The present invention generally relates to liquid crystal display devices and more particularly to a color liquid crystal display device that uses birefringence.
Color liquid crystal display devices generally use color filters for providing color picture representation. In a full color display device, color filters for three primary colors such as red, green and blue are provided in each pixel. On the other hand, such a construction generally has a problem of relatively coarse resolution associated with the use of three color filters in each of the pixels.
While such color liquid crystal display devices are successfully used as a display device of various information processing devices such as a personal computer that illuminates the display device from behind by a backlight, the use of color filters causes a problem of unwanted optical absorption particularly in the portable terminals that use reflected light for the display of information. In the liquid crystal display devices that use reflected light, high luminance of white is an essential factor in view point of relatively dark representation associated with the use of reflected light.
As a method to achieve color display of images without using color filters, it is known to use birefringence of liquid crystals. In the color liquid crystal display devices that use birefringence for the representation of color images, the light incident to a liquid crystal cell experiences a decomposition to an ordinary ray and an extraordinary ray having respective, different velocities. Thus, upon propagation through the liquid crystal cell, there occurs a retardation between the ordinary ray and the extraordinary ray. Upon exiting from the liquid crystal cell, the ordinary ray and the extraordinary ray, shifted with each other in phase, cause interference and an interference color is obtained.
Generally, the color liquid crystal display devices that use birefringence for the color representation include ECB (electrically controlled birefringence) devices, wherein the ECS devices produce interference color by controlling the birefringence of the liquid crystal. In other words, various colors are obtained by controlling the strength of the electric field.
For example, there is a device that operates based upon the birefringence caused in a liquid crystal cell of the super twisted nematic liquid crystals wherein the liquid crystal molecules are arranged with a twist angle of 180.degree.-360.degree.. By setting the plane of polarization to be offset from the direction of the principal axis of the liquid crystal molecule at the surface of the electrode that contacts with the liquid crystal layer, it is possible to obtain optical interference due to the birefringence of the liquid crystal.
Generally, the ECB device of the aforementioned type is suitable for providing red, green and blue colors in the reflection type display devices, while such a device has a drawback of poor performance in displaying white or black colors.
FIGS.1A and 1B show the principle of ordinary liquid crystal display panel.
Referring to FIGS. 1A and 1B, a liquid crystal layer 3c is sandwiched by a pair of glass substrates 3a and 3b having respective inner surfaces, wherein an alignment layer (not shown) is provided on each of the inner surfaces such that molecules 3c.sub.1 of the liquid crystal are aligned as indicated in FIG. 1A in the state that there is no electric field applied to the liquid crystal layer. Thus, the liquid crystal molecules 3.sub.1 adjacent to the glass substrate 3a are aligned in a first direction while the liquid crystal molecules 3.sub.1 adjacent to the glass substrate 3b are aligned in a second, perpendicular direction. Further, a first polarizer 4 is disposed above the glass substrate 3a with the plane of polarization coincident to the direction of alignment of the liquid crystal molecules 3.sub.1 adjacent to the substrate 3a, and a second polarizer 5 is provided below the glass substrate 3b with the plane of polarization coincident to the direction of alignment of the liquid crystal molecules 3.sub.1 adjacent to the substrate 3b.
Thus, the optical beam incident to the liquid crystal layer 3c from the upward direction as indicated in FIG. 1A by an arrow, experiences polarization in the polarizer 4 and propagates through the liquid crystal layer 3c while rotating the plane of polarization and exits through the polarizer 5. In other words, FIG. 1A shows the state that the liquid crystal panel is transparent to the transmitting light.
In the state of FIG. 1B, an electric field is applied to the liquid crystal layer and the liquid crystal molecules 3c.sub.1 are aligned in the direction perpendicular to the plane of the liquid crystal layer 3c. Thus, the optical beam incident to the liquid crystal layer 3c maintains the plane of polarization coincident to the plane of polarization of the polarizer 5 and is cutoff at the lower polarizer 5. In other words, FIG. 1B shows the state wherein the liquid crystal display panel shuts the transmission of optical beam.
FIG.2 shows a conventional DSTN (double-layered super twisted nematic) color display device 11 for use in a transmission type color liquid crystal display devices.
