Compensation films uniaxial

Crossed polarizer with compensation films. Consider two crossed O-type polarizers. A uniaxial a plate and a uniaxial c plate are sandwiched between the two polarizers. The a plate has its optical axis parallel to the transmission axis of the first polarizer and has the retardation of 2/4 [(Anzf)a=- 4]. The c plate has the retardation of 22/9 [(AnzOc = 22/9]. Calculate the transmission of the system at the azimuthal angle of 45° as a function of the polar angle 6. 

Birefringent films are used as compensation films to improve the viewing angle of hquid crystal displays. They can be divided into three groups according to the orientation of the uniaxis (c axis) with respect to the normal of the film. 

In the on-state of a TN cell, the LC directors in the upper half are reoriented along the rubbing direction with almost no twist and the lower half has a similar structure with the director plane orthogonal to that of the upper half Thus, a uniform phase compensation film, such as a uniaxial a plate, cannot compensate the upper and lower parts simultaneously. Instead, a pair of wide-view films need to be used separately on both sides of the TN LC cell in order to compensate each of the half layers. Fuji Photo has skillfully developed discotic LC films for widening the viewing angle of TN cells. The molecular stractures of the wide-view (WV) discotic material are shown below in Figure 8.4. 

Table 8.1 lists some commercially available compensation films, classified by their refractive indices. Different LC modes need different types of compensation films in order to obtain satisfactory compensation effect. For example, the IPS mode may require a biaxial compensation film with nx>n >riy [26], while the VA mode needs a compensation film with nx>riy> [27]. Theoretical analyses of biaxial film-compensated LCDs are rather difficult. Here, we focus on the uniaxial film-compensated wide view LCDs. 

Both a film and c film can be further divided into positive or negative films depending on the relative values of the extraordinary refractive index tie and the ordinary refractive index Table 8.1 lists aU the types of compensation films and their refractive index relationship. In our analyses, we focus on the uniaxial films. As a general rale, a positive uniaxial film means Tie > no, otherwise, rig < tig for a negative uniaxial film. 

Both uniaxial compensation film and nematic LC layer can be treated as uniaxial media. When a light propagates into a uniaxial film, generally two forward eigenwaves (one ordinary wave... 

When a discotic liquid crystal is sandwiched between two substrates (or exposed to air), the direction of the uniaxial axis can be controlled by alignment layers, external electric fields, and chiral dopants [46,47]. It is therefore possible to develop discotic compensation films with spatially varied uniaxial axis orientations. For example, Fuji Photo Film Co. developed discotic compensation films for TN LCDs. In both the TN display and discotic compensation film, the liquid crystal directors vary in the vertical direction. Each layer of nematic liquid crystal with a certain director orientation is compensated by a layer of discotic liquid crystal with the same director orientation. 

When a polymer film is drawn uniaxially, the induced birefringence is usually uniaxial. The produced compensation film is a positive a plate. The drawing can be carried out by the machine in a roll-to-roll process, as shown in Figure 15.18 [50]. The stretching rate can be adjusted by changing the relative speed between the feed rolls and the stretching rolls. 

In some liquid crystal displays, however, biaxial compensation films are needed. These can be made by drawing polymer films in the two directions in the film plane [50]. They can also be made by drawing uniaxial liquid crystal polymer films in the direction perpendicular to the uniaxial axis [52,53]. 

To suppress the light leakage at oblique angles and further widen the viewing angle, several phase compensation schemes using uniaxial films [20-22] and biaxial films [23-25] have been proposed. Computer simulation and experimental results have been reported. 

In this section, we focus on the analytical solutions for the uniaxial film-compensated wide view LCDs. With the analytical solutions, the interdependency between the LC cell and film parameters is clearly revealed. More importantly, analytical solutions provide a clear physical description of the compensation mechanisms. 

In the uniaxial-film-compensated LCDs, both a and c films are commonly used. In these two special cases. Equation (8.16) can be further simplified. 

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. 

In the following, we use the four-domain MVA-LCD as an example to demonstrate some uniaxial-film compensation schemes and provide each scheme with a comprehensive analytical solution. All of these compensation schemes are equally applicable to PVA mode LCDs. 

X. Zhu, Z. Ge, and S. T. Wu, Analytical solutions for uniaxial film-compensated wide-view liquid crystal displays, J. Display Technology, 2, 3 (March, 2006). 

X. Zhu and S. T. Wu, Super wide view in-plane switching LCD with positive and negative uniaxial a-films compensation, SID Digest Tech. Papers, 34, 1164 (2005). 

Photoalignment can be applied to the fabrication of retardation films as well. Retardation films are used widely in LCDs for viewing angle enhancement as well as dispersion compensation. They can be made from uniaxial or biaxial materials. Within the class of uniaxial materials, many types of films can be made and they serve different functions. For example, for uniform films where the optical axis is fixed throughout the entire film, one can have A-plates, C-plates and O-plates as shown in Figure 5.8. 

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