Reflective liquid crystal display device having bright light display and high contrast

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a reflective liquid crystal display device having a bright light display and high contrast among the characteristics of the bright light display, and, a wide angle of view on the display plane in vertical and horizontal directions, and superior visual characteristics.

2. Description of the Related Art

In general, liquid crystal display devices are either a semi-transmissive or transmissive type having a backlight, or a reflective type. Reflective liquid crystal display devices produce a display without a backlight by using only ambient light, such as sunlight or artificial light, and are widely used in, for example, portable information terminals or the like that need a low profile, light weight, and low power consumption.

FIG. 10

is a schematic cross-sectional view showing the configuration of a well-known type of reflective liquid crystal display device. In particular, the device in this example is a passive-matrix STN (Super-Twisted Nematic) type.

In this reflective liquid crystal display device, a liquid crystal cell

72

for a reflective STN mode and a retardation film

73

are placed in that order on the surface of a lower polarizing plate

70

of a reflection plate

71

, and an upper polarizing plate

74

is further placed on the retardation film

73

.

In the liquid crystal cell

72

, 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

placed opposing the lower alignment layer

79

with a space therebetween, an upper transparent electrode layer

81

, and an upper glass substrate

82

are laid in that order on the surface of the lower polarizing plate

70

. An STN liquid crystal layer

83

is interposed between the lower and upper alignment layers

79

and

80

. An overcoat layer (not shown) made of silica or acrylic resin is provided between the color filter

76

and the lower transparent electrode layer

78

.

The retardation film

73

compensates for the phase difference of light passing through the STN liquid crystal, thereby preventing the display from taking on a blue or yellow tint.

The liquid crystal display device is generally required to have high display performance, such as having high resolution, high contrast, a bright screen, vivid colors, high visibility, and a wide angle of view.

When the conventional reflective liquid crystal display device is applied to a display section of a portable information terminal or the like, however, a high-contrast region of the display screen in the horizontal direction is large, whereas a high-contrast region in the vertical direction is small. Therefore, the angle of view on the display screen in the vertical direction is narrow, the dependence on the angle of view is great, and visual characteristics are unsatisfactory. Moreover, light display (white display) is darker than that of a transmissive liquid crystal display device equipped with a high-intensity backlight.

Accordingly, it has been suggested that the white display during the application of a selection voltage be brightened by removing the lower polarizing plate

70

placed between the liquid crystal cell

72

and the reflection plate

71

, and by using only the upper polarizing plate

74

on the retardation film

73

. In such a reflective liquid crystal display device, however, removing one of the polarizing plates not only brightens the light display, but also brightens the dark display (black display), which results in a loss of contrast.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a reflective liquid crystal display device that having a bright white display and high contrast among the characteristics the of bright white display, a wide angle of view on the display plane in vertical and horizontal directions, and excellent visual characteristics.

In order to achieve the above object, according to an aspect of the present invention, there is provided a reflective liquid crystal display device, wherein a transparent electrode and an alignment layer are placed in that order from the inner side the of transparent substrates on opposite sides of a liquid crystal layer in a liquid crystal cell of a normally black display type, a transparent electrode and an alignment layer are placed in that order from the inner side of the other transparent substrate, and two retardation films and a polarizing plate are placed in that order from the outer side of the other transparent substrate in the liquid crystal cell, wherein a retardation Δnd

LC

of the liquid crystal cell ranges from 705 nm to 890 nm, and wherein, when an alignment direction a of the alignment layer on the side of the other transparent substrate and an alignment direction b of the alignment layer on the side of one of the transparent substrates are viewed from the light incident side, and the direction between the alignment directions a and b that bisects the interior angle, which is formed by an intersection O of the alignment directions a and b, and the alignment directions a and b, is designated a normal direction X, a retardation Δnd

RF1

of the retardation film adjacent to the other transparent substrate ranges from 195 nm to 280 nm, an angle φ

RF1

formed by a delayed phase axis of the retardation film with respect to the normal direction X ranges from 75° to 115°, counterclockwise, as viewed from the light incident side, a retardation Δnd

RF2

of the retardation film adjacent to the polarizing plate ranges from 310 nm to 415 nm, an angle φ

RF2

formed by a delayed phase axis of the retardation film with respect to the normal direction X ranges from 130° to 180°, counterclockwise, as viewed from the light incident side, and an angle formed by an absorption axis of the polarizing plate with respect to the normal direction X ranges from 80° to 130° or 170° to 220°, counterclockwise, as viewed from the light incident side.

According to the reflective liquid crystal display device of the present invention, in the liquid crystal cell of the normally black display type, the transparent electrode and the alignment layer are placed in that order from the inner side of one of the transparent substrates on opposite sides of the liquid crystal layer, the transparent electrode and the alignment layer are placed in that order from the inner side of the other transparent substrate, and two retardation films and the polarizing plate are placed in that order from the outer side of the other transparent substrate of the liquid crystal cell. Therefore, only a single polarizing plate is used, which makes it possible to improve transmittance in a voltage applied state (when a selection voltage is applied), and to brighten the white display. As a result, the contrast is improved, and excellent display characteristics are obtained.

In particular, Δnd

LC

of the liquid crystal cell ranges from 705 nm to 890 nm, Δnd

RF1

of the retardation film adjacent to the other transparent substrate ranges from 195 nm to 280 nm, and the angle φ

RF1

formed by the delayed phase axis of the retardation film with respect to the normal direction X ranges from 75° to 115°, counterclockwise, as viewed from the light incident side. Δnd

RF2

of the retardation film adjacent to the polarizing plate ranges from 310 nm to 415 nm, and the angle φ

RF2

formed by the delayed phase axis of the retardation film with respect to the normal direction X ranges from 130° to 180° counterclockwise, as viewed from the light incident side. The angle φ

Pol

formed by the absorption axis of the polarizing plate with respect to the normal direction X ranges from 80° to 130° or 170° to 220°, counterclockwise, as viewed from the light incident side. The above settings make the white display brighter, and improves contrast.

