Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-06-02
2002-08-06
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S113000, C349S117000
Reexamination Certificate
active
06429920
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This present invention relates to a reflecting type liquid crystal display device, inclusive of a translucent type, for carrying out display by reflecting ambient light or front light. More particularly, the invention relates to an STN (super-twist nematic) type liquid crystal display device.
2. Description of the Related Art
Technical features required of portable terminal apparatuses are low power consumption, light weight and compactness in size. Since a liquid crystal display device can be driven at a relatively low voltage, the liquid crystal display device can reduce electric power consumption. In particular, since a reflecting type liquid crystal display device utilizes ambient light, it does not need back-light and therefore realizes light weight and compactness in size easily. The reflecting type liquid crystal display device suitable for portable terminal devices can be divided into two types, i.e., one type using two polarizing plates and another type using one polarizing plate.
In the type using two polarizing plates, a polarizing plate provided with a reflecting plate is bonded to a surface of either one of a pair of light transmitting substrates, which surface is opposite to a liquid crystal layer which is sandwiched between the light transmitting substrates. On the other hand, light incident from the other substrate side passes through the other substrate, the liquid crystal layer and the one substrate in this order, are reflected by the polarizing plate provided with the reflecting plate, and outgo as the light passes again through the one substrate, the liquid crystal layer and the other substrate in this order. In this instance, the image becomes double due to parallax resulting from the thickness of the substrate, and visibility drops markedly. In a reflecting type color liquid crystal display device using micro-color filters, for example, the light passes through the color filters of different colors between the incoming route and the outgoing route due to the parallax, and color purity drops markedly.
In the type using one polarizing plate, the reflecting plate can be disposed inside the liquid crystal cell. Therefore, parallax resulting from the thickness of the substrates described above does not occur. The drop of visibility resulting from doubling of the image and the drop of color resulting from color mixture do not occur, either. Therefore, the type using one polarizing plate has become predominant in the reflecting type liquid crystal display devices. In the reflecting type liquid crystal display device which uses one polarizing plate, incident ambient light is converted to linearly polarized light by a polarizing plate
2
, then to elliptically polarized light by phase difference plates
3
and
4
, are modulated by an STN type liquid crystal layer
9
and are reflected by a reflecting layer
15
, as shown in FIG.
1
. The travel of reflected light is the reverse of incident light, namely, reflected light outgoes after being modulated , and is observed.
It is important in the liquid crystal display device to greatly modulate an intensity ratio of outgoing light in the full wavelength range of visible light by an applied voltage to a liquid crystal layer. In general, when reflected light is employed to display in white, a birefringence phase difference between reflecting means and a polarizing plate is set to &lgr; or 2/&lgr; and reflected light is converted to linearly polarized light by the reflecting means. And when reflected light is used for display in black, the birefringence phase difference between the reflecting means and the polarizing plate is set to &lgr;/4 or 3 &lgr;/4 and reflected light is converted to circularly polarized light by the reflecting means. Therefore, when a voltage is applied to the liquid crystal layer and the phase difference is modulated to &lgr;, or &lgr;/2 to &lgr;/4, or 3 &lgr;/4, in the full wavelength range, achromatic display can be accomplished with high contrast and high lightness.
However, the display operation of the STN type liquid crystal display device is carried out by virtue of birefringence and rotatory polarization of light. Moreover, the birefringence phase difference involves wavelength dispersion. For these reasons, it is difficult to obtain circularly polarized light and linearly polarized light with high accuracy in a visible light range of 400 nm to 800 nm. In the case of circularly polarized light, in particular, it is necessary to set not only the phase difference to &lgr;/4 or 3 &lgr;/4 but also to set the ratio of the major axis to the minor axis of an outgoing ellipse to 1:1. In other words, the angle of the absorption axis of the polarizing plate, &Dgr;n.d and the angle of the slow axis of each phase difference plate, &Dgr;n.d of the liquid crystal layer and the twist angle, must be optimized. Examples of them are disclosed in, for example, Japanese Unexamined Patent Publication JP-A 10-161110 (1998) and JP-A 10-170906 (1998).
According to the optical arrangement of the reflecting type STN type liquid crystal display devices using one polarizing plate, inclusive of the prior art technologies disclosed in JP-A 10-161110 and JP-A 10-170906, which have been proposed in the past, optimization of optical compensation is not complete. Therefore, optimization of the contrast, lightness and tone has not yet been achieved with good balance.
A mode for obtaining dark display under the state where liquid crystal molecules are aligned substantially parallel to the substrate surface, that is, NB (Normally Black) mode for obtaining black display without application of a voltage, can more easily compensate for dark display, can obtain more easily satisfactory dark display, and can therefore obtain more easily higher contrast than a mode for obtaining dark display while the liquid crystal molecules are kept upright from the substrate surface, that is, NW (Normally White) mode for obtaining white display without application of the voltage.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an STN reflecting type liquid crystal display device using one polarizing plate, which allows to obtain achromatic display of high contrast and high lightness, and provide excellent visibility.
The invention provides a reflecting type liquid crystal display device of NB mode, comprising an STN type liquid crystal cell; a first phase difference plate, a second phase difference plate and a polarizing plate, the first phase difference plate, second phase difference plate and polarizing plate being disposed in this order on one surface of the STN type liquid crystal cell; and reflecting means disposed on the other surface of the STN type liquid crystal cell, wherein a product &Dgr;nLC.dLC of birefringence &Dgr;nLC and thickness dLC of a liquid crystal layer of the liquid crystal cell is selected from a range of 660 nm to 830 nm, a product &Dgr;n1.d1 of birefringence &Dgr;n1 and thickness d1 of the first phase difference plate is selected from a range of 120 nm to 240 nm, and a produce &Dgr;n2.d2 of birefringence &Dgr;n2 and thickness d2 of the second phase difference plate is selected from a range of 300 nm to 430 nm, and wherein in the case where a twist direction of a major axis of liquid crystal molecules of the liquid crystal layer of the liquid crystal cell from the reflecting means side to the first phase difference plate side is assumed to be a positive direction, a twist angle &khgr; of liquid crystal molecules from a major axis of the reflecting means side liquid crystal molecules of the liquid crystal layer of the liquid crystal cell to a major axis of the first phase difference plate side liquid crystal molecules of the liquid crystal layer of the liquid crystal cell is selected from a range of 220° to 260°, and angle &thgr; from a major axis of the first phase difference plate side liquid crystal molecules of the liquid crystal layer of the liquid crystal cell to a slow axis of the first phase difference plate is selected from a range of &mi
Conlin David G.
Dike, Bronstein, Roberts & Cushman, IP Practice Group of Edwards
Nguyen Dung
Sharp Kabushiki Kaisha
Sikes William L.
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