Liquid crystal cells – elements and systems – Particular structure – Particular illumination
Reexamination Certificate
2002-08-20
2003-12-02
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Particular illumination
C349S113000
Reexamination Certificate
active
06657684
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a liquid crystal display device and an electronic apparatus, and more particularly relates to a reflective liquid crystal display device having an illumination device (a front light) and a directional reflector.
2. Description of Related Art
Reflective liquid crystal display devices have low power consumption since they do not have light sources, such as back lights, and conventionally, have frequently been used for, for example, accessory display portions for various portable electronic apparatuses and devices. The reflective liquid crystal display devices, however, utilize external light, such as natural light or illumination light to perform display, thus having a problem in that visibility of the display is reduced at a location where the amount of external light is limited. Thus, a liquid crystal display device having an illumination device (front light) at the front surface of a reflective liquid crystal cell has been proposed. The front light serves to introduce light from a light source, which is arranged adjacent to one end surface of a light guide plate provided at the front surface of the liquid crystal cell, into the light guide plate. The light introduced therein is then radiated from a plate surface of the light guide plate toward the liquid crystal cell. The liquid crystal display device having the front light allows the liquid crystal cell to be viewed through the transparent light guide plate at a bright location. Thus, at a bright location, the liquid crystal display device can be used as a typical reflective liquid crystal display device, and at a dark location, the front light is turned on to illuminate the liquid crystal cell, thereby allowing the display to be viewed.
FIG. 10
shows an example of a conventional front light. A front light
100
of this example includes a light source
101
, a light guide plate
102
for transmitting light from the light source
101
in the rightward direction of
FIG. 10
, and a reflector
103
arranged so as to surround the light source
101
. A plurality of projections
105
are formed on the upper surface of the light guide plate
102
at regular intervals, each projection
105
having a triangular cross-section and having a gentle-slope portion
104
a
with a gentle oblique angle and a steep-slope portion
104
b
with oblique angle steeper than the gentle-slope portion
104
a
. The projections
105
are formed in a striped pattern so as to extend in a direction perpendicular to the surface of the paper of
FIG. 10
, and serve to change the reflection direction of light propagating within the light guide plate
102
and to emit the light from a plate surface of the light guide plate
102
to the outside thereof. That is, in the front light
100
, when light emitted from the light source
101
is introduced into the light guide plate
102
from a side surface thereof, the light propagates while repeating total reflection within the light guide plate
102
. Meanwhile, light that was incident on the gentle-slope portion
104
a
of each projection
105
and that was totally reflected is emitted from the lower surface of the light guide plate
102
as an illumination light.
Meanwhile, in a reflective liquid crystal display device, a technology for providing a bright display within a certain degree of a viewing angle by forming a large number of projections or depressions on a surface of a reflective film to thereby scatter reflected light has been conventionally adopted. However, from the viewpoint of effectively utilizing reflected light in an environment where reflected light is limited and presenting a bright display to a user's eyes, in lieu of a conventional technology in which projections or depressions each having a symmetric cross-section are formed to thereby scatter light isotropically, a technology for a so-called directional reflector has been proposed. In the technology, for example, projections each having an asymmetric cross-section, i.e., projections each having a saw-toothed cross-section and having a gentle-slope with a gentle oblique angle and a steep-slope with a steep oblique angle are formed, to impart anisotropy (hereinafter referred to as “directivity”) to light scattering, thereby orienting reflected light to the user's eyes.
FIG. 11
is a view showing the effect of a directional reflector. For example, as shown in the figure, in a reflector
200
on which a large number of projections
201
each having a triangular cross-section are formed, the surfaces of the projections
201
serve as reflective surfaces. Designing the cross-sectional shape of each projection to have an asymmetric saw-tooth rather than to have an isosceles triangle can make the area of a gently-inclined reflective surface
202
a
larger enough than the area of a steeply-inclined reflective surface
202
b
. With an liquid crystal display device incorporating the reflector
200
, in a typical operating environment, external light, such as, sunlight or illumination light, is incident on the screen from the upper direction (the direction of arrow A
1
in
FIG. 11
) of the screen, and a user commonly views the screen from a generally frontward direction (the direction of arrow A
2
in
FIG. 11
) of the screen. Thus, the external light from the upper direction of the screen is reflected on the gentle-slope surfaces of the reflective layer, thus increasing the amount of reflected light especially in the frontward direction of the panel. As a result, the user can view a bright image.
SUMMARY OF THE INVENTION
The conventional reflective liquid crystal display device described above has the front light and the directional reflector and is thus superior in that, in theory, it can provide a bright image regardless of an operating environment. However, in practice, when a user views the screen, there are some problems in that, for example, uneven brightness is generated depending on a location or moirófringes are viewed. Thus, the conventional liquid crystal display device cannot necessarily be said to have good display quality.
The present invention has been made to overcome the foregoing problems, and an object thereof is to provide a reflective liquid crystal display device that can provide a bright image regardless of an operating environment without generating, for example, location-dependent uneven-brightness or moirófringes.
To achieve the above object, a reflective liquid crystal display device of the present invention includes a liquid crystal cell in which liquid crystal is sandwiched between a pair of substrates consisting of an upper substrate and a lower substrate which are arranged so as to oppose each other, an illumination device which is provided at the upper surface side of the liquid crystal cell and which has a light source and a light guide plate, and a reflective layer provided below the liquid crystal of the liquid crystal cell. A plurality of projections or depressions are provided at the light guide plate of the illumination device so as to extend in one direction and a plurality of projections or depressions are provided at the reflective layer so as to extend in one direction. The extending direction of the projections or depressions of the light guide plate and the extending direction of the projections or depressions of the reflective layer are not parallel to each other in plan view. The “projections or depressions of the light guide plate” herein serve to change the reflection direction of light propagating within the light guide plate so that reflected light is emitted from a plate surface of the light guide plate to the outside thereof. The “projections or depressions of the reflective layer” serve to constitute reflective surfaces that impart directivity.
The present inventors investigated the cause of generation of location-dependent uneven brightness or moirófringes in a liquid crystal display device having a front light and a directional reflector. As a result, we found that, in the conventional configuration, sinc
Kamijo Kimitaka
Maeda Tsuyoshi
Oliff & Berridg,e PLC
Seiko Epson Corporation
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