In the reflective liquid crystal display device having the above-described configuration, it is preferable that a coefficient N

Z

, given by the following equation (1), of the retardation film adjacent to the other transparent substrate be within the range of −0.5 to 2.0 and that a coefficient N

Z

of the retardation film adjacent to the polarizing plate be within the range of −0.5 to 0.9′, because this ensures a large high-contrast region of the display screen in vertical and horizontal directions, and thereby permits a wide angle of view in the vertical and horizontal directions of the display screen and yields good visual characteristics:

N

z

=(

n

x

−n

z

)/(

n

x

−n

y

) (1)

where n

x

is the refractive index in the X-axis direction of the retardation film, n

y

is the refractive index in the Y-axis direction, and n

z

is the refractive index in the Z-axis direction.

In order to obtain higher contrast and better display characteristics, it is preferable to use a liquid crystal for the liquid crystal layer which has a wavelength dispersion characteristic of the birefringence Δn

LC

less than the wavelength dispersion characteristics of the birefringence Δn

RF1

of the retardation film adjacent to the other transparent substrate and the birefringence Δn

RF2

of the retardation film adjacent to the polarizing plate.

According to another aspect of the present invention, there is provided a reflective liquid crystal display device, wherein a transparent electrode and an alignment layer are placed in that order from the inner side of one of the transparent substrates on opposing sides of a liquid crystal layer in a liquid crystal cell of a normally white display type, a transparent electrode and an alignment layer are placed in that order from the inner side of the other transparent substrate, and two retardation films and a polarizing plate are placed in that order from the outer side of the other transparent substrate of the liquid crystal cell, wherein a retardation Δnd

LC

of the liquid crystal cell ranges from 705 nm to 1250 nm, and wherein, when an alignment direction a of the alignment layer on the side of the other transparent substrate and an alignment direction b of the alignment layer on the side of one of the transparent substrates are viewed from the light incident side, and the direction between the alignment directions a and b that bisects the interior angle, which is formed by an intersection O of the alignment directions a and b, and the alignment directions a and b, is designated a normal direction X, a retardation Δnd

RF1

of the retardation film adjacent to the other transparent substrate ranges from 115 nm to 250 nm, an angle φ

RF1

formed by a delayed phase axis of the retardation film with respect to the normal direction X ranges from 65° to 95°, counterclockwise, as viewed from the light incident side, a retardation Δnd

RF2

of the retardation film adjacent to the polarizing plate ranges from 255 nm to 365 nm, an angle formed by a delayed phase axis of the retardation film with respect to the normal direction X ranges from 105° to 145°, counterclockwise, as viewed from the light incident side, and an angle φ

Pol

formed by an absorption axis of the polarizing plate with respect to the normal direction X ranges from 60° to 95° or 150° to 185°, counterclockwise, as viewed from the light incident side.

According to the reflective liquid crystal display device of the present invention, in the liquid crystal cell of the normally white display type, the transparent electrode and the alignment layer are placed in that order from the inner side of one of the transparent substrates on opposite sides of the liquid crystal layer, the transparent electrode and the alignment layer are placed in that order from the inner side of the other transparent substrate, and two retardation films and the polarizing plate are placed in that order from the outer side of the other transparent substrate of the liquid crystal cell. Therefore, only a single polarizing plate is used, which makes it possible to improve transmittance in a no-voltage applied state (when a non-selection voltage is applied), and to brighten the white display. As a result, the contrast is improved, and excellent display characteristics are obtained.

In particular, Δnd

LC

of the liquid crystal cell ranges from 705 nm to 1250 nm, Δnd

RF1

of the retardation film adjacent to the other transparent substrate ranges from 115 nm to 250 nm, and the angle φ

RF1

formed by the delayed phase axis of the retardation film with respect to the normal direction X ranges from 65° to 95°, counterclockwise, as viewed from the light incident side. Δnd

RF2

of the retardation film adjacent to the polarizing plate ranges from 255 nm to 365 nm, and the angle φ

RF2

formed by the delayed phase axis of the retardation film with respect to the normal direction X ranges from 105° to 145° counterclockwise, as viewed from the light incident side. The angle φ

Pol

formed by the absorption axis of the polarizing plate with respect to the normal direction X ranges from 60° to 95° or 150° to 185°, counterclockwise, as viewed from the light incident side. The above settings make the white display brighter, and improve contrast.

Further objects, features, and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1

is a cross-sectional view showing a first embodiment in which effective liquid crystal display device of the present invention is applied to a STN reflective color liquid crystal display device.

FIG. 2

is an exploded perspective view showing the principal part of the reflective liquid crystal display device of the first embodiment.

FIG. 3

is plan view showing a preferable positional relationship an absorption axis (Pol) of a polarizing plate, RF

1

of a first retardation film (adjacent to an upper glass substrate), RF

2

of a second retardation film (adjacent to the polarizing plate), an alignment direction a of an upper alignment layer (on the side of the upper glass substrate), and an alignment direction b of a lower alignment layer (on the side of a lower glass substrate), in the reflective liquid crystal display device of the first embodiment.

FIG. 4

is an exploded perspective view showing the principal part of a reflective liquid crystal display device according to a second embodiment of the present invention.

FIG. 5

is a plan view showing a preferable positional relationship among an absorption axis (Pol) of a polarizing plate, RF

1

of a first retardation film (adjacent to an upper glass substrate), RF

2

of a second retardation film (adjacent to the polarizing plate), an alignment direction a of an upper alignment layer (on the side of the upper glass substrate), and an alignment direction b of a lower alignment layer (on the side of a lower glass substrate), in the reflective liquid crystal display device of the second embodiment.

FIG. 6

is a graph showing visual characteristics of the normally black type reflective liquid crystal display device (first embodiment).

FIG. 7

is a graph showing visual characteristics of a normally white type reflective liquid crystal display device (Comparative Example 1).

FIG. 8

is a graph showing the wavelength dispersion characteristic of Δn

LC

of liquid crystals A, B, and C and the wavelength dispersion characteristics of Δn

RF1

and Δn

RF2

of the first and second retardation films.

FIG. 9

is a graph showing spectral reflectance characteristics in a black display state of reflective liquid crystal display devices according to third, fourth and fifth embodiments.

FIG. 10

is a cross-sectional view schematically showing the confirmation of a conventional reflective liquid crystal display device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A reflective liquid crystal display device according to a first embodiment of the present invention will be described below in detail.

