Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-10-17
2004-02-03
Whitehead, Jr., Carl (Department: 2813)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S117000, C349S119000
Reexamination Certificate
active
06686983
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a reflection type liquid crystal display device which meets at least the performance requirements for a bright display with improved brightness and high contrast among the following performance requirements: a bright display with improved brightness and high contrast, a wide viewing angle in the vertical and horizontal directions on the display surface for excellent visual angle characteristics.
2. Description of Related Art
Generally speaking, liquid crystal display devices are classified into two types by display methods: a semi-transmission or transmission type with backlight and a reflection type. A reflection type liquid crystal display device is a liquid crystal display device which makes displays using only external light such as sunlight and illumination light without backlight. This type of liquid crystal display is widely used in devices which should be thin, lightweight and consume less power, such as personal digital assistants.
FIG. 5
is a sectional view which shows the general structure of a conventional typical reflection type liquid crystal display device. This is an example of a passive matrix STN liquid crystal display device.
This reflection type liquid crystal display device has a laminated structure consisting of a reflector
71
with a lower polarizer
70
, a reflective mode STN (Super-Twisted Nematic) liquid cell
72
placed on top of the lowerpolarizer
70
, a phase difference plate
73
placed on top of the cell
72
and an upper polarizer
74
placed on top of the plate
73
.
The liquid crystal cell
72
generally comprises the following layers which are laminated in order from the lower polarizer
70
side: a lower glass substrate
75
, a color filter
76
, a lower transparent electrode layer
78
, a lower alignment layer
79
, an upper alignment layer
80
which is facing the lower alignment layer
79
with an STN liquid crystal layer
83
between the layers
79
and
80
, an upper transparent electrode layer
81
and an upper glass substrate
82
. There is an overcoat layer (not shown in the figure) of silica or acrylic resin between the color filter
76
and lower transparent electrode layer
78
.
The above phase difference plate
73
compensates for phase difference in the light which passes through the STN liquid crystal to prevent the display from being bluish or yellowish.
Generally, the required display features of a liquid crystal display are excellence in the following aspects: (1) resolution, (2) contrast, (3) screen brightness, (4) color vividness, and (5) visibility (wide viewing angle, etc).
However, when a conventional reflection type liquid crystal display is used as the display of a personal digital assistant or similar device, on the display surface (screen), the area with good contrast in the horizontal direction is wide but the area with good contrast in the vertical direction is relatively narrow, which means high dependence on the viewing angle or poor visual angle characteristics. Also, the bright display (white display) is darker than in a transparent liquid crystal display with high luminance backlight.
One suggested approach as a solution to this problem is that the lower polarizer
70
located between the liquid crystal cell
72
and reflector
71
is omitted, namely only one polarizer (upper polarizer
74
located on the phase difference plate
73
) is used to make the white display brighter at the time of application of selection voltage. However, in this type of reflection type liquid crystal display device, not only the bright display but also the dark display (black display) are brighter, causing a deterioration in contrast.
SUMMARY OF THE INVENTION
In view of the above circumstances, the present invention provides a reflection type liquid crystal display device which meets at least the performance requirements for improved brightness in the bright display (white display) and high contrast among the following performance requirements: improved brightness in the bright display and high contrast, wide viewing angles in the vertical and horizontal directions on the display surface for excellent visual angle characteristics.
To solve the above problem, the present invention provides a reflection type liquid crystal display which is characterized as follows. The reflection type liquid crystal display has a liquid crystal cell in which a transparent electrode and an alignment layer are provided on an inner side of one of two transparent substrates facing each other with a liquid crystal layer between them in the order of mention from the one substrate side while a transparent electrode and an alignment layer are provided on an inner side of the other transparent substrate in the order of mention from the other substrate side, and two phase difference plates and a polarizer which are provided on an outer side of the other transparent substrate in order from the other substrate side,
wherein the liquid crystal layer is twisted by 240 to 250 degrees in its thickness direction and the crystal cell has a retardation (&Dgr;nd
LC
) of 600 nm to 800 nm; and
wherein, viewing the alignment direction
a
of the alignment layer on the other transparent substrate side and the alignment direction b of the alignment layer on the one transparent substrate from the light incidence side, when a direction of a normal X is assumed to be a direction which lies between the alignment directions a and b and passes through both the intersection O of the alignment directions
a
and b and a direction angled by one half of an interior angle formed by the alignment directions
a
and b,
the retardation (&Dgr;nd
RF1
) of the phase difference plate adjacent to the other transparent substrate is from 100 nm to 200 nm, and the lagging axis &bgr; of the phase difference plate forms an angle (&phgr;
RF2
) of 60 to 100 degrees in the counterclockwise direction as viewed from the light incidence side with respect to the normal direction X,
wherein the retardation (&Dgr;nd
RF2
) of the phase difference plate adjacent to the polarizer is from 300 nm to 500 nm and the lagging axis &ggr; of the phase difference plate forms an angle (&phgr;
RF2
) of 90 to 140 degrees in the counterclockwise direction as viewed from the light incidence side with respect to the normal direction X, and
wherein the angle (&phgr;
po1
) formed by an absorption axis &agr; of the polarizer with respect to the normal direction X is set at a value between 20 to 70 degrees or between 110 to 160 degrees in the counterclockwise direction from the light incidence side.
According to one aspect of the present invention, the reflection type liquid display device uses a single polarizer as it has a liquid crystal cell in which a transparent electrode and an alignment layer are provided on an inner side of one of two transparent substrates facing each other with a liquid crystal layer between them in the order of mention from the one substrate side while a transparent electrode and an alignment layer are provided on an inner side of the other transparent substrate in the order of mention from the other substrate side, and two phase difference plates and a polarizer which are provided on the outer side of the other transparent substrate in the order of mention from the other substrate side. With this structure, transmissivity is higher at the OFF time of voltage (when non-selection voltage is applied) and the bright display (white display) is brighter, which results in a higher contrast and excellent display characteristics.
Furthermore, the above liquid crystal layer is twisted by 240 degrees to 250 degrees in its thickness direction and the above crystal cell has a retardation (&Dgr;nd
LC
) of 600 nm to 800 nm;
when, viewing the alignment direction
a
of the alignment layer on the other transparent substrate side and the alignment direction b of the alignment layer on the one transparent substrate from the light incidence side, a direction of a normal X is assumed to be a direction which lies between the alignm
Kano Mitsuru
Ohizumi Mitsuo
Tanada Tetsushi
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Jr. Carl Whitehead
Pham Thanhha
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