5. Liquid Crystal Display

Transcription

1 5. Liquid Crystal Display Twisted Nematic(TN) LC : Director is twisted by 90 o o o o Supertwisted Nematic(STN) LC : Director is twisted by 180, 40 or 70 Better contrast and viewing angle. Yeh; Twisted-nematic(TN) Displays A. Principle of Operation LC is sandwiched between glass plates with a gap of 5 10μm. The inner surface is deposited with ITO followed by polyimide Electrode For rubbing Polarization state at the input is parallel or perpendicular to the input director. Polarization state rotates according to the twist of LC molecules(waveguiding effect). Strong electric field by an applied voltage of 3-5V. LC molecules align to the electric field.

2 B. Transmission Properties of Field-OFF State The waveguiding effect is only valid for the total twist angle π φ << Γ = ( ne no) d Mauguin condition or the limit of slow twist λ Yeh; 5- The input/output relation in eo-coordinates, Eq. (4.3.13) Γ sin X sin X ' cos X i φ V e X X V e ' = where X V sin X sin X o Γ Vo φ cos X + i X X Γ = φ + V 1 e Using = V 0 and Mauguin condition o Γ sin X ' cos X i V 1 e X ' = V sin X 0 o φ X (1) Real LCD may not satisfy Mauguin condition. Reduction in brightness and contrast. Undesirable coloration. Normally Black (NB) Mode 90 o TN LC cell between parallel polarizers ' ' V is blocked, V is passed in Eq. (1) e o The transmission of unpolarized light, from (1) π sin 1 u 1 sin X 1 + T = φ = X 1+ u π Γ Δn φ =, u = = d, φ λ () T 0 as u Mauguin condition 0 for X=,, 3... T π π π =, 3, 15, 35,... u =

3 Normally White (NW) Mode 90 o TN LC cell between crossed polarizers ' ' V is passed, V is blocked in Eq. (1) e o Yeh; 5-3 T π sin u = 1+ u Transmitted Luminance It is given by a product of the illuminant spectral distribution D ( λ ), the transmission function T ( λ ), the photopic response of the human eyes ( ) P λ. L 780 = ( ) ( ) ( ) D λ T λ P λ dλ ( ) ( ) D λ P λ dλ

4 C. Transmission Properties of Field-ON State When a voltage is applied, LC molecules tilt toward the electric field. The elastic energy density is dθ dφ 1 1 UEL = ( k1cos θ + k3sin θ) + ( kcos θ + k3sin θ) cos θ dz dz k, k, k, elastic constants ; φ, twist angle ; θ, tilt angle. 1 3 Yeh; 5-4 The electrostatic energy density is 1 1 D UEM = Di E = ε sin θ + ε cos θ Minimize the total energy in LC cell Director distribution as a function of z, θ ( z), φ( z) Redistribution of TN LC requires a threshold voltage given by (5.1.13) Typically ~1V. Transmission at Normal Incidence (1) NB mode TN LC is optimized for green light using Eq. (). For field-off state T = 0 for green with u green = 3 T 0 for red and blue because u = 3 δ, u = 3 + δ red blue For field-on state Δ n and u decrease for increased voltage. T and T increase smoothly. green red T blue undergoes an undershoot. () Similar explanation for NW mode [Fig. 5.9]

5 5. Supertwisted Nematic(STN) Displays A. Steepness of Electrodistortion Curve of STN Cell Yeh; 5-5 For 70 o twist, LC can be right-handed 70 o twist or left-handed 90 o twist. LC should be chiral nematic Lower elastic energy Intrinsically twisted nematic. d Φ For =, p π d, cell spacing ; p, pitch ; Φ, twist angle. natural pitch = pitch in LC cell Increased steepness for increased cell twist angle (Higher contrast) B. Transmission Property of STN in Field-OFF state No waveguiding effect in STN cell Polarizers are in general not parallel nor perpendicular

6 5.3 Nematic Liquid Crystal Display(N-LCD) A. Parallel Aligned(PA) Cell Vertical Switching(E field LC layer) NW Vertical Switching Mode Yeh; 5-6 For Field-OFF state : ( ) π Γ= ne no d, Γ = π, 3 π... for NW λ π λ d n n dz, 0 For Field-ON state : Γ= ( e ( θ ) o) In-Plane Switching 1 sin θ cos θ n θ = n + n. ( ) e o e θ, tilt angle of the director NB operation of IPS of N-LCD (a) Field-OFF state, (b) Field-ON state

7 B. Vertically Aligned(VA) Cell Yeh; 5-7 Director distribution of VA cell. NB operation of VA-LCD (a) Field-OFF state, (b) Field-ON state C. Bend-Aligned(BA) Cell LC director has a bend distribution of a total 180 o The electric field is applied in the same direction as the beam direction

8 BA-LCD with NW operation Yeh; 5-8 Symmetrical viewing angle in the bending plane.

9 5.4 Polymer Dispersed Liquid Crystal Display(PD-LCD) LC in a bottle Milky colored Transparent Due to discontinuity of (1) Heating beyond the clearing point refractive index at boundary LC becomes isotropic () Poling into a single domain Yeh; 5-9 Suspension of LC droplets in polymer no < n < ne n o (a) Field-Off state : Scattering in incident light by LC droplets (b) Field-On state : LC droplets align, having n, to the field and no scattering o 5.5 Reflective LCD A reflector at the back of 45 o TN-LC cell Only one polarizer is needed Transmissive TN-LCD Left-right viewing angle symmetry Reflective TN-LCD No Left-right symmetr Self-phase-compensation property (Wide viewing angle) Controlling the twist angle and the polarizer Left-right symmetry and up-down symmetry also.

10 Yeh; Projection Displays Direct viewing system Fundamental energy loss of /3 due to color filters Projection system Reduced or elimination of the loss by dielectric mirrors [Fig 5.43] Projection display using reflective LCD Near normal incidence and use of polarizing BS. [Fig 5.44] Projection display using transmission LCD Dichroic mirrors and PBS are used 5.7 Other Display Systems