FIG. 1

is a cross-sectional view of the reflective liquid crystal display device of the first embodiment. The reflective liquid crystal display device of this embodiment generally comprises a liquid crystal cell

1

, a first retardation film

14

adjacent to an upper glass substrate

11

of the liquid crystal cell

1

, a polarizing plate

17

, a second retardation film

15

adjacent to the polarizing plate

17

, and a reflection plate

30

. The first retardation film

14

, the second retardation film

15

, and the polarizing plate

17

are placed in that order on the outer side of the upper glass substrate

11

, and the reflection plate

30

is disposed on the outer side of a lower glass substrate

12

of the liquid crystal cell

1

.

In the liquid crystal cell

1

, the upper and lower glass substrates

11

and

12

are opposed to each other across a liquid crystal layer

34

. From the inner side of the lower glass substrates

12

, a color filter

19

, a first overcoat

20

a

, a common electrode (transparent electrode)

23

, and a lower alignment layer

27

(on the side of the lower glass substrate

12

) are placed in that order. From the inner side of the upper glass substrate

11

, a segment electrode (transparent electrode)

24

, a topcoat

28

, and an upper alignment layer

26

(on the side of the upper glass substrate

11

) are placed in that order.

The liquid crystal cell

1

provided in the reflective liquid crystal display device of the first embodiment adopts a normally black display method.

The upper and lower alignment layers

26

and

27

are of a commonly used transparent type, and are formed, for example, by subjecting a high-polymer film of polyimide or the like to rubbing.

In this embodiment, when it is assumed that the counterclockwise direction as viewed from the light incident side is designated “+” and the clockwise direction is designated “−”, as shown in

FIG. 2

, an alignment direction (rubbing direction) a of the upper alignment layer

26

is set to be within the range of −35° to −25°, more preferably, to be −30°. Furthermore, an alignment direction (rubbing direction) b of the lower alignment layer

27

is set to be within the range of 25° to 35°, more preferably, to be 30°.

When the alignment direction a of the upper alignment layer

26

and the alignment direction b of the lower alignment layer

27

are viewed from the light incident side, as shown in

FIGS. 2 and 3

, the direction between the alignment directions a and b that bisects the interior angle, which is formed by an intersection O of the alignment directions a and b, and the alignment directions a and b, is designated a “normal direction X”.

In the figures, Z represents the direction that is orthogonal to the light incident surfaces of the liquid crystal cell

1

, the first and second retardation films

14

and

15

, and the polarizing plate

17

.

The liquid crystal layer

34

has a spiral structure twisted by 230° to 250° in the direction of thickness thereof, and includes liquid crystal molecules that are sealed in a region surrounded by the upper and lower alignment layers

26

and

27

disposed inside the upper and lower glass substrates

11

and

12

, and a sealing member (not shown) joining the alignment layers

26

and

27

at a predetermined distance, and that are in a nematic state at ordinary temperature. These liquid crystal molecules are of a super-twisted nematic (STN) type.

In order to obtain higher contrast and better display characteristics, it is preferable that the wavelength dispersion characteristic of the birefringence Δn

LC

of the liquid crystal in the liquid crystal layer

34

be less than the wavelength dispersion characteristic of Δn

RF1

of the first retardation film

14

and the wavelength dispersion characteristic of Δn

RF2

of the second retardation film

15

. The wavelength dispersion characteristic of Δn

LC

of the liquid crystal constituting the liquid crystal layer

34

can be altered by changing the liquid crystal material itself. Furthermore, the wavelength dispersion characteristics of Δn

RF1

and Δn

RF2

of the first and second retardation film

14

and

15

can be altered by changing the material of the retardation films.

The topcoat

28

is provided to ensure insulating properties, and is made of an inorganic material, such as silica or ZrO

2

.

Although the material of the upper glass substrate

11

varies depending on the type of the liquid crystal display device, soda lime glass or the like is employed in this embodiment. Although the thickness of the upper glass substrate

11

also varies depending on the type of the liquid crystal display device, a preferable thickness is within the range of 0.3 mm to 1.1 mm.

The first overcoat

20

a

is provided to smooth the uneven surface of the color filter

19

, and is made of an organic material that strongly adheres to the color filter

19

, e.g., polyvinyl alcohol or acrylic resin.

In the color filter

19

, pixels of the three primary colors, red, blue, and green, are formed in a desired pattern by photolithography, printing, or the like. The color filter

19

may also have a linear black matrix patterned around the pixels, and the adjoining pixels may partly overlap each other.

Although the material of the lower glass substrate

12

varies depending on the type of the liquid crystal display device, soda lime glass containing an oxide of an alkali metal, such as sodium, or the like is employed in this embodiment. A preferable thickness of the lower glass substrate

12

is within the range of 0.3 mm to 1.1 mm.

The reflection plate

30

serves to increase the angle of view by reflecting and diffusing incident light.

The reflection plate

30

in the reflective liquid crystal display device of the first embodiment is formed of a metal film of Al, Ag, or the like provided on the uneven surface of a glass substrate

35

disposed outside the lower glass substrate

12

.

Furthermore, the reflection plate

30

is bonded to the lower glass substrate

12

by a transparent bonding layer

36

made of an epoxy material containing fluorine.

The retardation Δnd

LC

, which is the product of the birefringence Δn

LC

and the thickness d of the above-described liquid crystal cell

1

, is set to be within the range of 705 nm to 890 nm (measured wavelength: 589 nm). When Δnd

LC

is out of the above range, the white display is darkened and the contrast decreases.

In order to obtain high. contrast and good monochrome display, it is generally preferable that Δnd

LC

be set to be within the range of 740 nm to 820 nm, more preferably, within the range of 760 nm to 800 nm.

The first and second retardation films

14

and

15

are formed of a uniaxially or biaxially-oriented film made of polyvinyl alcohol or polycarbonate, or the like. The direction of orientation serves as the delayed phase axis.

The retardation Δnd

RF1

of the first retardation film

14

is set within the range of 195 nm to 280 nm. As shown in

FIGS. 2 and 3

, the delayed phase axis RF

1

of the first retardation film

14

is set so that it forms an angle φ

RF1

of 75° to 115° with respect to the above-described normal direction X, counterclockwise, as viewed from the light incident side.

