest, p.1025

Optimized orientation of micro-holographic grating dots resulting in a wide range of conical grating directions

deflecting grating(conical angle) distribute light sideways

전자물리학과

Diffusing: Diffuser plate (I) Structure and Functions

•• Beads (2~30Beads (2~30 µµmm) of high refractive index are dispersed in transparent) of high refractive index are dispersed in transparentplate of PMMA, MS, PC or plate of PMMA, MS, PC or PSPS. . Volume scattering effect Volume scattering effect

““homogeneous dispersion is importanthomogeneous dispersion is important””•• Homogenize the white light from lamps in directHomogenize the white light from lamps in direct--lit BLU lit BLU

almost almost LambertianLambertian distribution on the diffuser platedistribution on the diffuser plate•• Support other optical films on itSupport other optical films on it *source: DSLCD website

positional dependence of luminance

angular dependence of luminance

almost Lambertian ondiffuser plate

* “광확산필름, 프리즘 보호필름, 반사필름” (SKC 세미나 자료)

전자물리학과

Diffusing: Diffuser plate (II) Reliability and Recent Trends

RequirementsRequirements

•• light weightlight weight•• heatheat--resistanceresistance•• humidityhumidity--resistanceresistance•• UVUV--resistance

TrendsTrends

•• Higher transmission Higher transmission higher luminancehigher luminance•• Larger, thinner dimensionsLarger, thinner dimensions•• patterned diffuserpatterned diffuser•• multimulti--functions: functions: diffusing+collimatingresistance diffusing+collimating

* Kim, Eur. Polym. J. 41, 1729 (2005)FFL

KR10-2006-0131430synchronization betweenchannels of FFL and lenspatterns of the plate

(Q) How to improve the luminance by increasing transmission without degradation of uniformity? (Q) How to combine diffusing and collimating functions into one plate?

Kuraray,FPD 2006

전자물리학과

Diffusing: Diffuser sheet (I) bead-type

•• Hide dot patterns and homogenize theHide dot patterns and homogenize thewhite light from LGP in edgewhite light from LGP in edge--lit BLU bylit BLU bysurface scatteringsurface scattering

•• Homogenize the white light and enhanceHomogenize the white light and enhancethe onthe on--axis luminance in directaxis luminance in direct--lit BLU bylit BLU bythe refraction on the surface (not necessarythe refraction on the surface (not necessaryif the luminance is high enough) if the luminance is high enough)

PET Substrate

Diffusion layer: diffuse and redirect white light toward the on-axis direction

Back layer: Anti-blocking between films, preventing static electricity

StructureStructure FunctionsFunctions

beads size: 5 ~ 20 µm(PMMA, PS, PU, SiO2...)

angular dependence of luminance on DS

* “광확산필름, 프리즘 보호필름, 반사필름” (SKC 세미나 자료)

전자물리학과

Diffusing: Diffuser sheet (II) Micro-lens type

Mntech(미래나노텍)

Shinwha(신화인터텍)

* http://www.mntech.co.kr/eng/product/ute.htm* http://www.shinwha.com

•• monolithic micromonolithic micro--lens films on which semilens films on which semi--spherical microspherical micro--lenses, whose diameter is a fewlenses, whose diameter is a fewtens of tens of µµm, are dispersed randomlym, are dispersed randomly

•• hybrid films having diffusing and collimatinghybrid films having diffusing and collimatingfunctionsfunctions

•• two films may replace two films may replace ““beadbead--type type DS+prismDS+prism filmfilm””

Parameters On-axis luminance

Viewing angle

Density ↑ ↑ ↓

Refractive index ↑ ↑ ↓

Aspect ratio# Optimum value near ~ 2/3

randomization Effective in removing Moire# the ratio between the height and diameter of microlens

전자물리학과

Diffusing: Diffuser sheet (III) holographic diffuser

* GE Plastics, Illuminex film

•• surfacesurface--relief microrelief micro--structured pattern:structured pattern:random, random, nonperiodicnonperiodic structures can bestructures can beconsidered randomized considered randomized microlensletsmicrolenslets. .

•• high transmission (up to 92%), no colorhigh transmission (up to 92%), no colordistortion, distortion, moiremoire--freefree

•• reshaping reshaping LambertianLambertian light source resulting inlight source resulting ingain of ongain of on--axis luminance: by varying theaxis luminance: by varying thesurface pattern, the angle distribution of thesurface pattern, the angle distribution of thelight beam can be controlled to the desiredlight beam can be controlled to the desiredpattern. pattern.

