1. Field of the Invention
The present invention relates to a liquid crystal display device and, in particular, to a reflection/transmission type liquid crystal display device capable of performing a display both in a reflection mode and a transmission mode.
2. Description of the Related Art
Conventionally, there have been a reflection type liquid crystal display device utilizing ambient light, a transmission type liquid crystal display device utilizing backlight, and a semi-transmission type liquid crystal display device equipped with a half mirror and a backlight.
In a reflection type liquid crystal display device, a display becomes less visible under dim environment, whereas in a transmission type liquid crystal display device, a display becomes hazy under strong ambient light (e.g., under outdoor sunlight). As a liquid crystal display device capable of functioning in both modes so as to perform a satisfactory display under any environment, a semi-transmission type liquid crystal display device is disclosed by Japanese Laid-Open Publication No. 7-333598.
However, the above-mentioned conventional semi-transmission type liquid crystal display device has the following problems.
The conventional semi-transmission type liquid crystal display device uses a half mirror in place of a reflective plate used in a reflection type liquid crystal display device, and has a minute transmission region (e.g., minute holes in a metal thin film) in a reflection region, thereby performing a display by utilizing transmitted light as well as reflected light. Since reflected light and transmitted light used for a display pass through the same liquid crystal layer, an optical path of reflected light becomes twice that of transmitted light, which causes a large difference in retardation of the liquid crystal layer with respect to reflected light and transmitted light. Thus, a satisfactory display cannot be obtained. Furthermore, a display in a reflection mode and a display in a transmission mode are superimposed on each other, so that the respective displays cannot be separately optimized. This results in difficulty in performing a color display, and causes a blurred display.
A liquid crystal display device according to the present invention, includes: a first substrate, a second substrate, a liquid crystal layer interposed between the first substrate and the second substrate, and a plurality of pixel regions defined by a pair of electrodes for applying a voltage to the liquid crystal layer, wherein each of the plurality of pixel regions includes a reflection region and a transmission region, and the liquid crystal layer is made of a liquid crystal material having positive dielectric anisotropy, the device further including: a first polarizing element provided on the first substrate opposite to the liquid crystal layer; a second polarizing element provided on the second substrate opposite to the liquid crystal layer; a first phase difference compensator provided between the first polarizing element and the liquid crystal layer; and a second phase difference compensator provided between the second polarizing element and the liquid crystal layer, a twist angle xcex8t of the liquid crystal layer being in a range of 0xc2x0 to 90xc2x0, wherein retardation Rd and the twist angle xcex8t in a visible light region of the liquid crystal layer in the reflection region are in ranges within curves respectively represented by the following Formulae (1) and (2), and Formulae (3) and (4), in ranges within curves respectively represented by the following Formulae (5) and (6) and Formulae (7) and (8) at the twist angle xcex8t in a range of 0xc2x0xe2x89xa6xcex8txe2x89xa654.3xc2x0, and in ranges within curves respectively represented by the following Formulae (5) and (8) at the twist angle xcex8t in a range of 54.3xc2x0 less than xcex8txe2x89xa690xc2x0, and wherein the retardation Rd and the twist angle xcex8t in a visible light region of the liquid crystal layer in the transmission region are in ranges within curves respectively represented by the following Formulae (9) and (10) and Formulae (11) and (12):
Rd=xe2x88x920.0043xc2x7xcex8t2xe2x88x920.065xc2x7xcex8t+1011.8xe2x80x83xe2x80x83(1)
Rd=xe2x88x920.0089xc2x7xcex8t2+0.1379xc2x7xcex8t+914.68xe2x80x83xe2x80x83(2)
Rd=xe2x88x920.0015xc2x7xcex8t2xe2x88x920.1612xc2x7xcex8t+737.29xe2x80x83xe2x80x83(3)
Rd=xe2x88x920.0064xc2x7xcex8t2xe2x88x920.0043xc2x7xcex8t+640.65xe2x80x83xe2x80x83(4)
Rd=xe2x88x920.0178xc2x7xcex8t2+0.2219xc2x7xcex8t+458.92xe2x80x83xe2x80x83(5)
Rd=xe2x88x920.0405xc2x7xcex8t2+0.4045xc2x7xcex8t+364.05xe2x80x83xe2x80x83(6)
