Mixed-mode twisted nematic (MTN)

NW 90

°

This mode has twist angle φt=90° and β=20°. The required phase retardation d∆n is about 240 nm for normally white operation. Figure 3.2 (a) shows the one-dimensional (1D) simulation results on the voltage-dependent normalized reflectance using broadband red (R:620-680 nm), green (G:520-560 nm) and blue (B:420-480 nm) lights. As shown in the figure, the maximum reflectance of this mode is only about 88%. Based on the curve, we let the turned-on voltage (Von) to be 5 volts and the turned-off voltage (Voff) to be 0.7 volts.

Figure 3.2 (b) shows the calculated iso-contrast contour viewing diagram for the broadband green channel. The polar angle corresponding to 1000:1 contrast ratio exceeds 8°. Although the property of viewing angle is less critical in projection displays, a wide acceptance angle allows a small F-number projection lens to be used which greatly improves the display brightness [12].

Fig. 3.2 Simulated results of (a) the broad band RV curve, (b) the broad band green light iso-contrast viewing diagram, and (c) the LC director and the reflectance profiles of NW 90°-MTN mode with β=20° and d∆n==240 nm.

Figure 3.2 (c) shows the 2D simulation results of the LC director profile and the reflectance calculated by Jones matrix method for the dark-bright-dark pixel configuration with 2.8 µm cell gap and 2° pretilt angle at green band (540 nm). In this condition, the fringing fields are the strongest. As shown in the 2D LC director profile, the fringing fields at the pixel edges will penetrate into the adjacent pixels and induce light leakage in the dark pixels. This effect will degrade the contrast ratio as well as the image sharpness.

The advantages of this mode are the high contrast ratio owing to the natural surface phase compensation of the two orthogonal boundary layers and the relatively small fringing-field effect. The calculated 2D flat field contrast ratio exceeds 6000:1 for the normally incident light. The shortcoming of this 90^o MTN mode is that its maximum optical filled factor is ~84.7% at the dark-bright-dark configuration. Here we define the optical filled factor as

∫

= R_bright

F S1

, (3.1)

where S denotes the dimension of the turned-on pixels and R_bright. is the normalize reflectance within the bright pixel area. Changing the twist angle to 80° would boost the filled factor to ~95%. However, the contrast ratio decreases drastically because of incomplete surface phase compensation.

NW Film-Compensated 63.6

°

The normalized reflectance R^⊥ of a reflective twisted LC cell under crossed-polarizer condition can be given as*:

( ) d∆n/λ= 2/4. This mode has the advantage of high reflectance; however, a very high voltage is needed to obtain a good dark state due to the absence of intrinsic phase compensation. In order to operate at lower voltage, a uniaxial film is employed. Figure 3.3 (a) shows the 1D simulated results of the voltage-dependent reflectance curve under broadband incident light when a uniaxial film with α=110° (referring to Fig. 2.6) and (d∆n)film=24 nm is applied. The required phase retardation d∆n of the LC cell is 203 nm and entrance polarizer angle is set at β=0°. As shown in Fig. 3.3 (a), the maximum reflectance slightly decreases to 97% due to the effect of the compensation film. Figure 3.3 (b) plots the calculated results of iso-contrast viewing diagram under broadband green light with Von=5 V and Voff=0.7 V. The contour line for 1000:1 contrast ratio reaches 10°

viewing cone. Figure 3.3 (c) demonstrates the 2D simulated results of the LC director profile and the reflectance for the dark-bright-dark pixel configuration with 2.4 µm cell gap.

The light leakages still exists at the dark pixels due to the influence of fringing fields.

Fig. 3.3 Simulated results of (a) the broad band RV curve, (b) the broad band green light iso-contrast viewing diagram, and (c) the LC director and the reflectance profiles of 63.6°-MTN mode with β=0°, d∆n==203 nm, (d∆n)film=24 nm, and 110° film anglerelated to x-axis.

In addition, the optical filled factor is only about 92.7% since the maximum R⊥ can not reach 100%. When setting β=4°, d∆n=212 nm, (d∆n)film=15 nm, and α=136°, the maximum R⊥ can be boosted up to 99%. In this circumstance, more birefringence effect is introduced to enhance the reflectance. However, a stronger fringing field occurs and results in a lower F. Thus, this tradeoff is not worth taking.

NW Film-Compensated 45

°

The film-compensated 45° MTN cell has twist angle φt=45° and β=78°. The required phase retardation d∆n is only 195 nm for normally white operation. To obtain a good dark state at lower voltage, a uniaxial film with α=110° and (d∆n)film=27 nm is added. Figure 3.4 (a) shows the 1D simulated results of the voltage-dependent reflectance curve under broadband R, G and B lights. The maximum reflectance of this mode can reach 100% and on-state voltage is 5V. Figure 3.4 (b) shows the calculated results of iso-contrast viewing diagram under broadband green light with Von=5.0 V and Voff=0.7 V. We can see that the contour line for 1000:1 contrast ratio exceeds 10° polar angle in all directions. This large viewing cone results from the small d∆n value. Figure 3.4 (c) demonstrates the 2D simulation results of the LC director profile and the reflectance for the dark-bright-dark pixel configuration with 2.3 µm cell gap. It is shown that its response to fringing fields is similar to other two MTN modes as we have discussed. Among them, 90°-MTN cell has the smallest and 63.6°-MTN has the largest light leakages in the dark-pixel areas.

The advantages of the film-compensated 45°-MTN mode are twofold: high optical filled factor (up to 96.7%) and low d∆n value which, in turn, leads to a thin cell gap and fast response time. Therefore, this mode is particularly attractive for color sequential displays using a single LC panel. However, the dark state of this mode is not as good as that of 90°-MTN mode since the complete compensation only occurs at certain wavelengths.

Besides, the required compensation film has a relatively small d∆n value which is not easy to fabricate. The film needs to be laminated onto the surface of the polarizing beam splitter.

Any artifacts or bubbles during the lamination process would be magnified and projected to the screen. Moreover, the film has to withstand high power illumination from the arc lamp.

Fig. 3.4 Simulated results of (a) the broad band RV curve, (b) the broad band green light iso-contrast viewing diagram, and (c) the LC director and the reflectance profiles of 45°-MTN mode with β=78°, d∆n==195 nm, (d∆n)_film=27 nm, and 110° film angle related to x-axis.