The retardation Δnd

RF2

of the second retardation film

15

is set within the range of 310 nm to 415 nm. As shown in

FIGS. 2 and 3

, the delayed phase axis RF

2

of the second retardation film

15

is set so that it forms an angle φ

RF2

of 130° to 180° with respect to the above-described normal direction X, counterclockwise, as viewed from the light incident side.

When Δnd

RF1

and Δnd

RF2

of the first and second retardation films

14

and

15

are out of the above ranges, it is impossible to obtain a high contrast or a bright white display.

When RF

1

and RF

2

of the first and second retardation films

14

and

15

are out of the above ranges, it is also impossible to obtain a high contrast or a bright white display.

In order to obtain high contrast and a good monochrome display, it is more preferable that RF

1

of the first retardation film

14

be set to form an angle φ

RF1

of 90° to 110° counterclockwise, as viewed from the light incident side, with respect to the above-described normal direction X.

For the same purpose, it is more preferable that RF

2

of the second retardation film

15

be set to form an angle φ

RF2

of 130° to 165°, counterclockwise, as viewed from the light incident side, with respect to the above-described normal direction X.

Furthermore, it is preferable that the first retardation film

14

have a coefficient N

z

ranging from −0.5 to 2.0 that is given by the following equation (1), and that the second retardation film

15

have a coefficient N

Z

ranging from −0.5 to 0.9, because this ensures a large high-contrast region of the display screen in vertical and horizontal directions, and thereby permits a wide angle of view in the vertical and horizontal directions of the display screen and good visual characteristics:

N

z

=(

n

x

−n

x

)/(

n

x

−n

y

) (1)

where n

x

is the refractive index in the X-axis direction of the retardation film, n

y

is the refractive index in the Y-axis direction, and n

z

is the refractive index in the Z-axis direction.

As shown in

FIGS. 2 and 3

, it is generally preferable that the absorption axis Pol of the polarizing plate

17

be set to form an angle φ

Pol

ranging from 80° to 130° or 170° to 220°, counterclockwise, as viewed from the light incident side, with respect to the normal direction X.

When Pol of the polarizing plate

17

is out of the above range, it is impossible to obtain a high-contrast or good monochrome display. In order to obtain a higher contrast and better monochrome display, it is more preferable that the angle φ

Pol

be set within the range of 80° to 110° or 170° to 220°.

In the reflective liquid crystal display device of the first embodiment having the above-described configuration, a single polarizing plate is used. This makes it possible to improve the transmittance in the voltage applied state, and to brighten the light display. As a result, the contrast is improved, and good display characteristics are obtained.

In particular, Δnd

LC

of the liquid crystal cell

1

, Δnd

RF1

and the angle φ

RF1

of the first retardation film

14

formed by RF

1

and the normal direction X, Δnd

RF2

and the angle φ

RF2

of the second retardation film

15

formed by RF

2

and the normal direction X, and the angle φ

Pol

formed by Pol of the polarizing plate

17

and the normal direction X, are set within the above ranges, respectively. Therefore, the white display becomes brighter, and the contrast is increased.

Furthermore, the first retardation film

14

has a coefficient N

Z

ranging from −0.5 to 2.0, and the second retardation film

15

has a coefficient N

Z

ranging from −0.5 to 0.9. Therefore, it is possible to obtain a large high-contrast region of the display screen in vertical and horizontal directions, a wide angle of view in the vertical and horizontal directions of the display screen, and good visual characteristics.

When the liquid crystal that forms the liquid crystal layer

34

has a wavelength dispersion characteristic of birefringence Δn

LC

thereof that is less than the wavelength dispersion characteristics of Δn

RF1

and Δn

RF2

of the first and second retardation films

14

and

15

, respectively it is possible to obtain improved contrast and good display characteristics.

While the topcoat

28

is interposed between the upper alignment layer

26

and the segment electrode

24

, the first overcoat

20

a

is interposed between the common electrode

23

and the color filter

19

, and the color filter

19

is placed inside the liquid crystal cell

1

in the above description of the reflective liquid crystal display device of the first embodiment, the topcoat

28

, the first overcoat

20

a

, and the color filter

19

need not always be provided, and may be appropriately provided according to the type of the reflective liquid crystal display device, and required characteristics.

While the color filter

19

is placed inside the liquid crystal cell

1

in the reflective liquid crystal display device of the first embodiment, it may also be placed outside the liquid crystal cell

1

.

While the reflection plate

30

in the first embodiment is of an exterior type that is placed outside the liquid crystal cell

1

, it be of an interior type placed inside the liquid crystal cell

1

.

The alignment directions a and b of the upper alignment layer

26

and the lower alignment layer

27

, respectively, are not limited to the above, and may by appropriately set according to the type of the reflective liquid crystal display device and required characteristics.

Next, a reflective liquid crystal display device according to a second embodiment of the present invention will be described.

The reflective liquid crystal display device of the second embodiment is different from that of the first embodiment in the following ways: A liquid crystal cell

1

adopts a normally white display method, Δnd

LC

of the liquid crystal cell

1

ranges from 705 nm to 1250 nm, as shown in

FIGS. 4 and 5

, Δnd

RF1

of a first retardation film

14

ranges from 115 nm to 250 nm, RF

1

of the first retardation film

14

forms an angle φ

RF1

of 65° to 95° with respect to the normal direction X, counterclockwise, as viewed from the light incident side, Δnd

RF2

of a second retardation film

15

ranges from 255 nm to 365 nm, RF

2

of the second retardation film

15

forms an angle φ

RF2

of 105° to 145° with respect to the normal direction X, counterclockwise, as viewed from the light incident side, and Pol of the polarizing plate

17

forms an angle φ

Pol

of 60° to 95° or 150° to 185° with respect to the normal direction X, counterclockwise, as viewed from the light incident side.

When Δnd

LC

of the liquid crystal cell

1

is out of the above range, the white display is darkened and the contrast decreases.

When Δnd

RF1

and Δnd

RF2

of the first and second retardation films

14

and

15

are out of the above ranges, it is impossible to obtain a high contrast or a bright white display.

When RF

1

and RF

2

of the first and second retardation films

14

and

15

are out of the above ranges, it is also impossible to obtain a high contrast or a bright white display.

