Nowadays, liquid crystal display devices are in widespread use in various monitors for personal computers or the like, display devices for cellular phones, and so forth, and are expected to increasingly spread hereafter, and in fact, it is being planned to develop their use in large-screen television sets. In such liquid crystal display devices, what is called a micro color filter system is widely used as a color display system.
With this system, one pixel is divided into at least three subpixels, for each of which red, green and blue color filters are formed to perform full-color display. This system has such an advantage that high color reproduction performance can be materialized with ease. However, in this system, there is a disadvantage such that transmittance comes to be ⅓ so that light utilization efficiency is lowered.
In such a low light utilization efficiency, a problem is raised in that it is necessary to increase the luminance of a back light or a front light in an attempt to achieve bright display so that visibility can be improved, resulting in a high power consumption, in the case of a transmission type liquid crystal display device or semi-transmission type liquid crystal display device having a back light, or a reflection type liquid crystal display device having a front light.
That low light utilization efficiency results in a more serious problem in the case of a reflection type liquid crystal display device making use of no front light. That is, a reflection type liquid crystal display device having RGB color filters can secure sufficient visibility in the very bright outdoors, but it is difficult to secure sufficient visibility in dark places and even in an environment of offices or homes.
Meanwhile, as a color liquid crystal display device which performs display without use of any color filters, the liquid crystal display device of an ECB type (electrically controlled birefringence effect type) is conventionally known in the art.
This ECB type liquid crystal display device includes a transmission type in which a liquid crystal cell holding a liquid crystal is interposed between a pair of substrates and a polarizing plate is disposed on each of its front side and back side, and a reflection type including a single polarizing plate type in which the polarizing plate is disposed only on one substrate, and a double polarizing plate type in which the polarizing plate is disposed on both the substrates and a reflecting plate is provided on the outside of one polarizing plate.
Here, in the case of, e.g., the transmission type ECB type liquid crystal display device, the linear polarization transmitted through one polarizing plate to have entered becomes light, each wavelength light of which has turned to elliptical polarization in a different polarization state by the birefringent action of a liquid crystal layer while being transmitted through the liquid crystal cell. This light enters the other polarizing plate, and the light transmitted through the other polarizing plate becomes colored light in a color corresponding to the ratio of light intensity of each wavelength light which constitutes that light.
That is, the ECB type liquid crystal display device colors light by utilizing the birefringent action of a liquid crystal layer of the liquid crystal cell and the polarization action of at least one polarizing plate. Since it is free of absorption of light that takes place when color filters are used, it can perform bright color display at high light transmittace.
Moreover, the ECB type liquid crystal display device varies in birefringence of the liquid crystal layer depending on the state of alignment of liquid crystal molecules in accordance with the voltage applied across the electrodes of both the substrates of the liquid crystal cell. Following such variations, the polarization state is varied with each wavelength light entering the other polarizing plate, so that the color of colored light can be changed by controlling the voltage applied to the liquid crystal cell. Thus, a plurality of colors can be displayed in the same pixel.
FIG. 10 is a view showing retardation levels and colors corresponding thereto where the ECB type liquid crystal display device is driven under crossed Nicol (cross polarization). As shown in FIG. 10, colors change in accordance with birefringence levels. Here, when using as a liquid crystal mode, e.g., a material having negative Δε which stands perpendicularly aligned when no voltage is applied is used, black is displayed when no voltage is applied, and colors are so changed as to be black→gray→white→yellow→red→purple→blue→yellow→purple→light blue→green with an increase in voltage.
However, the ECB type liquid crystal display device can display any desired colors in the same pixel, but has such a problem that because of its mode that utilizes the coloring relying on retardation, display colors may be varied with changes in retardation.