Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-05-24
2004-06-22
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S117000, C349S119000, C349S123000, C349S130000
Reexamination Certificate
active
06753939
ABSTRACT:
RELATED APPLICATION DATA
The present application claims priority to Japanese Application(s) No(s). P2001-158527 filed May 28, 2001, which application(s) is/are incorporated herein by reference to the extent permitted by law.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device widely used in, for example, a notebook-size personal computer, a portable terminal, a portable video tape recorder (VTR), or a digital still camera. The present invention is one related to a liquid crystal display device including a reflection function and a transmission function among the different types of liquid crystal display devices. When the liquid crystal display device has a reflection function and a transmission function, it has excellent visibility both indoors and outdoors. Accordingly, such a liquid crystal display device is suitable for use in, for example, a portable terminal, a portable VTR, or a digital still camera.
2. Description of the Related Art
Various types of flat displays where the content of display can be rewritten by modulating light using an electrical signal have been developed and produced. Self-luminous display devices which have been researched and developed a great deal in recent years include OLED (organic light emitting diode), a plasma display, and an FED (field emission display).
A self-luminous display has excellent visibility indoors compared to a liquid crystal display because the self-luminous display is brighter than the liquid crystal display. However, since the outdoor environment is often brighter than a self-luminous display disposed outdoors, the visibility of the self-luminous display device becomes considerably poor in such an environment.
In addition, in a self-luminous display, electrical current and electrical voltage required to drive it are relatively large, so that electrical power consumption thereof tends to become large. When electrical power consumption becomes large, the display device is not suitable for use in, for example, a portable terminal of a digital camera, a portable VTR, a cellular phone, or the like.
On the other hand, a reflective liquid crystal display device uses external light rather than light generated by itself for a displaying operation, so that its visibility becomes good due to increased brightness, rather than becoming poorer even when the reflective liquid crystal display device is used outdoors where there is sufficient brightness. In addition, since, unlike a transmissive liquid crystal display, a reflective liquid crystal display does not need a backlight, electrical power consumption thereof is small, so that it is most suitable for use in small portable devices.
However, although a reflective liquid crystal display device has excellent visibility in a bright outdoor environment, its visibility obviously becomes poor when it is used in a dark environment at night because it does not generate light. This occurs naturally in a reflective liquid crystal display because the brightness of the surrounding environment is directly reflected in the brightness of the display.
Accordingly, a display device of, for example, a portable terminal needs to provide excellent visibility in any environment, both indoors and outdoors. One type of display device for achieving this object is a transmissive-and-reflective liquid crystal display device. In general, the transmissive-and-reflective liquid crystal display device is realized in the following two ways.
One method is disclosed in Japanese Unexamined Patent Application Publication No. 59-218483. In this method, a transmission/reflection mode is provided by disposing a transflective film, which is a thin metallic film, between a backlight and a liquid crystal layer. However, in principle, in this transmission/reflection mode, the transmission mode and the reflection mode cannot be optimized at the same time. More specifically, since the same liquid crystal layer is used in the transmission mode and the reflection mode, when optical designing is carried out by giving precedence to the transmission mode, the visibility in the reflection mode is reduced, whereas, when optical designing is carried out by giving precedence to the reflection mode, optical characteristics during transmission of light become poor.
The other method is disclosed in Japanese Unexamined Patent Application Publication No. 11-242226. In this method, a reflection/transmission mode is achieved by separating a reflection section and a transmission section within a pixel area. The orientation states of liquid crystals of the transmission section and the reflection section are made different, so that thought is put in obtaining good optical characteristics during reflection and transmission of light.
More specifically, optimization is achieved by changing the thicknesses of the liquid crystals at the reflection section and at the transmission section within a pixel. In other words, by setting the phase differences of wavelengths in the visible wavelength region when the voltage is turned on and when it is turned off at &lgr;/2 in the transmission section and at &lgr;/4 in the reflection section, a high reflection ratio, a high transmission ratio, and high contrast are achieved at both the reflection section and the transmission section. That is, the thickness of the liquid crystals at the transmission section is twice the thickness of the liquid crystals at the reflection section.
Japanese Patent Application Nos. 9-359036 and 10-364247 introduce as liquid crystal modes, such as: (1) a guest host mode in which doping of a dichroic coloring material is carried out, (2) a twist orientation mode, and (3) a homogeneous orientation mode. Emphasis is put in making the thickness of the portion of the liquid crystal layer at the transmission section and that of the portion of the liquid crystal layer at the reflection region different from each other.
Here, by changing the thickness of the portion of the liquid crystal layer at the transmission region and the thickness of the portion of the liquid crystal layer at the reflection region, panels in the liquid crystal modes (1), (2), and (3) were experimentally produced and inspected. In particular, the mode (3) was studied at great length because it was closest to being used for practical purposes.
A reflective-and-transmissive liquid crystal device in which the guest host mode (mode (1)) was used was produced and evaluated. The result of the evaluation makes it possible to confirm that the reflection ratio and the transmission ratio are high because a polarizing plate is not used. However, it has been found that contrast is not sufficient because the black level cannot be made sufficiently low due to insufficient ratio between the two colors of the dichroic coloring material.
The twist orientation mode (mode (2)) was used in a reflective-and-transmissive liquid crystal display device in order to produce and evaluate a panel. The result of the evaluation showed that, when a twist orientation process is carried out, it is difficult to control the orientation of the liquid crystals at the boundary between the transmission region and the reflection region.
An evaluation in the homogeneous mode (mode (3)) was carried out. In the homogeneous mode, liquid crystal molecules are orientated horizontal (parallel) to a substrate. The directions of orientation of the liquid crystals within a plane are often controlled to one direction by rubbing or the like. The vertical rubbing directions are anti-parallel directions. In the case where a horizontal mode is used, when there is a step at the transmission section and the reflection section, there is the advantage that retardation can be precisely obtained in proportion to the difference in the level. More specifically, if the thickness of the portion of the liquid crystal layer at the reflection section is made half that of the portion of the liquid crystal layer at the transmission section, the difference in retardation is halved. In the horizontal mode, liquid crystal mat
Jisaki Makoto
Kida Yoshitoshi
Yamaguchi Hidemasa
Chowdhury Tarifur R.
Sonnenschein Nath & Rosenthal LLP
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