Recently, as markets on watches, notebook PCs, cellular phones, televisions and monitors have been tremendously extended, the demand for displays of low weight and low power consumption has been greatly increased. A liquid crystal display (LCD), which is light and thin and requires low power consumption, has been widely applied to such products.
However, an LCD has a disadvantage of viewing angle dependency. In other words, an LCD shows variations in color or light/darkness depending on viewing directions or angles. Also, as a size of such an LCD increase, a viewing angle decreases. As compared to a conventional CRT (cathode ray tube) device having a viewing angle of about 180° a TFT-LCD with no viewing angle compensation shows a viewing angle of merely about ±50°.
In order to solve the above described problems, various methods have been used such methods including a multi-domain method in which pixels are divided in liquid crystal cells to control liquid crystal alignment, a method of controlling voltage, and a method of utilizing an optical compensation film.
The above-mentioned viewing angle dependency of an LCD is caused by the incident light having a tilt angle to an LCD panel, which shows a birefringence effect different from that of the vertical incident light. In order to compensate for this, a method of using an optical compensation film has been widely used in which retardation films having opposite birefringence indexes to a panel are attached onto both surfaces of the panel. Also, as the size of a display panel has increased a high-quality liquid crystal compensation film has been needed.
A retardation film is obtained by coating an aligned transparent support with liquid crystal, and aligning the liquid crystal along a predetermined direction to the direction of an aligning layer, followed by curing. After aligning, the liquid crystal has a direction opposite to the direction of liquid crystal cells upon application of voltage, so that light leakage in a black state can be minimized. When such retardation films are combined with a liquid crystal panel so that light is allowed to penetrate through the panel, it is possible to compensate for a light phase difference caused by a difference of light paths because paths of the incident light are similar to each other in all directions. In addition, it is also possible to perform compensation of a difference in birefringence indexes in upper/lower/left/right directions by optimizing the magnitude of birefringence of each film, an angle formed between films, a rubbing direction and an angle to a polarizer.
Therefore, there is a need for a new liquid crystal compound used for manufacturing a viewing angle compensation film having high-quality characteristics of improving a contrast ratio, and minimizing color variations in a black state depending on viewing angles.
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