“holographically recorded micro-structures through laser speckling techniques”

~10 µm

Diffusion + Micro-lens

* Wavefront Tech., TMD films

전자물리학과

Collimating: Prism film (I) Structure and Function

•• Acrylic prisms are formed on a PET substrateAcrylic prisms are formed on a PET substrate•• Enhance the onEnhance the on--axis luminance using refraction at the surface of microaxis luminance using refraction at the surface of micro--prisms at prisms at the expanse of viewing anglethe expanse of viewing angle

•• interference of periodic structure with that of LCD pixel interference of periodic structure with that of LCD pixel moiremoire patternpattern

* http://cms.3m.com/ml

• Recycling is very important for higher on-axis luminance.• For better recycling, the apex angle of prisms should be 90o.• Recycling is dependent on the condition of the lower part of BLU on which downward rays will be reflected and recycled.

contribute to the formation of side lobes

Rays incident on PET substrate are confined within an angle of ±42o according to the Snell’s law.

randomized prism pattern reduced Moire

전자물리학과

Collimating: Prism film (II) Angular distribution of luminance

Angular distribution of luminance measured on a directAngular distribution of luminance measured on a direct--lit BLUlit BLU

(Q) How to reduce side lobes while maintaining the same efficiency?

side lobe* Figures: G.J. Park, IDW’06 Digest, p.953 (2006)

전자물리학과

Collimating: Prism film (III) Recent Developments

•• Randomization or modulation of oneRandomization or modulation of one--dimensionaldimensionalprism structure has been adopted for removingprism structure has been adopted for removingmoiremoire pattern, increasing handling robustness andpattern, increasing handling robustness anddefect hiding. defect hiding.

•• Many companies entered the market of collimatingMany companies entered the market of collimatingfilms for BLU (LG, SKC, films for BLU (LG, SKC, KolonKolon, , DoosanDoosan, GE, , GE, EfunEfun,,ReflexiteReflexite... ). ... ).

•• New prism structures have been tried forNew prism structures have been tried forincorporating diffusing function into the prism sheet.

* Eastman Kodak, SID’06 Digest

incorporating diffusing function into the prism sheet.overlapped, interlocked microlenses composed of one planar and one curved surfaces

* GE Plastics, Information Display 2006

nonlinear, randomized prisms

전자물리학과

Polarization Control: Reflective polarizer

Multi-layer type CLC type Wire-grid type

Structure

- PEN-coPEN multi-layer stack total 4 groups - each group consisting of 120~150 layers

- three CLC layers serving as reflection layers in the R, G, B ranges- additional λ/4 plate

- sub-wavelength metal (usually Al) grating structure

Process Co-extrusion stretching CLC coating lamination attach λ/4 plate Nano-imprinting technology

Status Commercialized Commercialized Under development for BLU

Properties

- generalization of Fresnelequation free adjustment of Brewster angle- each layer-group controls reflectivity of p-polarization over ~ 100nm 4 layer-groups cover visible range

- adjust pitch/alignment of CLC for achieving circular dichroism in visible range- additional λ/4 plate necessary for change of circularly polarized light to linearly polarized light

- wire-grid acts as a metal mirror to the s-polarization- performances are dependent on the period, linewidth, and depth of the grid elements- lower transmission in the blue range of visible light

Company 3MNitto-Denko, Samsung Fine Chemicals

LG Electronics, NanoOpto Co., Eastman Kodak, MoxTek, ....

* PEN: polyethylene naphthalate* CLC: Cholesteric Liquid Crystal

전자물리학과

Polarization Control: Reflective polarizer (I) -DBEF

•• Transmit the polarization component parallel to the polarizationTransmit the polarization component parallel to the polarization direction of the bottomdirection of the bottompolarizer of LCD panel while reflect the orthogonal component polarizer of LCD panel while reflect the orthogonal component down to the BLUdown to the BLUresulting in polarization recycling via change of the polarizaresulting in polarization recycling via change of the polarization state. tion state.

