Voltage-dependent transmittance

To compare different operating modes, let us focus on the normalized transmittance by ignoring the optical losses from polarizers and indium-tin-oxide (ITO) layers, and the interface reflections from substrates. The normalized transmittance TV) of a TN cell can be described by the following Jones matrices as = mV [5]  

Here p is the angle between the polarization axis and the front LC director, (p is the twist angle, 

Equation (8.2) is a general formula describing the light transmittance of a TN cell (without voltage) as a function of twist angle, beta angle, and dtSrdX. For a 90° TN cell, = nil and Equation (8.2) is simplified to 

Equation (8.3) has a special solution, that is cos X= 1. When cosX= l (i.e. X=mn m = integer), then sin X = 0 and the second term in Equation (8.3) vanishes. Therefore, TT = 1, independent of p. By setting X = mjr and knowing that T = IndLnlX, the Gooch-Tairy s condition is found as 

For the lowest order m = 1, dAnJX = Vlll. This is the Gooch-Tarry s first minimum condition for the 90° TN cell. For the second order m = 2 and dAn = v/l5/2. The second minimum condition is used only for low-end displays such as wrist watches, because a large cell gap is easier to fabricate, and the cyano-biphenyl LCs are less expensive. For notebook TFT-LCDs, the first minimum is preferred because fast response time is required. 

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Figure 8.2 Voltage-dependent transmittance of a normally white 90° TN cell, dhn = 480 nm.

For simplicity but without losing generality, let us assume that in eaeh pixel the LC direetors form a four-domain orientation profile, as Figure 8.21(a) shows. Figure 8.21(b) depicts the calculated voltage-dependent transmittance curve of a typical MVA-LCD using Merck MLC-6608 LC material whose parameters are listed in Table 8.3. Here, the absorption loss of polarizers has been taken into consideration. In the film-compensated MVA eells, the refractive indices of the uniaxial films and polarizers are still the same as those listed in Table 8.2. 

Figure 8.30 Voltage-dependent transmittance curves of a r cell. dAn=436nm, uniaxial film rfAn = 53.3 nm and its optic axis is perpendicular to that of LC cell.

A homogeneous cell is useful as a tunable phase retardation plate. Figure 8.35 plots the voltage-dependent transmittance curve of a homogeneous LC cell at X = 633 nm. The polarizers are crossed and the angle between the front polarizer and the LC rubbing direction is 45°. 

Figure 9.11 Transflective TN LCDs (a) schematic device configuration, and (b) voltage-dependent transmittance and reflectance curves. Zhu 2006. Reproduced with permission from IEEE.

Figure 9.11(b) plots the voltage dependent transmittance and reflectance curves of a typical transflective TN LCD. Here, twist angle (p = 90° and the first Gooch-Tairy minimum condition d n = 476 nm are employed, where d is the cell gap and An is the LC birefiingence. The grayscales of both T and R modes overlap well with each other. This is because the reflection beam in the R-mode experiences the bottom polarizer, LC layer, and top polarizer successively in turn, as the transmission beam does in the T-mode. 

Unless identical display modes are adopted in both T- and R-modes, otherwise there are always some discrepancies between their voltage dependent transmittance and reflectance curves. This is the reason that none of the abovementioned transflective mixed-mode LCDs has perfectly matched voltage dependent transmittance and reflectance curves. Different from the mixed display modes employed between transmission and reflection regions as described above. Sharp Corp. also introduced the dual-cell-gap concept for transflective LCDs [57]. 

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