Liquid crystal cells – elements and systems – Particular structure – Particular illumination
Reexamination Certificate
2002-04-26
2004-06-01
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Particular illumination
C349S064000
Reexamination Certificate
active
06744480
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device, and particularly relates to a transflective liquid crystal display device capable of a display in a reflection mode utilizing external light and a display in a transmission mode in which backlight is turned on.
2. Description of the Related Art
Following the widespread use of cellular phones and personal digital assistants, portable information processing devices are on the market in volume. While reduction in size and weight is underway, high performance and color display are in demand in these portable information processing devices. In the display devices of these portable information processing devices, transflective liquid crystal display devices, which is capable of a reflection mode display by external light with less power consumption and a transmission mode display by backlight to improve visibility during night and the like, are mainly used.
In a conventional transflective liquid crystal display device, a polarizer and a transflective reflector are placed at a side opposite to a visible side of a liquid crystal cell. However, this transflective reflector uses a semitransparent plastic film or a thin metal film of half mirror, and its reflectivity R is expressed by
Reflectivity R=100−(transmittance T+light absorption ratio A) [%],
and even if a material with a very low light absorption ratio A is used, the reflectivity R becomes low when the transmittance T is made high, and one of the transmission mode display and the reflection mode display becomes a dark display.
Thus, for example, as disclosed in JP, 2000-330107, A, a liquid crystal display device, which includes a scattering layer with a polarization maintaining property, a light guide film formed with a special projection and depression form on a top face thereof and a reflector on a side opposite to a visible side of a liquid crystal cell including a pair of polarizers, is proposed, and according to this, a bright reflection display is made possible though the transmittance is very high.
The conventional liquid crystal display device is constituted of an upper polarizer
110
, a liquid crystal cell
100
, a lower polarizer
120
, a scattering layer
90
, a transparent light guide film
30
and a light source
40
, and a reflector
70
, as shown in FIG.
13
. Prismatic projections and depressions constituted by slant faces
30
b
each with an inclination angle of 35 to 40 degrees and flat faces
30
a
each with an inclination angle of 10 degrees or less are formed on a top face of the light guide film
30
. The light guide film
30
and the light source
40
constitute a backlight.
In this liquid crystal device, when the light source
40
is turned on, incident light I
1
to the light guide film
30
from the light source
40
is reflected downward by the slant face
30
b
and is reflected again by the reflector
70
, it is transmitted through the flat face
30
a
, further transmitted though the scattering layer
90
, the lower polarizer
120
, the liquid crystal cell
100
and the upper polarizer
110
to be emitted light O
1
and emitted to the visible side (the upper side in FIG.
13
). Since no transflective films such as half mirrors are not provided in an emission route of the light from the above-described light source
40
, in this liquid crystal display device, a bright display in the transmission mode can be obtained as in an ordinary transmission type liquid crystal display device.
Meanwhile, in the case of the reflection mode display by external light, incident light I
2
from the visible side is transmitted through the upper polarizer
110
, the liquid crystal cell
100
, the lower polarizer
120
and the scattering layer
90
, and reaches the light guide film
30
. Further, the light which reaches the flat face
30
a
of the light guide film
30
travels in a straight line inside the light guide film
30
, reflected by the reflector
70
, emitted from the flat face
30
a
of the light guide film
30
, transmitted through the scattering layer
90
, the lower polarizer
120
, the liquid crystal cell
100
and the upper polarizer
110
, and emitted to the visible side as emitted light O
2
. In the case in which a mirror tuning reflector such as a thin metal film is used for the reflector
70
, the area other than that in the specular reflection direction becomes dark, and therefore the scattering layer
90
is provided.
Here, if the scattering layer
90
has a high degree of depolarization, namely, if it has the property of changing the polarizing state of the incident light to a large extent, the incident light I
2
and the reflected light which reaches the lower polarizer
120
have different polarizing states, and the amount of light transmitted through the lower polarizer
120
, namely, the amount of light of the emitted light O
2
decreases, thus making the display dark. However, by using the scattering layer
90
having a polarization maintaining property, a bright display in the reflection mode is made possible.
However, it is necessary to design the light guide film
30
for each liquid crystal cell because a pitch of the prismatic projections and depressions on the surface of the light guide film
30
and a pixel pitch of the liquid crystal cell
100
interfere with each other to cause moire, thus causing a disadvantage of increasing the cost.
If a prism sheet is placed between the light guide film
30
and the scattering layer
90
for the purpose of increasing transmission luminance, polarization of the light reflected by the reflector
70
is depolarized when it passes through the prism sheet in the case of the reflection mode, and a part of the reflected light is absorbed in the lower polarizer
120
and is not emitted to the visible side, thus causing the disadvantage that the display in the reflection mode becomes dark.
Next, another prior art by which the same effect can be obtained without using the light guide film in a complicated shape as in the above prior art will be explained. This is, for example, a liquid crystal display device disclosed in JP, 2001-83502, A, and a schematic configuration thereof is shown in FIG.
14
. In
FIG. 14
, the parts corresponding to those in
FIG. 13
are given the same reference numerals and symbols.
As a light guide film composing backlight with the light source
40
by LED, this liquid crystal display device comprises a transparent light guide film
50
, which is formed to have fixed thickness by a transparent resin such as acrylic resin, polycarbonate resin, and amorphous polyolefin resin, an inorganic transparent material such as glass, or a composite of them, and which has a number of small projections (omitted in the drawing) formed on its surface.
It also comprises a reflection type polarizer
60
having a reflection axis and a transmission axis between the light guide film
50
and the scattering layer
90
.
In this reflection type polarizer
60
, the reflection axis and the transmission axis intersect each other orthogonally, and the transmission axis is placed to substantially correspond to a transmission axis of the lower polarizer
120
, whereby a bright display in the reflection mode is obtained.
Namely, the incident light I
2
from an upper side, part of which is depolarized when transmitted through the light guide film
50
, is reflected by a reflector
70
in a state in which the polarization direction is rotated. Then, the reflected light of which polarization direction is not rotated is transmitted through the reflection type polarizer
60
, the lower polarizer
120
, the liquid crystal cell
100
and the upper polarizer
110
and emitted to the visible side as emitted light O
21
. Further, the light of which polarization direction is rotated inside the light guide film
50
is reflected downward by the reflection type polarizer
60
, transmitted through the light guide film
50
again, reflected by the reflector
70
and reaches the reflection type polarizer
60
again. On this occas
Chung David Y.
Citizen Watch Co. Ltd.
Kim Robert H.
Westerman Hattori Daniels & Adrian LLP
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