In order to obtain a high-contrast and good monochrome display, it is more preferable that RF

1

of the first retardation film

14

form an angle φ

RF1

of 70° to 85° with respect to the normal direction X, counterclockwise, as viewed from the light incident side.

For the same purpose, it is more preferable that RF

2

of the second retardation film

15

form an angle φ

RF2

of 110° to 130° with respect to the normal direction X, counterclockwise, as viewed from the light incident side.

When Pol of the polarizing plate

17

is not set to be within the above range, it is impossible to obtain a high-contrast and good monochrome display. In order to obtain a higher-contrast and better monochrome display, it is more preferable that Pol of the polarizing plate

17

form an angle φ

Pol

of 65° to 85° or 155° to 175° with respect to the normal direction X, counterclockwise, as viewed from the light incident side.

The wavelength dispersion characteristic of birefringence Δn

LC

of the liquid crystal that forms a liquid crystal layer

34

in the second embodiment is not limited to the range described in the first embodiment. Furthermore, the coefficients N

z

of the first and second retardation films

14

and

15

, respectively, in the second embodiment are not limited to the ranges described in the first embodiment.

According to the reflective liquid crystal display device of the second embodiment having the above-described configuration, a single polarizing plate is used, which increases the transmittance the no-voltage applied state (when a non-selection voltage is applied), and brightens white display. As a result, the contrast increases, and good display characteristics are obtained.

In particular, Δnd

LC

of the liquid crystal cell

1

, Δnd

RF1

and the angle φ

RF1

formed by RF

1

of the first retardation film

14

and the normal direction X, Δnd

RF2

and the angle φ

RF2

formed by RF

2

of the second retardation film

15

and the normal direction X, and the angle φ

Pol

formed by Pol of the polarizing plate

17

and the normal direction X, are set within the above ranges, respectively. Therefore, the white display becomes brighter, and the contrast is increased.

The present invention will be described in more detail in conjunction with the following examples and comparative examples. However, the present invention is not limited to these examples.

EXPERIMENTAL EXAMPLE 1

Examinations were made of display characteristics of a reflective liquid crystal display device of a normally black display type having almost the same configuration of the reflective liquid crystal display device shown in

FIGS. 1

to

3

.

In this case, PSI-2501 (trade name; manufactured by Chisso Corp.) was used for upper and lower alignment layers constituting a liquid crystal cell, and was subjected to orientation so that the twist angle of the liquid crystal was 240°. The alignment direction a of the upper alignment layer was −30°, and the alignment direction of the lower alignment direction b was 30°. As the liquid crystal for a liquid crystal layer, AP-4268LA (trade name; manufactured by Chisso Sekiyu Kagaku Kabushiki Kaisha) was used. The liquid crystal cell did not include a color filter.

NRZ-420 (trade name; manufactured by Nitto Denko Corp., made of polycarbonate) was used as a first retardation film, NRZ-430 (trade name; manufactured by Nitto Denko Corp., made of polycarbonate) was used as a second retardation film, and NPF-EG1225DU (trade name; manufactured by Nitto Denko Corp.) was used as a polarizing plate.

Δnd

LC

of the liquid crystal cell (measured wavelength: 589 nm), Δnd

RF1

of the first retardation film, the angle φ

RF1

formed by RF

1

of the first retardation film with respect to the normal direction X, Δnd

RF2

of the second retardation film, the angle φ

RF2

formed by RF

2

of the second retardation film with respect to the normal direction X, and the angle φ

Pol

formed by Pol of the polarizing plate with respect to the normal direction X, were set as shown in the following Table 1 (Examples, Sample Nos. 1 to 14).

For comparison, Δnd

LC

of the liquid crystal cell (measured wavelength: 589 nm), Δnd

RF1

of the first retardation film, the angle φ

RF1

formed by RF

1

of the first retardation film with respect to the normal direction X, Δnd

RF2

of the second retardation film, the angle φ

RF2

formed by RF

2

of the second retardation film with respect to the normal direction X, and the angle φ

Pol

formed by Pol of the polarizing plate with respect to the normal direction X, were set as shown in the following Table 2 so that they are out of the ranges of the present invention (Comparative Examples, Sample Nos. 15 to 22).

Display characteristics of Samples Nos. 1 to 22 were examined in the following way. That is, the light source, the polarizing plate, the second retardation film, the first retardation film, the liquid crystal cell, and the reflection plate were placed in that order. Light was incident on the liquid crystal cell at an azimuth angle of 180° from the Z-direction shown in

FIG. 1

(in the direction of 0°, from directly above), and reflected light in the Z-direction was received at a receiving angle of 0°. Y (brightness) and CR (contrast) in this case were examined in a white display state (a voltage of 2.25 V was applied) in the normally black mode (N/B). The results are shown in the following Tables 1 and 2:

TABLE 1

(N/B)

Sam-

ple

Δnd

LC

φPol

Δnd

RF2

φRF

2

Δnd

RF1

φRF

1

Con-

No.

(nm)

(°)

(nm)

(°)

(nm)

(°)

Y

trast

1

720.7

104.5

315

156.5

195

99.0

34.6196

21.26

2

720.7

104.5

315

156.5

205

99.0

33.2533

38.07

3

720.7

104.5

310

156.5

215

99.0

30.1372

38.65

4

720.7

104.5

315

156.5

215

99.0

31.4705

44.55

5

782.3

77.5

355

132.0

230

83.0

31.2818

20.04

6

782.3

82.5

355

137.0

230

88.0

32.9755

48.28

7

782.3

88.5

355

143.0

230

94.0

34.9742

43.54

8

819.3

107.5

375

161.5

245

107.0

37.0051

27.44

9

819.3

122.5

365

172.0

245

107.5

36.1975

34.69

10

819.3

126.0

365

175.5

245

110.0

36.4970

29.43

11

819.3

129.5

365

179.0

245

114.5

36.8707

20.09

12

883.3

107.0

410

155.0

275

105.0

36.3191

23.29

13

883.3

107.0

410

155.0

280

105.0

36.4432

20.83

14

883.3

107.0

415

155.0

275

105.0

35.9418

21.55

TABLE 2

(N/B)

Sample

Δnd

LC

φPol

Δnd

RF2

φRF

2

Δnd

RF1

φRF

1

No.

