The present invention relates to a liquid crystal display device, and, more particularly, to a liquid crystal display device having a viewing angle which is widened and a light utilization efficiency which is improved by re-utilization of light using polarizing conversion and polarizing wave length selectivity.
Currently, the technical advancement in liquid crystal display devices, particularly in color liquid crystal display devices, is significant, such that display devices having almost the same image quality as a CRT have been realized. The liquid crystal display device has experienced an enlarged commercial market based on such features as thinness, lightness in weight, and low power consumption. However, the liquid crystal display itself is still inferior to a CRT in display performance, such as a moving image display, viewing angle, and color reproduction. Therefore, the liquid crystal display device still has areas of its display performance which require improvement, as well as for its production cost to be reduced.
The direct view type color liquid crystal display devices which are available on the present market can be divided roughly into two types, i.e. an active matrix driven liquid crystal display device using TFT (thin film transistor) and a multiplex driven STN (super twisted nematic) liquid crystal display device. In both of these display devices, polarizers are arranged at both sides of an element, which is composed of a liquid crystal layer held by glass substrates, and a display is produced by modulating the polarization of linearly polarized light.
In the liquid crystal display device using the TFT, a TN (twisted nematic) mode is a representative mode of operation. However, both of the TN and STN modes have a narrow viewing angle, and other problems, such as image reversal in a grayscale display and a multicolor display, and a decrease in contrast ratio.
As a way of widening the viewing angle using the TFT, various viewing angle widening modes, such as a VAN (vertical aligned nematic) mode, an IPS (in-plane switching) mode, and others, are used. In the above VAN and IPS modes for widening the viewing angle, grayscale reversal depending on viewing angle is scarcely generated, but color shift and a decrease in contrast ratio are generated.
A method using a composition of a collimated light source and a screen arranged on a liquid crystal display element has been disclosed in PCT/US94/7369 as a prior proposal for realizing a display with a widened viewing angle. Regarding screen technology for achieving a widened viewing angle, a method is disclosed in U.S. Pat. No. 2,378,252.
Conventional liquid crystal display devices display images by controlling polarized light obtained by polarizing light transmitted from an illumination device. In estimating the light loss in a color liquid crystal display device, it has been found that the light loss by the polarizer alone is approximately 60%. In the case of a color display, the color filter loss in a display device provided with plane-divided color filters is equal to or more than 70%. Approximately 88% of light is lost by the arrangement including the polarizer and the color filters. Accordingly, even if the light loss generated for any other reason is eliminated, only approximately 12% of the projected light from the illumination device can be utilized because of the absorption loss by the polarizer and the color filters.
On the other hand, demands for the liquid crystal display device of note-type personal computer are not only thinness and lightness in weight, but also low power consumption and high brightness in the display. Furthermore, a demand for a decrease in power consumption for the display of a desk top computer and a work station is high. Accordingly, decreasing the power consumption of the liquid crystal display device is indispensable, in addition to the widening of the viewing angle thereof.
Regarding the above issues, methods for decreasing the absorption loss of the polarizer and color filter in order to realize an improvement in brightness are disclosed in JP-A-6-130424 (1994) and JP-A-6-167718 (1994). In accordance with these methods, the efficiency of light utilization is improved by re-utilizing reflected light by controlling the reflection-transmission of circular polarized light in a specified direction of a specified wavelength by use of a cholesteric liquid crystal layer in order to utilize the light of the specified wavelength efficiently.
In order to realize an improvement in brightness, a method relating to the polarizing conversion using a cholesteric liquid crystal is disclosed in JP-A-3-45906 (1991). Another approach, wherein a composition using a cholesteric filter for a back light composition, is disclosed in JP-A-7-36032 (1995).
FIG. 32 illustrates a cross section of a liquid crystal display having a widened viewing angle, such as disclosed in PCT/US94/7369. The display has a problem in that the power consumption of the back light has been significantly increased for obtaining a brighter display, because the transmission factor of the screen is low, in addition to the complexity in the collimating structure and the screen structure. The liquid crystal display element comprises an arrangement wherein a liquid crystal layer 13 is interposed between two transparent substrates 11A, 11B, and two polarizers are arranged on either side thereof (not shown in the figure). A screen 10AA has transparent portions in the shape of a quadrangular pyramid at the displaying plane side and black absorbing bodies covering the intervals therebetween. A collimated illumination device, comprising lamps 51, is provided at both sides of a waveguide, and transparent media 65 in the shape of a quadrangular pyramid are adhered onto the waveguide. In the liquid crystal display device having the above structure, a decrease in resolution caused by thickness of the substrate 11 is suppressed by the collimated illumination device, the viewing angle of which is widened by the screen 10AA. In order to obtain a high resolution with the above structure, a strict collimation is required for the back light depending on the thickness and the index of refraction of the transparent substrate 11A. Simultaneously, a further decrease in the consumption power, a further widening of the viewing angle, and a further improvement in the resolution are required. It has been understood that an increase in the input power to the lamps has an undesirable effect on the display, such as an increase in the temperature due to heating (for instance, providing an inferior image quality and a shortening of the life of the lamp), in addition to an increase in the power consumption.