Referring to FIG.2, the color liquid crystal display device 11 includes a stacking of a first liquid crystal panel 12 and a second liquid crystal panel 13, with a first polarizer plate 14 disposed above the panel 12 and a second polarizer plate 15 disposed below the panel 13. As usual, the liquid crystal panel 12 is formed of a pair of glass substrates 12a and 12b sandwiching a liquid crystal layer 12c. Similarly, the liquid crystal panel 13 is formed of a pair of glass substrates 13a and 13b sandwiching a liquid crystal layer 12c. In the illustrated example, the liquid crystal panel 13 acts as a driver panel and transparent electrodes 13a.sub.1 are provided on the glass substrate 13.sub.1. Similarly, transparent electrodes 13b.sub.1 are provided on the glass substrate 13b. Further, a color filter (RGB filter) 16 is provided on the glass substrate 13b.
In the panel of FIG.2, it should be noted that the optical beam that is incident to the display device 11 is colored according to the color of the filter 16 upon passage therethrough, and a desired color image is displayed. The upper liquid crystal panel 12 is provided for eliminating unwanted coloring that may occur when the optical beam passes through the liquid crystal layer 13c as a result of birefringence. In order to avoid unwanted coloring, the panels 12 and 13 are formed to have a thickness d set smaller than about 6 .mu.m to minimize retardation represented as .sub..DELTA. n.multidot.d, wherein .sub..DELTA. n represents the birefringence. Generally, the thickness d is set to satisfy a relationship of .sub..DELTA. n.multidot.d.ltoreq.1000 nm.
While the liquid crystal display device 11 of FIG. 2 is successfully used in the liquid crystal color display devices of transmission type, the device has a drawback when used in the reflection type liquid crystal display devices that the color filter 16 provides unwanted absorption of light and the displayed image tends to become dark as described already. It should be noted that luminance of light decreases by two-thirds upon passage through the RGB color filter 16. In such reflection type devices that lacks backlight, a high luminance of white is particularly important.
FIG.3 shows another known color liquid crystal display device 20 that uses birefringence as disclosed in the Japanese Laid-open Patent Publication 4-188113.
Referring to FIG.3, the device is of the ECB (electrically controlled birefringence) type and includes a main substrate 21 on which a data electrode 21b is provided, wherein the data electrode 21b acts also as a reflector and is covered by a molecular alignment layer 21a.
Above the substrate 21, there is provided another, second substrate 22 acting as a polarizer, wherein the polarizer 22 carries a transparent electrode 22 on each of upper and lower major surfaces such that the electrode 22 is covered by a molecular alignment film 22a. Further, there is provided still other, third substrate 23 above the polarizer 22 such that the substrate 23 carries a transparent electrode 23b covered by a molecular alignment layer 23a. Thus, a liquid crystal layer 24a is sandwiched between the substrate 21 and the substrate 22, and a liquid crystal layer 24b is sandwiched between the substrate 22 and the substrate 23. Further, another polarizer 25 is disposed above the substrate 23, wherein the plane of polarization is set orthogonal in the polarizer 22 and the polarizer 25. A liquid crystal that changes refractive index in response to the electric field is used for the liquid crystal layers 24a and
As indicated in FIG.3, the molecular alignment layers 21a, 22a and 23a provide a vertical alignment to the liquid crystal molecules in the liquid crystal layers 24a and 24b when in the unbiased state as indicated in a region A of FIG.3, and the optical beam passes through the liquid crystal layers 24a and 24b without experiencing substantial birefringence. As the plane of polarization is set orthogonal between the polarizer 22 and the substrate 23, the optical beam incident to the region A is effectively cutoff at the polarizer 22 and the region A shows a black appearance. When a voltage is applied-across the electrodes 21a and 22a, and between the electrodes 22a and 23a, the liquid crystal molecules are no longer vertical in the layers 24a and 24b, and retardation occurs in the liquid crystal layers 24a and 24b. Thus, a color is obtained in the optical beam exiting from a region B in FIG.3.
By stacking two liquid crystal layers 24a and 24b, it is possible to improve the purity of color. On the other hand, the device of FIG.3 has a drawback in that it cannot provide desired bright white that is needed by the reflection type liquid crystal display device.
In the Japanese Laid-open Patent Publication 4-188113, it is also described that the direction of the liquid crystal molecules may not be perpendicular to the plane of the substrate. Even in such a case, the reference merely proposes to stack two liquid crystal cells with identical direction of twisting of the liquid crystal molecules.