(nm)

(°)

(nm)

(°)

(nm)

(°)

Y

Contrast

15

702.2

95.0

310

146.5

205

92.0

29.4453

110.50

16

720.7

104.5

315

156.5

190

99.0

35.1609

15.95

17

720.7

104.5

305

156.5

215

99.0

28.6149

25.79

18

782.3

73.5

355

128.0

230

79.0

30.0582

10.77

19

819.3

131.5

365

181.0

245

116.5

36.9544

16.47

20

883.3

107.0

410

155.0

285

105.0

36.4439

17.38

21

883.3

107.0

420

155.0

275

105.0

35.4455

17.54

22

895.7

123.0

420

172.0

280

104.0

34.8080

17.36

As the results in Tables 1 and 2 show, in some of Sample Nos. 15 to 22 (comparative examples), in which Δnd

LC

of the liquid crystal cell (measured wavelength: 589 nm), Δnd

RF1

of the first retardation film, the angle φ

RF1

formed by RF

1

of the first retardation film with respect to the normal direction X, Δnd

RF2

of the second retardation film, the angle φ

RF2

formed by RF

2

of the second retardation film with respect to the normal direction X, and the angle φ

Pol

formed by Pol of the polarizing plate with respect to the normal direction X, were out of the ranges of the present invention, the contrast is less than 20, or the Y-value in the white display state is less than 30, which yields a dark display. In contrast, Sample Nos. 1 to 14 as the examples of the present invention exhibited a contrast of more than 20, and a Y-value in the white display state that is greater than 30, which yields a bright display. This reveals that the white display is bright, the contrast is high, and display characteristics are good, compared with the comparative examples.

EXPERIMENTAL EXAMPLE 2

Examinations were made of display characteristics of a reflective liquid crystal display device of a normally white display type having almost the same configuration of the reflective liquid crystal display device shown in

FIGS. 1

,

4

, and

5

.

The materials for forming upper and lower alignment layers and a liquid crystal layer, which constitute a liquid crystal cell, and the alignment directions a and b of the upper and lower alignment layers were the same as those in the above Experimental Example 1. A first retardation film, a second retardation film, and a polarizing plate were also the same as those in the above Comparative Example 1.

Δnd

LC

of the liquid crystal cell (measured wavelength: 589 nm), Δnd

RF1

of the first retardation film, the angle φ

RF1

formed by RF

1

of the first retardation film with respect to the normal direction X, Δnd

RF2

of the second retardation film, the angle φ

RF2

formed by RF

2

of the second retardation film with respect to the normal direction X, and the angle φ

Pol

formed by Pol of the polarizing plate with respect to the normal direction X, were set as shown in the following Table 3 (Examples, Sample Nos. 31 to 42).

For comparison, Δnd

LC

of the liquid crystal cell (measured wavelength: 589 nm), Δnd

RF1

of the first retardation film, the angle φ

RF1

formed by RF

1

of the first retardation film with respect to the normal direction X, Δnd

RF2

of the second retardation film, the angle φ

RF2

formed by RF

2

of the second retardation film with respect to the normal direction X, and the angle φ

Pol

formed by Pol of the polarizing plate with respect to the normal direction X, were set as shown in the following Table 4 so that they are out of the ranges of the present invention (Comparative Examples, Sample Nos. 43 to 49).

Display characteristics of Sample Nos. 31 to 49 were examined in the following way. That is, the light source, the polarizing plate, the second retardation film, the first retardation film, the liquid crystal cell, and the reflection plate were placed in that order. Light was incident on the liquid crystal cell at an azimuth angle of 180° from the Z-direction shown in

FIG. 1

(in the direction of 0°, from directly above), and reflected light in the Z-direction was received at a receiving angle of 0°. Y (brightness) and CR (contrast) in this case were examined in a white display (a voltage of 2.10V was applied) in the normally white mode (N/W). The results are shown in the following Tables 3 and 4:

TABLE 3

(N/W)

Sample

Δnd

LC

φPol

Δnd

RF2

φRF

2

Δnd

RF1

φRF

1

No.

(nm)

(°)

(nm)

(°)

(nm)

(°)

Y

Contrast

31

709.8

66.5

255

111.5

115

69.5

31.3838

23.91

32

709.8

63.0

265

108.0

115

66.0

32.2205

22.25

33

709.8

66.5

265

111.5

115

69.5

30.0548

43.99

34

819.3

72.0

270

117.0

150

76.0

35.2273

129.91

35

906.9

71.5

295

116.5

160

74.5

35.1888

85.26

36

1009.5

73.0

310

118.0

180

76.5

32.3237

102.18

37

1199.5

89.0

340

134.0

230

87.5

31.7062

160.34

38

1199.5

92.0

340

137.0

230

90.5

30.0208

36.13

39

1201.2

81.5

350

126.5

230

83.0

32.1525

335.26

40

1201.2

81.5

350

126.5

250

83.0

34.0465

28.54

41

1201.2

81.5

365

126.5

230

83.0

30.2457

25.66

42

1214.5

86.5

340

132.0

225

86.0

32.9130

563.68

TABLE 4

(N/W)

Sam-

ple

Δnd

LC

φPol

Δnd

RF2

φRF

2

Δnd

RF1

φRF

1

Con-

No.

(nm)

(°)

(nm)

(°)

(nm)

(°)

Y

trast

43

702.2

72.0

250

117.0

130

75.0

29.2508

79.24

44

709.8

66.5

265

111.5

110

69.5

29.8705

42.51

45

709.8

66.5

250

111.5

115

69.5

32.0402

16.27

46

709.8

59.5

265

104.5

115

62.5

34.1090

10.57

47

1201.2

81.5

350

126.5

255

83.0

34.4865

19.27

48

1201.2

81.5

370

126.5

230

83.0

29.4519

36.41

49

1199.5

97.0

340

142.0

230

95.5

27.1540

13.18

As the results in Tables 3 and 4 show, in some of Sample Nos. 43 to 49 (comparative examples), in which Δnd

LC

of the liquid crystal cell (measured wavelength: 589 nm), Δnd

RF1

of the first retardation film, the angle φ

RF1

formed by RF

1

of the first retardation film with respect to the normal direction X, Δnd

RF2

of the second retardation film, the angle φ

RF2

formed by RF

2

of the second retardation film with respect to the normal direction X, and the angle φ

Pol

formed by Pol of the polarizing plate with respect to the normal direction X, were out of the ranges of the present invention, the contrast is less than 20, and the Y-value in the white display state is less than 30, which yields a dark display. In contrast, Sample Nos. 31 to 42 as the examples of the present invention provide a contrast higher than 20, and a Y-value in the white display state that is greater than 30, which yields a bright display. This reveals that the white display is bright, the contrast is high, and display characteristics are good, compared with the comparative examples.

