Liquid Crystal Displays

T.N. RuckmongathanRaman Research InstituteBangalore 560080

IntroductionLiquid crystal displays are popular because of:

Low power consumptionLow voltage operationFlat panel with thin profileFlexible size, format and shape

Classification of LCDsPassive matrix LCD:Intrinsic nonlinear characteristic is exploited for matrix addressing

Active matrix LCD:A nonlinear element (thin film transistor) is used in each pixel

Electro-optic EffectsTwisted Nematic (TN) Super Twisted Nematic(STN)Electrically Controlled Birefringence (ECB) Polymer Dispersed Liquid Crystals Pi- CellIn Plane SwitchingBi Stable devices: ferro-electric, Zeninthal, Chiral Nematic, etc.

Molecular alignment

Twisted Structure

Twisted Nematic LCD

Electro Optic Response

Driving LCDsStatic or Direct (Non-multiplexed)Matrix Addressing (Multiplexing)Line-by-line addressingMulti-line addressingActive addressing (all the rows)

Active MatrixDiodesThin Film TransistorsMIM, Varistors etc.

Static (Direct) Drive

Matrix LCDs

Line by Line Addressing

LCD with Drive Electronics

Passive Matrix LCDThe Contrast and viewing angle decreases with multiplexingSupply voltage also increases with multiplexing

Multi-line addressing techniqueReduced supply voltageGood brightness uniformity

Color in LCDsUsing color filtersRGB sub-pixelsReduces light throughput

Gray shades1-bit2-bits3-bits4-bits5-bits6-bits7-bits8-bits

Gray scale in images 1-bit 2-bits 3-bits 4-bits 5-bits6-bits

Gray Shades in PMLCDFrame modulationPulse width modulationAmplitude modulationSuccessive approximationSpatial ditheringArea modulation

PMLCD eight gray shades

Active Matrix LCD

AMLCD Color

Active Matrix LCDThe contrast is independent of matrix sizeHigh contrastComplex manufacturing process

a-Si TFTIntegration of drivers with poly- silicon

LCD TV

Some gadgets using LCDCamcordersLCD monitorsLCD Screens CamerasMobile phones PDAs

LCD TV (23 and 30)

LCD TV 54