Transmissive Liquid Crystal Displays From optics of liquid

Transmissive Liquid Crystal Displays From optics of liquid crystal display, Chap. 5, 6 P. Yeh and C. Gu 1

Chap. 5 Liquid crystal displays • Most of displays produced recently envolve the use of either TN or STN • The STN was introduced to improve the performance of LCD without the use of TFT. • Here we discuss the principles of operation of various displays. 2

5. 1 twisted nematic displays • The operation of TN based on waveguiding property. 3

5. 1. 2 Transmission properties of field-off state • Using Jones matrix method. • Consider a 90 degree TN cell with front local director // the transmission axis of the polarizer. • The waveguiding is valid only if ψ<<2πΔn d/λ. – φ is the twisting angle. – Known as the limit of slow twist. – Also known as Mauguin(莫吉恩) condition. – For a 90 degree TN, the condition reduces to λ/2<< Δn d. – In general cases when the condition is not satisfied, the output beam is elliptical polarized-reduce the transmission 4

• Consider the eo coordinate system, the input beam is • the output beam is (5. 1 -4) – Mauguin condition equal to φ<< Γ Ex: Dn=0. 23, l=0. 63 mm, φ=p/4 – Output beam ~linearly polarized along the local director(e axis). This explains the waveguiding phenomenon. 5

Normally black mode • In most TN cells, Mauguin condition is not always satisfied. This leads to a series of performance degradation, ex: brightness, contrast, color shift…. . • NB mode: TN cell sandwiches between a pair of parallel polarizer – Input end: LC director // polarizer – Output end: LC director ⊥ polarizer – According eq. (5. 1. 4), the transmission for unpolarized light is given by o component 6

Normally with mode • NW mode: TN cell sandwiches between a pair of cross polarizers • According eq. (5. 1. 4), the transmission for unpolarized light is given by e component 7

Nornally Black mode Dn decreases with voltage Normally White mode Dn decreases with voltage Gooch Tary 1 st minimum principle Voff-bright state, Normally white Voff-dark state, Normally black 8

5. 1. 3 Transmission properties of field-on state -TN mode • The final distribution of the director [θ, φ] as functions of z can be obtained by minimizing the total energy integrated over the LC cell. This require the technique of variational calculus. 9

1. 2. 3. 4. Middle layer director Cell director distribution Subdivide TN director Jones matrix calculation Φ is the total twisting angle 10

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Twisted Nematic Liquid Crystal Displays: Normally White (e-mode) 12

Twisted Nematic Liquid Crystal Displays: Normally White (o-mode) 13

Twisted Nematic Liquid Crystal Displays: Normally Black (e-mode) 14

Transmission of TN LCDs: Normally Black Dn: varied by applied field first minimum second minimum third minimum V increases 15

Transmission of TN LCDs: Normally White 16 V increases

High Contrast TN LCDs: First Minimum First minimum condition find d for Dn=0. 095 5 mm is a typical cell gap for first minimum displays 17

Contrast at normal incidence Contrast ratio contrast Viewing angles 18

• NB: limited contrast ratio, due to the slightly elliptical output light. • NW: high contrast ratio, because the homeotropically aligned LCs at high voltage. 19

Gray scale inversion 20

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Isotransmittance Viewing Diagrams: Normally Black TN LCD V=0 Volts V=2. 10 Volts V=5. 49 Volts Reference: Yeh and Gu, Optics of Liquid Crystal Displays 22

Isotransmittance Viewing Diagrams: Normally White TN LCD V=0 Volts V=2. 10 Volts V=5. 49 Volts Reference: Yeh and Gu, Optics of Liquid Crystal Displays 23

Transmitted luminance • The transmission of a TN cell depends on the wavelength of light. • The integrated transmission luminance is obtained by integrating the transmission function T(l) with the photonic response of the human eyes P(l) and the illuminat spectral distribution D(l). 24

• transmission function T(l) • the photonic response of the human eyes P(l) • the illuminat spectral distribution D(l). 25

Super-Twisted Nematic (STN) 26

Super-Twisted Nematic (STN) 27

Super-Twisted Nematic (STN) Multiplexing Example for STN D 0. 5 T(%) 0. 4 STN LCD 0. 3 0. 2 0. 1 VTH 1 2 3 4 5 V Notice the very steep threshold Medium resolution is possible 28

Threshold of STN Define nematic director Calculate the elastic free energy Express elastic free energy in terms of q and f Calculate the electric field contribution to free energy, Dz is displacement 29

Threshold of STN And taking the tilt angle, qo, to be zero. f. T (twist angle) 90 o 180 o 270 o 360 o d/P (gap/pitch) 0. 25 0. 50 0. 75 1. 00 VTH (volts) 1. 10 1. 63 2. 20 2. 70 30

270 o Super-Twisted Nematic (STN) 90 VR = 3. 15 80 225 70 60 180 VR = 2. 04 50 135 40 30 90 20 45 10 0 VR = 1. 83 VR = 0. 11 0 0. 2 0. 4 0. 6 z/L 0. 8 1 Twist Angle f (Degrees) Tilt Angle q (Degrees) 270 0 31 q f