EXPERIMENTAL EXAMPLE 3

Examinations were made of visual characteristics of the reflective liquid crystal display device (first embodiment) of a normally black display type having a configuration similar to that of the liquid crystal display device used in the above Experimental Example 1, except that Δnd

LC

of a liquid crystal cell was 800 nm (measured wavelength: 589 nm), Δnd

RF1

of a first retardation film was 235 nm, the angle φ

RF1

formed by RF

1

of the first retardation film with respect to the normal direction X was 99.5°, N

z

was 0.5, Δnd

RF2

of a second retardation film was 360 nm, the angle φ

RF2

formed by RF

2

of the second retardation film with respect to the normal direction X was 151°, N

z

was 0.1, and the angle φ

Pol

formed by Pol of the polarizing plate with respect to the normal direction X was 96.5°.

As the visual characteristics, the reflectance (%) in the direction of the polar angle was measured when the direction in which the black display on the display screen of the liquid crystal display device is viewed from the light incident side changes to 0°, 45°, 90°, and 135°. The results are shown in FIG.

6

. In

FIG. 6

, V

ns

indicates a non-selection state.

For comparison, examinations were made, in a similar manner, of visual characteristics of the reflective liquid crystal display device (Comparative Example 1) of a normally white display type having a configuration similar to that of the liquid crystal display device used in the above Experimental Example 2, except that Δnd

LC

of a liquid crystal cell was 800 nm (measured wavelength: 589 nm), Δnd

RF1

of a first retardation film was 155 nm, the angle φ

RF1

formed by RF

1

of the first retardation film with respect to the normal direction X was 78°, N

z

was −1.5, Δnd

RF2

of a second retardation film was 265 nm, the angle φ

RF2

formed by RF

2

of the second retardation film with respect to the normal direction X was 120°, N

z

was −1.5, and the angle φ

Pol

formed by Pol of the polarizing plate with respect to the normal direction X was 75°. The results are shown in FIG.

7

. In

FIG. 7

, V

s

indicates a selected state.

As the results in

FIGS. 6 and 7

show, in Comparative Example 1 of a normally white display type, in which Δnd

LC

of the liquid crystal cell (measured wavelength: 589 nm), Δnd

RF1

of the first retardation film, the angle φ

RF1

formed by RF

1

of the first retardation film with respect to the normal direction X, N

z

, Δnd

RF2

of the second retardation film, the angle φ

RF2

formed by RF

2

of the second retardation film with respect to the normal direction X, N

z

, and the angle φ

Pol

formed by Pol of the polarizing plate with respect to the normal direction X are out of the ranges of the present invention, the reflectance changes with the changes in viewing direction. That is, light leakage is large in the black display, and the contrast is low. In contrast, in the first embodiment, the reflectance hardly changes even when the viewing direction changes. That is, light leakage is not serious in the black display, the contrast is high, and the visual characteristics are good.

EXPERIMENTAL EXAMPLE 4

A reflective liquid crystal display device (third embodiment) of a normally black display type was produced, which had the same configuration as that of the liquid crystal display device used in the above Experimental Example 1, except that Δnd

LC

of a liquid crystal cell was 820 nm (measured wavelength: 589 nm), Δnd

RF1

of a first retardation film was 250 nm, the angle φ

RF1

formed by RF

1

of the first retardation film with respect to the normal direction X was 96.5°, Δnd

RF2

of a second retardation film was 375 nm, the angle φ

RF2

formed by RF

2

of the second retardation film with respect to the normal direction X was 160.5°, and the angle φ

Pol

formed by Pol of the polarizing plate with respect to the normal direction X was 115.5°, and that Δn

LC

of liquid crystal in the liquid crystal layer exhibited the wavelength dispersion characteristic shown by the curve A in FIG.

8

.

A reflective liquid crystal display device (fourth embodiment) of a normally black display type was produced, which had the same configuration as that of the liquid crystal display device used in the above Experimental Example 1, except that Δnd

LC

of the liquid crystal cell was 820 nm (measured wavelength: 589 nm), Δnd

RF1

of the first retardation film was 245 nm, the angle φ

RF1

formed by RF

1

of the first retardation film with respect to the normal direction X was 107°, Δnd

RF2

of the second retardation film was 375 nm, the angle φ

RF2

formed by RF

2

of the second retardation film with respect to the normal direction X was 161.5°, and the angle φ

Pol

formed by Pol of the polarizing plate with respect to the normal direction X was 107.5°, and that Δn

LC

of liquid crystal in the liquid crystal layer exhibited the wavelength dispersion characteristic shown by the curve B in FIG.

8

.

A reflective liquid crystal display device (fifth embodiment) of a normally black display type was produced, which had the same configuration as that of the liquid crystal display device used in the above Experimental Example 1, except that Δnd

LC

of the liquid crystal cell was 820 nm (measured wavelength: 589 nm), Δnd

RF1

of the first retardation film was 250 nm, the angle φ

RF1

formed by RF

1

of the first retardation film with respect to the normal direction X was 97°, Δnd

RF2

of the second retardation film was 375 nm, the angle φ

RF2

formed by RF

2

of the second retardation film with respect to the normal direction X was 157°, and the angle φ

Pol

formed by Pol of the polarizing plate with respect to the normal direction X was 109.5°, and that Δn

LC

of liquid crystal in the liquid crystal layer exhibited the wavelength dispersion characteristic shown by the curve C in FIG.

8

.

In the third to fifth embodiments, the duty ratio of the liquid crystal cell was 1:240, the bias ratio was 1:13, and the twist angle of liquid crystal was 240°.