Rd=0.0347xc2x7xcex8t2xe2x88x920.4161xc2x7xcex8t+186.53xe2x80x83xe2x80x83(7)
Rd=0.0098xc2x7xcex8t2xe2x88x920.1912xc2x7xcex8t+89.873xe2x80x83xe2x80x83(8)
xe2x80x83Rd=xe2x88x920.0043xc2x7xcex8t2xe2x88x920.065xc2x7xcex8t+995.66xe2x80x83xe2x80x83(9)
Rd=xe2x88x920.0058xc2x7xcex8t2xe2x88x920.0202xc2x7xcex8t+665.8xe2x80x83xe2x80x83(10)
Rd=xe2x88x920.0248xc2x7xcex8t2+0.6307xc2x7xcex8t+439.58xe2x80x83xe2x80x83(11)
Rd=0.0181xc2x7xcex8t2xe2x88x920.6662xc2x7xcex8t+109.51xe2x80x83xe2x80x83(12)
In one embodiment of the present invention, the retardation Rd is in a range within the curves respectively represented by Formulae (7) and (8) at the twist angle xcex8t in the reflection region in a range of 0xc2x0xe2x89xa6xcex8txe2x89xa654.3xc2x0, and in a range within the curves respectively represented by Formulae (5) and (8) at the twist angle xcex8t in the reflection region in a range of 54.3xc2x0 less than xcex8txe2x89xa690xc2x0, and the retardation is in a range within the curves respectively represented by Formulae (11) and (12) at the twist angle xcex8t in the transmission region in a range of 0xc2x0xe2x89xa6xcex8txe2x89xa690xc2x0.
In another embodiment of the present invention, the reflection region and the transmission region include a liquid crystal layer made of the same liquid crystal material, and a thickness of the liquid crystal layer in the reflection region is smaller than a thickness of the liquid crystal layer in the transmission region.
In another embodiment of the present invention, the first phase difference compensator has a first phase difference plate, the twist angle xcex8t of the liquid crystal layer is 0xc2x0, the retardation Rd of the reflection region is 90 nmxe2x89xa6Rdxe2x89xa6187 nm, the retardation Rd of the transmission region is 110 nmxe2x89xa6Rdxe2x89xa6440 nm, and the retardation Rd of the first phase difference plate is 30 nmxe2x89xa6Rdxe2x89xa6250 nm.
In another embodiment of the present invention, the first phase difference compensator further has a second phase difference plate, and the retardation Rd of the second phase difference plate is in a range of 220 nmxe2x89xa6Rdxe2x89xa6330 nm.
In another embodiment of the present invention, the second phase difference compensator has a third phase difference plate, and the retardation Rd of the third phase difference plate is in a range of 120xe2x89xa6Rdxe2x89xa6150 nm.
In another embodiment of the present invention, the second phase difference compensator further has a fourth phase difference plate, and the retardation Rd of the fourth phase difference plate is in a range of 240xe2x89xa6Rdxe2x89xa6310 nm.
Hereinafter, the function of the present invention will be described. First, the terms used herein will be described. In a reflection/transmission liquid crystal display device, a region where a display is performed by using transmitted light is referred to as a transmission region, and a region where a display is performed by using reflected light is referred to as a reflection region. The transmission region and the reflection region respectively include a transparent electrode region and a reflective electrode region formed on a substrate and a liquid crystal layer interposed between a pair of substrates. The transparent electrode region and the reflective electrode region on the substrate respectively define two-dimensional areas of the reflection region and the transmission region. The transparent electrode region is typically defined by a transparent electrode. The reflective electrode region is defined by a reflective electrode or a combination of the transparent electrode and the reflective electrode.
The liquid crystal display device of the present invention has a reflection region and a transmission region per pixel region. Thus, retardation of the liquid crystal layer can be optimized independently in the reflection region and the transmission region. More specifically, by prescribing the retardation of the liquid crystal layer in the reflection region to be those which (hatched regions (including double-hatched regions) in FIG. 5) are within curves represented by Formulae (1) and (2), Formulae (3) and (4), Formulae (5) and (6), and Formulae (7) and (8), and by prescribing the retardation of the liquid crystal layer in the transmission region to be those which (hatched regions (including double-hatched regions) in FIG. 6) are within curves represented by Formulae (9) and (10) and Formulae (11) and (12), the brightness (reflectivity) in the reflection region can be set to be about 70% or more, and the brightness (reflectivity) in the transmission region can be prescribed to be about 30% or more.