30~65% increase in luminance30~65% increase in luminance

• The light has to be depolarized and re-emitted.• The luminance gain depends on the condition of

the lower part of BLU onto which downward rayswill be reflected, depolarized and recycled.

alternatively-stacked multilayer of isotropic and birefringentmaterials

stretching

* M. F. Weber et al., Science Vol.287, 2451 (2000)* http://cms.3m.com/cms/US/en/2-136/cRciRFF/view.jhtml

전자물리학과

Polarization Control: Reflective polarizer (I) –DBEF (Continued)

one reflection band for p-polarized light in the green range centered at 550 nm

extinction ratio of 300:1

( )( )

22

2

1 4, arcsin1

NH L H L

DBEF NDBEF H LH L

n n n nRn nn n

λλ π

⎡ ⎤− − ⎛ ⎞∆ −⎡ ⎤= =⎢ ⎥ ⎜ ⎟⎢ ⎥ +⎣ ⎦− +⎢ ⎥ ⎝ ⎠⎣ ⎦

4 layer-groups

(N: number of film pairs)

nHnL

nL nL

Giant Giant birefringentbirefringent optics--. generalization of . generalization of FresnelFresnel equations for equations for birefringentbirefringent

materials materials --. improved control of the reflectivity of p. improved control of the reflectivity of p--polarized light:polarized light:

all of the characteristics of the normal incidence reflectionall of the characteristics of the normal incidence reflectionband are maintained for highband are maintained for high--incidence pincidence p--polarized light polarized light

--. free adjustment of the Brewster. free adjustment of the Brewster’’s angle between 0 ~ 90s angle between 0 ~ 90o

optics

o

* M. F. Weber et al., Science Vol.287, 2451 (2000)* W. J. Jeong, IDW’06 Digest, p.999 (2006)

전자물리학과

Polarization Control: Reflective polarizer (II) - CLC

2 22

2 2

2 2

2tanh2

2

e oCLC

e o

e o

CLC e o

n nR Ln n

n nn n

πλ

λλ

⎡ ⎤−⎢ ⎥=⎢ ⎥+⎣ ⎦

⎛ ⎞−∆⎡ ⎤ ⎜ ⎟=⎢ ⎥ ⎜ ⎟⎣ ⎦ +⎝ ⎠

* T. Nakajima, Nitto Denko Tech. Rep. Vol.41, 30 (2003) * W. J. Jeong, IDW’06 Digest, p.999 (2006)* P. Yeh and C. Gu, “Optics of Liquid Crystal Displays” (Wiley-Interscience)

B

G

R

(L: film thickness)

* http://www.chelix.com/

overlap of three bands

Effect of CLC polarizer

전자물리학과

Polarization Control: Reflective polarizer (III) – wire-grid polarizer

* www.moxtex.com, ProFlux

* J. J. Wang, Appl. Phys. Lett., Vol.89, 141105 (2006)

* E. Hecht, “Optics” (Addison Wesley)

* J. J. Wang, Appl. Phys. Lett., Vol.89, 141105 (2006)* X.-D. Mi, SID’05 Digest, 1004 (2005)* S. H. Kim, Nanotechnology, Vol.17, 4436 (2006)

•• It is important to achieve as high a It is important to achieve as high a PP as possible while maintainingas possible while maintainingenough CR. enough CR. LinewidthLinewidth of the grid is critical to obtaining high of the grid is critical to obtaining high PP..

•• The shorter the period, the shorter the operating wavelength. The shorter the period, the shorter the operating wavelength.

double-side Al WGP

Parameters Definition Remarks

Transmission contrast ratio (CR) Tp / Ts

tradeoff relationship with energy efficiency

Total energy efficiency (P) Tp + Rp (or 1-lossTM) always less than 1 due to

the internal loss.

TM transmission (Tp)Power transmission coefficient for p-polarized light

very sensitive to the linewidth of the grid

전자물리학과

Polarization Control: Polarized LGP

Brewster angle selective TIR Sub-wavelength grating

Anisotropic scattering

Operating principle

• utilize either reflected or transmitted light incident at the Brewster’s angle

• polarization selection via polarization-dependent TIR at an isotropic-anisotropic interface

• extraction of p-polarized light via grating • conversion from s- to p-polarized light via λ/4 plate

• anisotropic scattering centers transmit index-matched polarization while scatter the orthogonal component

Structure

• coupled prismatic polarizer light pipe•Wedge-shaped LGP whose angle is controlled based on the Brewster’s angle

• a bicomponent structure consisting of an isotropic LGP on which a micro-structured birefringentpolymer film is coated

• sub-wavelength grating formed on LGP• λ/4 plate is put below LGP

• composite materials such as oriented polymer and an isotropic dispersed phase

Properties

• not enough CR• low enhancement• poor angular characteristics

• CR up to 64~100• gain factor ~1.6• Smooth surface is important.

