Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-04-20
2003-06-03
Dudek, James (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06573959
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical element, namely reflector that alters the properties of light intentionally interacting with it. The present invention also relates to an optical element incorporated in to a display device and a method of manufacturing such a display device.
2. Description of the Related Art
Reflective display devices are well known. In principle, these consist of a light-modulating element, and a reflector disposed behind the light-modulating element. Light incident on the front of the light-modulating element passes through the element, is reflected by the reflector, and passes back through the light-modulating element. A reflective device had the advantage that, under suitable illumination conditions, it can utilise ambient light and does not require its own light source. For such a display to operate effectively, it is necessary that sufficient of the ambient light incident on the display is directed towards the observer so that a sufficiently bright display is produced.
Blazed reflectors can be used to redirect ambient light impinging on a reflective display at an oblique angle so that, after reflection, it exits the display substantially at normal incidence. This is advantageous since viewers of a display generally view the display from the normal direction, or from a near-normal direction, and the use of a blazed reflector creates a higher reflectance of the display towards such a viewer.
The principle of operation of a blazed reflector is illustrated in FIGS.
1
(
a
) and
1
(
b
).
In FIG.
1
(
a
), light is incidence on a block
1
of material with a refractive index n
i
which has upper and lower surfaces that are parallel to one another. If light is incident on the front surface of the block
1
at an angle &thgr;
0
to the normal, it will undergo refraction at the front surface of the block
1
. It will propagate within the block at an angle &thgr;
i
to the normal, where &thgr;
0
and &thgr;
i
are connected by Snell's Law, namely sin &thgr;
i
=sin &thgr;
0
i
(assuming that the medium outside the block has a refractive index n
0
=1).
A reflective layer
1
′, such as a metallic or dielectric layer, is disposed on the lower face of the block
1
of FIG.
1
(
a
). Light transmitted through the block
1
is specularly reflected by the reflective layer
1
′ when it reaches the lower surface of the block, and is again refracted at the upper surface of the block so that it leaves the block at an angle &thgr;
0
to the normal. It can thus be seen that if light is incident on the upper surface of the block
1
at an oblique angle to the normal to the block, it will be reflected at an oblique angle of the same absolute value, and so will not reach an observer viewing the block from the normal direction.
The advantages of using a blazed reflector are illustrated in FIG.
1
(
b
). In FIG.
1
(
b
), the lower surface of the block is not a plane surface parallel to the upper surface of the block, but is in the form of a blazed reflector. As is well known, the reflective surface of a blazed reflector consists of segments, with each segment being inclined at an angle &thgr;
m
(known as “the angle of blaze”). A reflective layer
1
′ is disposed on the lower face of each segment.
Since the lower surface of the block is a series of inclined segments, light transmitted through the block at an oblique angle will be reflected closer to the normal of the display. Indeed, if the angle of blaze is chosen such that &thgr;
m
=&thgr;
i
/2, then the reflected light will be reflected in the normal direction. Use of a blazed reflector will thus increase the brightness of the display in a normal or near-normal direction.
If the refractive index of the block
1
is assumed to be n
i
=1.5, the angle of blaze required to direct reflected light in the normal direction is &thgr;
m
=10° for the case &thgr;
0
30°. If &thgr;
0
=45°, then the required angle of blaze is &thgr;
m
=14°.
FIG.
2
(
a
) is a polar diagram showing the preferred reflection cone
2
from a reflective display device for collimated light that is incident on the display at an azimuthal angle of 90° and a polar angle of +30°. If light is reflected within this cone, it will reach an observer viewing the display at a normal, or near-normal, angle.
FIG.
2
(
b
) is a polar diagram showing a typical range
3
of possible positions for a light source for use with a reflective display device that incorporates a blazed reflector.
Reflective display devices are known which consist of a conventional liquid crystal display device, and a blazed reflector disposed behind the liquid crystal display device. A blazed reflector suitable for this application is typically produced by embossing a thermoplastic polymer film
4
disposed on a substrate
5
, by moving a suitable embossing tool over the layer of photopolymer. This is illustrated schematically in FIG.
3
. Once the photopolymer layer
4
has been embossed, a metallic film is then disposed over the photopolymer
4
to produce the blazed reflector. Manufacture of a blazed reflector in this way to described in U.S. Pat. Nos. 5,245,454 and 5,128,787.
Simply disposing a blazed reflector behind a conventional passive matrix LCD is only satisfactory if the extension of the second substrate normal to the display plane is small compared to the extension of the picuture element (pixel) in the display plane. This approach is not compatible with active matrix LCDs, since components of the active matrix LCD, for example such as thin film transistors (TFTs), will shade the blazed reflector. A significant amount of light passing through the LCD will thus not reach the blazed reflector, but will instead be reflected at an oblique angle by the pixel electrodes or absorbed by other components of the LCD. Furthermore, the use of a blazed reflector that is external to the LCD can give rise to optical cross-talk between adjacent pixels, and it can also cause parallax problems. This leads to a loss of resolution of the display.
It is desirable for a blazed reflector to be disposed within a LCD, so that the problems of shading of the reflector and of optical cross-talk and parallax are eliminated or at least reduced in severity.
A further desired property of a reflector for use in a reflective display is that it reflects light into a range of angles around the exact direction of specular reflection. If such a reflector is used, it is then possible for the direct specular reflection of a light source to be directed away from the position of an observer while still ensuring that a significant amount of light is directed to the observer. This means that the glare at the observer's position is reduced, as the observer will not see an image of the light source.
U.S. Pat. Nos. 5,204,765, 5,408,345 and 5,576,860 in the name of Sharp Kabushiki Kaisha describe an internal electrode for a liquid crystal display which has diffuse reflecting properties. The reflector will direct light into a range of angles around the direction of specular reflection, and it also will preserve polariation of light incident on the reflector. However, this reflector is a symmetric reflector, and this technology has not yet been extended to produce an asymmetric diffuse reflector. Indeed, no-one has yet produced a blazed reflective TFT electrode suitable for internal use in a high resolution active matrix display. Furthermore, no-one has yet produced an asymmetric reflector which has diffuse reflecting properties.
It is, in principle, possible to create an internal blazed reflector using the embossing technique illustrated in
FIG. 3
, but in practice it is difficult to do this. One particular problem that arises with an active matrix LCD in which the blazed reflector is acting as a pixel electrode is that the metallic layer disposed over the photopolymer must be electrically connected to a thin film transistor (TFT) disposed on the substrate of the LCD. This requires that a th
Dudek James
Sharp Kabushiki Kaisha
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