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

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.

5.1 twisted nematic displays The operation of TN based on waveguiding property.

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

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=0.63mm, φ=p/4 Output beam ~linearly polarized along the local director(e axis). This explains the waveguiding phenomenon.

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

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

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

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.

Middle layer director Cell director distribution Subdivide TN director Jones matrix calculation Φ is the total twisting angle

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

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

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

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

Transmission of TN LCDs: Normally White 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

Contrast at normal incidence Contrast ratio contrast Viewing angles

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

Gray scale inversion

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

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

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).

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

Super-Twisted Nematic (STN)

Super-Twisted Nematic (STN)

Super-Twisted Nematic (STN) Multiplexing Example for STN D 0.5 0.4 STN LCD 0.3 T(%) 0.2 0.1 V TH 1 2 3 4 5 V Medium resolution is possible Notice the very steep threshold

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

Threshold of STN And taking the tilt angle, qo, to be zero. fT (twist angle) d/P (gap/pitch) VTH (volts) 90o 0.25 1.10 180o 0.50 1.63 270o 0.75 2.20 360o 1.00 2.70

270o Super-Twisted Nematic (STN) 10 30 40 60 70 90 80 50 20 45 90 135 180 225 270 VR = 3.15 VR = 2.04 Tilt Angle q (Degrees) Twist Angle f (Degrees) VR = 1.83 VR = 0.11 0.2 0.4 0.6 0.8 1 q f z/L

270o Super-Twisted Nematic (STN) 90 80 70 tilt angle (deg) 60 270o STN 50 40 Midlayer 30 20 10 V V ON OFF 0.5 1 1.5 2 2.5 3 Reduced Voltage

5.3 nematic liquid crystal display (N-LCD) 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.

TN LCDs Twist angle, complex Wavelength insensitive

5.3.1 parallel aligned (PA) cells

Parallel Aligned Cells x y

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 birefringence mode (ECB-LCD).

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

TN- LCD is less sensitive to wavelength variation than 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.

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.

In-Plane Switching

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.

5.3.2 vertical aligned (VA) cells

Rubbing direction

Backflow effect in LC devices

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

Backflow Effect in LC Devices TN CHLC

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.

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

ECB-Backflow Effects g1* is effective rotation viscosity For small angles g1* is effective rotation viscosity ai are the Leslie viscosity coefficients

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

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

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

OCB (Pi) cell (Splay) (Bend) (Homeotropic) V0=0 V1=Vc V2>>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)

l/2

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

5.4 polymer dispersed liquid crystal displays ne~np

5.6 projection displays

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 HN22, HN32, HN38S and HN42HE 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:HN42HE ~42%, the deviation of 8% from perfect polarizer is due to residual absorption for the transmission polarization.

Cholesteric liquid crystal polarizer RHC LHC

Reflectivity of Cholesteric LC Displays Theory Experiment

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.

廣視角技術 CRT顯示器 V.S. 薄膜電晶體液晶顯示器 當你從某個角度觀看TFT-LCD時，你將發現顯示器的亮度急遽的損失(變暗)及變色。較舊型的平面顯示器通常只有90度的視角，也就是左/右兩邊各45度。 影片中快速的移動畫面是常出現的，但這樣的需求卻是目前響應時間慢的LCD所無法提供的。太慢的響應時間會導致畫面失真及次序錯亂。Ex.股票市場中的交易顯示器及飛機飛過村莊的畫面。 改善視角特性新技術。 Film(視角補償膜） IPS(也稱為超級液晶顯示器) MVA。

TN+Film(TN+視角擴大膜)-configuration TN+Film的顯示器對準液晶於基版的垂直方向，與標準的TFT-LCD一樣。有了在上層表面的一層膜就可以增加視角。 從技術的觀點來看，TN+Film是廣視角技術中容易實現的方法。液晶顯示器的製造商使用較成熟的標準TFT-Twisted Nematic (扭轉向列式)液晶。一層特殊的薄膜(轉向膜或是)加在面板的上表面就可以將水平視角從90度改善到140度。但是，低對比及響應速度慢這兩大問題依舊無法改善。TN+Film法也許不是最佳的廣視角解決方案但它是最簡單的方法並且良率極高(幾乎與標準TFT-TN一樣)。

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

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

IPS (In-Plane Switching or Super-TFT) IPS起初是由Hitachi所發展，但現在NEC及Nokia也採用這項技術。IPS與使用TN+Film(扭轉向列液晶+視角擴大膜組合)技術不同的地方是電場方向平行於玻璃基版。使用IPS或Super TFT技術可以使視角擴大到170度，就如同CRT視角一樣好。但是這項技術也有缺點，電極必須製作成像梳子裝狀的排列在下層的表面。這樣做會導致對比降低，因此必須加強背光源的亮度。IPS模式的對比及響應時間與傳統的TFT-TN 比較起來並無改善。

MVA(Multi-Domain Vertical Alignment,畫素分割垂直配向)- Configuration

MVA(Multi-Domain Vertical Alignment,畫素分割垂直配向) 從技術的觀點來看，MVA是針對視角及響應時間最好的解決方案。依我們的看法，富士通已經找到最好的解決方案。MVA可以獲得160度的視角，而且，也可提供高對比及快速響應的優秀表現。 在MVA中，M代表 “multi-domain“(多象限)，是就一個子像素(cell)來說。圖3說明了以突出物來形成多象限。富士通最近已經開發出四個象限的畫素分割技術。VA 代表“Vertical Alignment“ (垂直配向)，但事實因為有突出物的關係，液晶分子在靜態時並不是真的垂直配向(見上圖，在off的時候)。當施加電壓時，液晶分子變成水平配向因此允許背光通過。對遊戲機及動畫的應用上，MVA可以提供比TN+視角擴大膜及IPS技術較短的響應時間。在對比方面，也比其他廣視角技術高，當然，對比也隨視角而變。

各種廣視角技術評估 MVA提供較快的響應及非常大的視角

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.