It is preferable that the conditions of the retardation are satisfied with respect to a central wavelength (high visibility) of visible light of about 550 nm. Furthermore, it is more preferable that the conditions of the retardation are satisfied in the entire wavelength range (about 400 nm to about 800 nm) of visible light.
Furthermore, since the twist angle xcex8t is in a range of about 0xc2x0 to about 90xc2x0, the same twist angle can be obtained in both the reflection region and the transmission region having different thickenesses of the liquid crystal layer by single rubbing treatment. In order to render the twist angle different between the reflection region and the transmission region, rubbing is required to be conducted separately for two regions, which complicates a production process.
Furthermore, by prescribing the retardation Rd in a region within the curves represented by Formulae (7) and (8) at the twist angle xcex8t of the reflection region in a range of 0xc2x0xe2x89xa6xcex8txe2x89xa654.3xc2x0, and in a region (double-hatched region in FIG. 5) within the curves represented by Formulae (5) and (8) at the twist angle xcex8t of the reflection region in a range of 54.3xc2x0xe2x89xa6xcex8txe2x89xa690xc2x0, and by prescribing the retardation Rd in a region (double-hatched region in FIG. 6) within the curves represented by Formulae (11) and (12) at the twist angle xcex8t of the transmission region in a range of 0xc2x0xe2x89xa6xcex8txe2x89xa690xc2x0, retardation of a liquid crystal layer in the reflection region and the transmission region becomes 0 in the presence of an applied voltage. If a black display is set to be performed at this time, a satisfactory black display is realized by applying the same voltage to the reflection region and the transmission region.
Furthermore, the above-mentioned condition corresponds to the case where a white region in which retardation is closest to 0 (i.e., the first peak from the lowest retardation side in FIGS. 7 and 8) is selected as a condition of realizing a white display. Thus, a gray-scale display is also satisfactorily performed. More specifically, in a gray-scale state in which a white display is changed to a black display, brightness (reflectivity and transmissivity) is monotonously decreased, so that a satisfactory gray-scale display is obtained. If a white display is performed by using the second peak from the lowest retardation side in FIGS. 7 and 8, the first peak is present in a region for a gray-scale display. Thus, a satisfactory gray-scale display cannot be performed.
When the liquid crystal layer in the transmission region and the reflection region are made of the same liquid crystal material, a structure and a production method will be simplified, compared with the case where the kind of a liquid crystal material is varied. It is effective to vary the thickness of the liquid crystal layer in the reflection region and the transmission region, so as to set different retardation in the reflection region and the transmission region. Furthermore, in order to match the length of an optical path with respect to light which contributes to a display in the reflection region with that in the transmission region, it is effective to prescribe the thickness of the liquid crystal layer in the transmission region to be larger than that in the reflection region. It is most preferable that the thickness of the liquid crystal layer in the transmission region is twice that in the reflection region.
If the first phase difference compensator has a first phase difference plate, the twist angle xcex8t of the liquid crystal layer is 0xc2x0, the retardation Rd of the reflection region is 90 nmxe2x89xa6Rdxe2x89xa6187 nm, the retardation Rd of the transmission region is 110 nmxe2x89xa6Rdxe2x89xa6440 nm, and the retardation Rd of the first phase difference plate is 30 nmxe2x89xa6Rdxe2x89xa6250 nm, a bright display of a normally white mode can be realized in the reflection region with a high contrast ratio.
If the first phase difference compensator has a second phase difference plate as well as the first phase difference plate, and the retardation Rd of the second phase difference plate is in a range of 220 nmxe2x89xa6Rdxe2x89xa6330 nm, wavelength characteristics in the reflection region can be alleviated, so that a display with a higher contrast can be obtained.
If the second phase difference compensator has a third phase difference plate, and the retardation Rd of the third phase difference plate is in a range of 120 nmxe2x89xa6Rdxe2x89xa6150 nm, a dark display is optimized even in the transmission region, so that a display with a higher contrast can be obtained.
If the second phase difference compensator has a fourth phase difference plate as well as the third phase difference plate, and the retardation Rd of the fourth phase difference plate is in a range of 240 nmxe2x89xa6Rdxe2x89xa6310 nm, wavelength characteristics of the transmission region are alleviated, so that a display with a higher contrast can be obtained.
Thus, the invention described herein makes possible the advantages of providing a liquid crystal display device which has outstanding mass-productivity and is capable of performing a satisfactory display irrespective of the brightness of ambient light.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.