• gain factor ~ 1.7• additional λ/4 required.• performances are sensitive to the conditions of grating

• CR ~ 20• lower efficiency• lower uniformity

Research Group IBM Japan Philips, Eindhoven Univ.,

National Chiao Tung Univ.National Chiao TungUniv., Tsinghua Univ.

Philips, EindhovenUniv. ...

전자물리학과

Polarization Control: Polarized LGP (I) based on Brewster’s angle

* E. Hecht, “Optics” (Addison Wesley)

highly colli-mated light

TIR

more p-polarized

s-polarized

stack of lots of plates

* M. Suzuki, SID Digest

21 0 1

2 21 0 1

cos / 2n nn n n

φ −⎛ ⎞

= ⎜ ⎟⎜ ⎟+⎝ ⎠λ/4 plate

•• Contrast ratio ~ 5 with 20 platesContrast ratio ~ 5 with 20 plates•• enhancement factor ~ 12%enhancement factor ~ 12%•• too complex structuretoo complex structure

* H. Tanase, SID’97 Digest, p.365

1500 µm

n=1.514

n=1.538

highly colli-mated light

Sample 1: laser input•• not enough contrast rationot enough contrast ratio•• poor angular characteristicspoor angular characteristics•• collimation of light source is collimation of light source is

necessarynecessary

전자물리학과

Polarization Control: Polarized LGP (II) Based on selective TIRPlease remind the picture of Nicol prism from “Optics” by E. Hecht. •• Polarization separation is based on the polarizationPolarization separation is based on the polarization--

dependent TIR at an isotropicdependent TIR at an isotropic--anisotropic interface. anisotropic interface. •• LGP based on microLGP based on micro--structured structured birefringentbirefringent foil showedfoil showed

better performances with a maximum CR of 64~100 andbetter performances with a maximum CR of 64~100 andenhancement factor of 1.6~1.78. enhancement factor of 1.6~1.78.

•• Higher birefringence is required for better performance. Higher birefringence is required for better performance.

Surface micro-relief structure for outcoupling

PMMA o en n n<

* K.-W. Chien, Appl. Opt. Vol.43, p.4672 (2004)* H. Jagt, SID’02 Digest, p.1236 (2002)* N. Stutzmann, Jpn. J. Appl. Phys. Vol.40, p.5966 (2001)

전자물리학과

Polarization Control: Polarized LGP (III) Based on sub-wavelengthgrating

Polarization separation at the subPolarization separation at the sub--wavelength grating wavelength grating

Polarization conversion from sPolarization conversion from s-- to pto p--polarization polarization via double pass through via double pass through λλ/4 plate/4 plate λλ/4 plate can be replaced by LGP in which /4 plate can be replaced by LGP in which

stressstress--induced birefringence is induced by induced birefringence is induced by stress freezing technique. stress freezing technique.

y xn n n Cσ σ σ∆ = − = ∆

stress difference

* Yang, Appl. Phys. Lett., Vol.88, p.221109 (2006)

* K.-W. Chien, Appl. Opt. Vol.43, p.1830 (2004)

전자물리학과

Polarization Control: Polarized LGP (IV) Based on anisotropic scattering

•• Polarization separation is based on the anisotropic scattering: Polarization separation is based on the anisotropic scattering: ss--polarized light is polarized light is strongly scattered and coupled out of the LGP linearly polarizstrongly scattered and coupled out of the LGP linearly polarized while the ped while the p--polarized light is much less coupled out and recycled in the Lpolarized light is much less coupled out and recycled in the LGP. GP.

•• There exist inherent difficulties in achieving enough uniformitThere exist inherent difficulties in achieving enough uniformity. It also suffers fromy. It also suffers fromlow CR and efficiency. low CR and efficiency.

* H. J. B. Jagt, IDW’00 Digest, p.387 (2000)* Figure from I. Amimori, J. Appl. Phys. Vol.93, 3248 (2003)

전자물리학과

Polarization Control: Summary

Absorbing polarizerAbsorbing polarizer Reflective polarizerReflective polarizer Scattering polarizerScattering polarizer

• Iodine-doped PVA films• absorb more than 50 %

of incident light• contribute to the large

loss in LCD along with that from color filter

• (1) isotropic-anisotropic multi-layer stack type, (2) CLC stacktype, (3) wire-grid type

• reasonably good performances • costly in BLU, thus tends to try

to omit this technology

• anisotropic scattering centers dispersed in isotropic matrix, or viceversa

• lower performances andlower cost than reflectivetypes

* Figures from I. Amimori, J. Appl. Phys. Vol.93, 3248 (2003)

전자물리학과

Hybrid Films Integrating Multi-functions

Prism sheet + reflective polarizer:- fewer components, thinner design • How to combine collimating

function and diffusing function?