In the structures disclosed in previously described JP-A-3-45906 (1991) and JP-A-736032 (1995) for improving the efficiency of light utilization, the reflected light is re-utilized using the cholesteric liquid crystal operating as a reflective polarizer. on the other hand, a light control element is used for the liquid crystal display of the note type personal computer in order to improve the brightness at a normal angle toward the display surface with a decreased power consumption. As the light control element used most generally, BEF (commercial name) of the 3M Company is one example. In the light control element described above, the illumination device has a directivity at a normal angle toward a display surface in order to obtain a highly bright display with a low power consumption. However, in the above-mentioned device, the efficiency of the polarizing conversion has not been considered, especially when these light control elements are used for improving the brightness at a normal angle. Furthermore, the efficiency of the polarizing conversion has not been considered when the light control elements are used.
In the light control element, a film having stripes, the cross section of which is a triangle shape, is used. Generally, PET (polyethylene terephthalate) is used as the material for the film, and has a biaxial birefringence. Accordingly, when its optical axis is shifted from the incident angle of incident linearly polarized light, the polarization is changed, and, as the result, a decrease in the efficiency of the polarizing conversion results. Furthermore, it was found that the efficiency of the polarizing conversion was decreased if two light control elements were arranged so as to intersect at right angles.
Compositions for decreasing the absorption loss by the color filter and for improving the efficiency of light utilization are disclosed in previously described JPA-6-130424 (1994) and JP-A-6-167718 (1994). A feature of the above compositions resides in the arrangement of a color selective layer at the outside and the inside of the substrate. Examples of the above-mentioned devices are indicated in FIG. 37 and FIG. 38. In accordance with the structure indicated in FIG. 37, a liquid crystal 503 is interposed between glass substrates 501, 504, a selective layer 500 is arranged at the light projection side, a cholesteric layer 506, i.e. a color selective layer, and a filter layer 505 are arranged at the light incident side, and a light source 507 and a reflector 508 are arranged at rear side of the cholesteric layer 506. In a case of this arrangement, wherein the cholesteric layer 506, i.e. the color selective layer, is arranged outside of the glass substrate 504, as indicated in FIG. 37, the projected light 510 viewed at an angle normal to the display surface does not have any problems, such as mixing of colors in a color display, because the projected light passes through a pixel, wherein the cholesteric layer 506 and the liquid crystal 503 are the same (a region displaying the same color). However, in a case where obliquely projected light 509 is viewed at an oblique angle, for instance, the light transmitted through a red (or green, blue) color selective layer 506 is controlled by a modulating signal for green (or blue), i.e. an adjacent pixel. Accordingly, when viewing at an oblique angle, the correct color is not necessarily displayed depending on the viewing angle, because of the thickness of the substrate 504 (generally the thickness of the glass substrate is 1.1 mm, or 0.7 mm, and the pixel pitch is approximately 100 μm).
In order to avoid the influence of the thickness of the glass substrate 504, an arrangement wherein the color selective layer 512 and a retardation film 511 are built-in has been proposed, as indicated in FIG. 38. Other constituents are the same as those indicated in FIG. 37. However, any problems concerning oblique incident light relating to the characteristics of the light source have not been considered. In the arrangement indicated in FIG. 38, the display is produced by controlling the polarization to the liquid crystal layer 503 by the color selective layer 512 and the retardation film 511, and controlling the polarization by the liquid crystal layer 503. However, the cholesteric liquid crystal layer used as the color selective layer 512 has an undesirable degree of polarization to the oblique incident light, and, moreover, unnecessary light leakage of color is generated. That means that with respect to the oblique incident light, a polarization other than a desired polarization is generated, leakage of light via a color other than a desired color is generated, and so a deterioration in display quality represented by decreases in contrast ratio, color reproduction and viewing angle characteristics results. Furthermore, any uses of the polarized light effectively have not been considered at all.