Midlayer tilt angle (deg) 270 o Super-Twisted Nematic (STN) 90 80 70 60 50 270 o STN 40 30 20 10 0 VON VOFF 0. 5 1 1. 5 2 2. 5 Reduced Voltage 3 32

5. 3 nematic liquid crystal display (NLCD) modes • TN, STN: undergo twist in the cell, displays show asymmetric viewing characteristics. Birefringence phase compensation for improving viewing characteristics is difficult. • Now we consider parallel aligned cell, vertical aligned cell and bend aligned cell. 33

TN LCDs Twist angle, complex Wavelength insensitive 34

5. 3. 1 parallel aligned (PA) cells 35

Parallel Aligned Cells x y 36

Vertical switching (E-field perpendicular to LC layer) Normally White: When E-field applied • Γ is chosen to be odd integral of π, usually the lowest order of Γ is usually chosen for the best viewing characteristics at large view angle. • This mode of switching is sometimes referred to as the electrically controlled 37 birefringence mode (ECB-LCD).

Normally black • In the field-off state, LC cell behaves like a half -wave plate. 38

• TN- LCD is less sensitive to wavelength variation that of N -LCDs. • Because TN-LCD is a result of waveguiding in the field-off state, but N-LCDs the transmission is based on polarization interference, which is often sensitive to wavelength. 39

In-plane switching (IPS) Normally black operation • Δφ is the twisting angle relative to the transmission axis of the polarizer due to the applied field. 40

In-Plane Switching 41

• The transmission is sensitive to wavelength. • For IPS LCDs to respond uniformly to all wavelength, pixels with different colors need to have different cell gaps. • Variation of birefringence is small. 42

5. 3. 2 vertical aligned (VA) cells 43

Rubbing direction 44

Backflow effect in LC devices 45

E E Twist (T-Mode) splay bend De>0 Classical Fredericks Transition 46

Backflow Effect in LC Devices TN CHLC 47

ECB-Backflow Effects • S and B deformations are always accompanied by a macroscopic flow of liquid crystal with velocity V=(V(Z), 0, 0), due to a change in position of the centers of gravity of the molecule. (reorientation of molecules out of plane) • The pure T mode is not accompanied by backflow because it is not accompanied by change in centers of gravity. 48

ECB-Backflow Effects Tough Problem: V(z) V z=0=V z=d=0 Boundary conditions Solve coupled differential equations 49

ECB-Backflow Effects For small angles g 1* is effective rotation viscosity ai are the Leslie viscosity coefficients 50

5. 3. 3 bend-aligned (BA) cells • Also known as the pi cell, or optically compensated bend (OCB) mode cell • Originally proposed by Bos and Koehler/Beran. • Fastest response time among nematic LC modes, TN, STN, IPS, MVA, . • w/o backflow • Suitable to Moving picture • Color sequential application 51

Slow Response Time Image blurring http: //www. cmo. com. tw 52

Color Sequential LCDs • Basic concept of color sequential (CS) – Separate the primary colors in temporal domains (not in spatial domains) • Requirement: fast response LC (> 500 Hz) to avoid color breakup 53

OCB (Pi) cell V 0 V 1 (Splay) V 0=0 V 2 (Bend) V 1=Vc (Homeotropic) V 2>>Vc Fast switching Symmetric view A critical field for splay bend transition P. J. Bos et al. MCLC, 113, 329 (1984); Uchida, SID 25, 927 (1994) 54

l/2 55

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Fast response (w/o backflow) ECB cell OCB cell 57

5. 4 polymer dispersed liquid crystal displays ne~np 58

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5. 6 projection displays 60

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6. 3 optical throughput of TFT-LCDs • 6. 3. 1 polarizer – A beam of unpolarized light suffers 50% energy loss due to a perfect polarizer. – In practice, sheet polarizers are used for LCDs. – Commercially available polarizeer materials such as HN 22, HN 38 S and HN 42 HE are used. – They are made of stretched PVA (polyvinyl alcohol) films consisting of highly concentrated iodine dyes. – The transmission of polarizer for LCDs is ~45% or less for visible wavelength. – Ex: HN 42 HE ~42%, the deviation of 8% from perfect polarizer is due to residual absorption for the transmission polarization. 62

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Cholesteric liquid crystal polarizer RHC LHC 64

Reflectivity of Cholesteric LC Displays Theory Experiment 65

6. 3. 2 color filter • Color filters: dyed gelatin, dyed polyimide, and color inks. • Dyed gelatin is by far the most widely used material. • Other types shown in the next page. 66

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IPS (In-Plane Switching or Super-TFT)- Configuration 73

In-Plane Switching Conventional Twisted Nematic In-Plane Switching Mode 74

MVA(Multi-Domain Vertical Alignment, 畫素分割垂直配向)Configuration 富士通所發展的MVA 76

Optical compensators for LCDs • From Chap. 9 • Discuss the principle of phase retardation compensation using birefringent thin film to achieve high contrast ratios and gray level stability. 79