- How to integrate collimating function into the diffuser plate? - Can holographic diffuser or other diffuser of new concept replace “prism film + diffuser sheet”?

전자물리학과

Contents

I. Introduction – Definition and Structure

II. Light Sources for Backlight

III. Optical Components for Backlight

IV. Advanced Backlight Technologies

전자물리학과

* Source: Displaybank

•• BLU occupies the largest portion among the total cost of BLU occupies the largest portion among the total cost of largelarge--size LCD module. size LCD module.

•• Lamps, an inverter and optical films are the mainLamps, an inverter and optical films are the maincomponents of largecomponents of large--size BLU from the viewpoint of the cost. size BLU from the viewpoint of the cost.

Cost structure of the LCD panel and BLU

전자물리학과

* Source: “2006년, BLU 산업과 기술 이슈”, Displaybank

David Hsieh(DisplaySearch), “Large Are TFT LCD Backlight Unit Market Outlook”(CVCE, 2005)

• Rapid expansion of LCD TV market rapid expansion of BLU market for LCD TV• Low-cost technology is the core of present technological trends of BLU: hybrid-components, reduce number of components…• Low-power-consumption technology is becoming more and more important. • Next-generation technologies: driving/ assembly innovations, FFL, LED, FEL, OLED... But, CCFL BLU will remain as a dominant technology.

LCD TV Price Trends (US)

* Source: Displaybank (2006)

Market outlook for large-size BLU

BLU Market Trend

전자물리학과

(Source : Displaybank’s Report, ‘Optical Films II - Backlight Unit’)

전자물리학과

Hybrid Backlight - Sharp

•• CCFL + RCCFL + R--LED(630 nm) LED(630 nm) possible to express deeppossible to express deep--red colorsred colors•• color gamut: NTSC 95 %color gamut: NTSC 95 %•• costcost--effective: 33% increase effective: 33% increase w.r.tw.r.t. CCFL BLU. CCFL BLU•• applied to 57applied to 57”” LCD TV of SharpLCD TV of Sharp•• power consumption increased by only 7% power consumption increased by only 7% w.r.tw.r.t. CCFL BLU. CCFL BLU

RED LED

전자물리학과

Color Filter-less LCD

•• Removal of color filterRemoval of color filterefficiency increaseefficiency increase

•• 180 color frames / sec180 color frames / sec

•• fast switching speed of liquid crystals fast switching speed of liquid crystals OCB mode or ferroelectric LCOCB mode or ferroelectric LC

•• short response time of BLU short response time of BLU LEDLED* Source: Nokia seminar material

* D. R> Selviah, SID’04 Digest, p.487 (2004)

전자물리학과

Blinking BLU technology

•• Turn on the BLU for, for example, the half time during one framTurn on the BLU for, for example, the half time during one frame e effective in reducing motion blur of moving pictureseffective in reducing motion blur of moving pictures

•• It remains the problem how to compensate for the luminance decrIt remains the problem how to compensate for the luminance decrease. ease.

전자물리학과

Adaptive dimming technology

•• An APL (average picture level) in a TV field time of 16.7 ms is An APL (average picture level) in a TV field time of 16.7 ms is detected and then thedetected and then theluminance of the backlight is adjusted according to the pictuluminance of the backlight is adjusted according to the picture level. re level.

•• The hybrid grayThe hybrid gray--scale expression of LCD transmission and backlight intensity allscale expression of LCD transmission and backlight intensity allows forows forimproved picture quality, especially at dark luminance level.improved picture quality, especially at dark luminance level.

•• Since the backlight does not have to operate at full luminance Since the backlight does not have to operate at full luminance all the time, the powerall the time, the powerconsumption is reduced. (* T. Shiga, J. SID 10/4 (200consumption is reduced. (* T. Shiga, J. SID 10/4 (2002) 3432) 343--346)346)

전자물리학과

Adaptive dimming with 0D, 1D, 2D controls

In case of moving pictures, (rising sun)

Relative power consumption

0D control 1D control 2D control

100 % 72 % 43 %

* Shiga, FPD International Pre-seminar 2006

전자물리학과

감사합니다!