Compensation film

Organic Polymer Compensation Films for Liquid Crystal Displays " ... 

There is an almost infinite potential to combine different kinds of compensation films in many different ways to achieve a given effect, e.g. compensation layers of negative birefringence with the optical axes in the plane of the cell situated on either side of a TN-LCD improve the viewing angle dependence in the horizontal plane, but not in the vertical plane. However, if the optical axis of the compensation layer is tilted with respect to the plane of the cell, then superior optical properties in the vertical direction are also achieved. Compen-... 

There are many other processing methods which can reduce the viewing angle dependence of the optical properties of LCDs, e.g. pixel-divided cells and multidomain cells. However, these are beyond the scope of this monograph and will not be discussed further. Optical retardation sheets are often preferred by LCD manufacturers and systems integrators of FPDs, because commercially available compensation films can simply be laminated to the LCD using contact bonding. 

A series of other examples are given in the literature. " The method of preparation of the optical compensation film has been described in detail... 

K. Tasaka, T. Yajima, N. Kuzuhara, and H. Umeda. Optical compensation film, viewing angle compensation integral type polarizing plate and liquid crystal display apparatus. US Patent 7105213, assigned to Konica Minolta Holdings, INC (Tokyo, JP), September 12, 2006. 

J.-J. Lyu, C.-H. Lee, and H.-S. Chang. Liquid crystal display with a wide viewing angle using a compensation film. US Patent 7 265 802, assigned to Samsung Electronics Co., Ltd. (KR), September 4,2007. 

Also, the chemistry and polymerization of discotic monomers has been reviewed [10]. Their current and emerging uses are in optical compensation films for liquid crystal displays, carbon nanostructures, organic electronics, solar cells, light-emitting diodes, and field-effect transistors. 

As mentioned above, LC elastomers, especially LSCE with monodomain order, anisotropic LC networks, and gels, may be optimized to make a kind of smart material for molecular switching and piezoelectric or pyroelectric sensors, because of their sensitivity to environmental conditions (electric or stress fields, temperature, and radiation, etc.) and memory effect. Other applications include wave-guide, polarizers, optical filters, alignment, and compensation films for LCD displays. 

Polymerizable hexabenzoates of triphenylene 3, 4, and 5 (Scheme 3.3) are the revolutionary materials, which have made the less abundant discotic nematic liquid crystalline materials more ubiquitous in a world with LCDs more than people by acting as the compensation films for widening the viewing angle [28]. Among the polymerizable compounds, the acryloyl derivative 3 is compatible for photopolymerization speed and thermal stability. 

FIGURE 3.5 (a) Natural orientation of a film of an No. (b) and (c) are controlled by the use of additives and surface treatment (above) along with a key to the molecular structure of the photopolymerizable No exploited in the Fuji optical compensation film. Examples of the types of materials used to control the orientation of the director at the air-liquid crystal and solid-liquid crystal interface (below). Reprinted with permission from Reference 14. Copyright 2002 Elsevier. 

SCHEME 3.3 Chemical structures of the revolutionary DLC compounds which have found real practical application in LCD devices as optical compensation films to enlarge the viewing angle and enhance the contrast ratio. 

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... 

Table 8.1 Different types of compensation films used for wide view LCDs.

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