FIG. 8

shows the wavelength dispersion characteristics of Δn

LC

of the liquid crystals A, B, and C, and the wavelength dispersion characteristics of Δn

RF1

and Δn

RF2

of first and second retardation films used herein. The vertical axis indicates Δn(λ)/Δn(550) that is obtained by dividing birefringences of the liquid crystal and the first and second retardation films with respect to wavelengths by a birefringence of 550 nm.

Examinations were made of spectral reflectance characteristics in a black display state of the produced reflective liquid crystal devices of the third to fifth embodiments. The examination results are shown in FIG.

9

.

As the results in

FIG. 8

show, the reflectance of the fifth embodiment including the liquid crystal C, which has a wavelength dispersion characteristic of Δn

LC

less than the wavelength dispersion characteristics of Δn

RF1

and Δn

RF2

of the first and second retardation films, is closer to 0 than those of the third and fourth embodiments including liquid crystals having wavelength dispersion characteristics of Δn

LC

greater than the wavelength dispersion characteristics of Δn

RF1

and Δn

RF2

of the first and second retardation films, which achieves a good black display state. This reveals that high contrast can be obtained by using a liquid crystal having a wavelength dispersion characteristic of Δn

LC

less than the wavelength dispersion characteristics of Δn

RF1

and Δn

RF2

of the first and second retardation films.

EXPERIMENTAL EXAMPLE 5

Reflective liquid crystal display devices of a normally black display type (Sample Nos. 50 to 58) were produced, which had the same configuration as that of the liquid crystal display device used in the above Experimental Example 1, except that Δnd

LC

of the liquid crystal cell was 828.1 nm (measured wavelength: 589 nm), Δnd

RF1

of the first retardation film was 250 nm, the angle φ

RF1

formed by RF

1

of the first retardation film with respect to the normal direction X was 97°, N

z

of the first retardation film was set as shown in the following Table 5, Δnd

RF2

of the second retardation film was 375 nm, the angle φ

RF2

formed by RF

2

of the second retardation film with respect to the normal direction X was 157.0°, N

z

of the second retardation film was set as shown in the following Table 5, and the angle φ

Pol

formed by Pol of the polarizing plate with respect to the normal direction X was 109.5°.

For comparison, reflective liquid crystal display devices of a normally black display type (Sample Nos. 59 to 62) were produced, which had the same configuration as that of the liquid crystal display device used in the above Experimental Example 1, except that Δnd

LC

of the liquid crystal cell was 828.1 nm (measured wavelength: 589 nm), Δnd

RF1

of a first retardation film was 250 nm, the angle φ

RF1

formed by RF

1

of the first retardation film with respect to the normal direction X was 97°, N

z

of the first retardation film was set as shown in the following Table 6, Δnd

RF2

of the second retardation film was 375 nm, the angle φ

RF2

formed by RF

2

of the second retardation film with respect to the normal direction X was 157.0°, N

z

of the second retardation film was set as shown in the following Table 6, and the angle φ

Pol

formed by Pol of the polarizing plate with respect to the normal direction X was 109.5°.

The display characteristics of the liquid crystal display devices, Sample Nos. 50 to 62 were examined in the following way.

That is, the light source, the polarizing plate, the second retardation film, the first retardation film, the liquid crystal cell, and the reflection plate were placed in that order. Light was incident on the liquid crystal cell at an azimuth angle of 180° from the Z-direction shown in

FIG. 1

(in the direction of −30°, from directly above), and reflected light in the Z-direction was received at a receiving angle of 30°. In this case, CR (contrast) in the direction of the polar angle was examined in a white display state in the normally black mode (N/B). The results are shown in the following Tables 5 and 6:

TABLE 5

N

z

Co-

N

z

Co-

Efficient

Efficient

Contrast

of Second

of First

Azi-

Azi-

Azi-

Azi-

Sample

Retarda-

Retarda-

muth

muth

muth

muth

No.

tion Film

tion Film

0°

45°

90°

135°

50

0.1

−0.5

5.7960

10.6173

3.0487

7.7776

51

0.1

−0.1

9.3514

9.7563

4.8023

8.5194

52

0.1

0.5

17.8986

8.3539

10.3848

9.0527

53

0.1

1.0

11.7226

7.2168

16.3350

8.9278

54

0.1

1.5

4.8534

6.1660

14.1036

8.3165

55

0.1

2.0

2.3603

5.2316

8.7300

7.4297

56

−0.5

0.5

18.7971

5.1006

3.2849

25.1979

57

0.5

0.5

5.9352

7.2998

20.3535

4.2921

58

0.9

0.5

2.1953

5.4898

21.4019

2.3008

TABLE 6

N

z

Co-

N

z

Co-

Efficient

Efficient

Contrast

of Second

of First

Azi-

Azi-

Azi-

Azi-

Sample

Retarda-

Retarda-

muth

muth

muth

muth

No.

tion Film

tion Film

0°

45°

90°

135°

59

0.1

−1.0

3.6695

11.4039

1.9131

6.6421

60

0.1

2.5

1.4153

4.3979

5.5070

6.5214

61

−0.1

0.5

7.8369

2.1473

1.4520

18.6115

62

1.0

0.5

1.7557

5.0945

19.5446

2.0019

The results shown in Tables 5 and 6 reveal that Samples Nos. 50 to 58, in which the values N

z

of the first and second retardation films are within the ranges of the present invention, provide a larger high-contrast region of the display plane in the vertical and horizontal directions, a wider angle of view of the display plane in the vertical and horizontal directions, and better visual characteristics, than those of Sample Nos. 59 to 62 in which the values N

z

of the first and second retardation films are out of the ranges of the present invention.

While the present invention has been described with reference to what are presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

Number	Name	Date
5528400	Arakawa	Jun 1996
5548426	Miyashita et al.	Aug 1996
5793455	Nakamura	Aug 1998
6115095	Suzuki et al.	Sep 2000
6141070	Kaneko	Oct 2000
6144431	Yamahara et al.	Nov 2000

Reflective liquid crystal display device having bright light display and high contrast

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

Agents

CPC

US Classifications

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US

International Classifications

Abstract

Description

Claims

Priority Claims (1)

US Referenced Citations (6)

Non-Patent Literature Citations (1)