Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-02-17
2003-05-06
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S098000, C349S009000
Reexamination Certificate
active
06559911
ABSTRACT:
This application claims the benefit of Japanese Application No. 09-48562, filed on Feb. 18, 1997, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light splitter and, more particularly, to a polarization light splitting film for use in a liquid crystal display device and the like.
2. Discussion of the Related Art
Due to recent technological development, liquid crystal displays (LCDs) are becoming increasingly popular for use in the display component of a personal computer and the like. Advantages of the liquid crystal display, such as the fact that they are thinner and lighter than the conventional display devices, for example, largely contributed to their gain in popularity. Moreover, a narrow viewing angle in conventional LCDs, which was previously considered to be a disadvantage, has been recently overcome by newly developed LCDs with a wide viewing angle. Accordingly, a wide variety of usages of the LCDs beyond personal-computer-use is expected to emerge.
Since a liquid crystal panel itself is not a light-emission device, the LCD needs an illumination light source. Reflection type LCDs use external illumination as the illumination light source. However, transmission type LCDs equipped with an internal backlight system are more popular.
FIG. 13
shows a typical structure of a conventional transmission type LCD. In this example, a backlight system
20
includes a light source
21
, light guide
22
, light diffusion elements
23
, reflective sheet
24
, diffusion sheet
25
, and a prism sheet
26
. Light emitted from light source
21
is incident on light guide
22
. A cone-like shaped object indicates a viewing position of the LCD. The incident light propagates in the light guide
22
by experiencing multiple total internal reflections. A portion of the light in the light guide
22
is diffused (or scattered) upwards by light diffusion elements
23
and emerges from the light guide
22
. A portion of the light emitted downwards from the light guide
22
is reflected by reflection sheet
24
and is returned to the light guide
22
. The light emitted upwards from the light guide
22
is diffused by diffusion sheet
25
and converged by prism sheet
26
. The resultant light is used as illumination light for a liquid crystal cell
30
, which is sandwiched by polarizing plates
31
and
32
.
In most LCDs, polarized light that is obtained by transmitting light through a polarizing plate is modulated in the liquid crystal layer. Approximately half of the incident light is absorbed at the polarizing plate and, accordingly, the efficiency in light usage is low. In order to produce sufficient luminance, more light needs to be incident on the polarizing plate. However, increasing the light intensity causes a variety of problems, such as increased power consumption of the light source and adverse effects on the liquid crystal material due to heat generated from the thus powered up light source, which degrades the display quality.
To solve the above-mentioned problems, the following technique has been proposed. First, unpolarized light from a light source is split by a polarization light splitter into two linearly polarized light beams having the polarization directions orthogonal to each other. Then, one of the linearly polarized light beams is directly used for illumination while the other polarized light beam is used indirectly. In other words, one of the polarization components, which are separated by the polarizing plate, is directly incident on the liquid crystal cell, whereas the other polarization component, which progresses towards the light source, is re-directed toward the polarizing plate by reflection or the like. This way, the efficiency in light usage can be improved. Some of the recent developments along this direction are as follows.
(1) In Japanese Application Laid-Open Publication No. 04-184429, an unpolarized light beam from a light source is split by a polarization light splitter into two orthogonally polarized light beams. One of the polarized light beams is directed towards the liquid crystal cell. The other beam, which progresses towards the light source, is converged and then reflected towards the liquid crystal cell.
(2) A backlight system disclosed in Japanese Application Laid-Open Publication No. 06-265892 includes a beam deflector that transmits light in a direction substantially normal to the light emitting surface of a planar light guide. The deflector is disposed on the light emitting side of the planar light guide, and a polarization light splitter is disposed above the deflector.
(3) In a backlight system disclosed in Japanese Application Laid-Open Publication No. 07-261122, a polarized light splitter is located on the light emitting side of a parallel light generating device. The parallel light generating device is constructed of a light scattering conductor including a portion having a wedge-shaped profile.
(4) Similar systems are proposed in Japanese Application Laid-Open Publications No. 06-289226, No. 07-49496. All the proposed polarized light splitting systems, including the above-mentioned (1) to (3), employ a multi-layer film utilizing the Brewster law (Brewster's angle) as polarization splitting means.
FIGS. 11 and 12
show cross-sectional views of conventional polarization light splitting films. In
FIG. 11
, polarization light splitting film
40
has a multi-layer structure formed by alternately laminating light transmission layers
41
having a large refractive index and light transmission layers
42
having a small refractive index. Using Brewster's law, the polarization light splitting film
40
is designed to transmit the p-polarized light component and reflect the s-polarized light component.
Polarization light splitting film
40
in
FIG. 12
also has a laminated structure of two types of layers
43
,
44
having different refractive indices. The thickness of the layers
43
is designed to cause interference with respect to visible light. In this construction, if the refractive indices of the layers
43
,
44
and the thickness of layer
43
satisfy a certain predetermined relationship, the transmission contrast between the p-polarized component and the s-polarized component becomes large with respect to a certain incident angle. The polarization light splitting film
40
of
FIG. 12
utilizes this property to transmit polarized light.
However, the above-mentioned conventional techniques have the following disadvantages. The backlight system of (1) is intended for use with projection LCD devices. The structure of such an illumination system requires a large amount of space. Therefore, it cannot be applied to thin panel-type LCD devices.
The backlight system of (2) is applicable to thin LCD devices. If a polarized light splitting layer is fabricated on the inclined sides of a columnar prism array having a plurality of triangular-shape prism units, a relatively high efficiency can be achieved in light usage. However, the structure of the polarized light splitting layer becomes complicated. In particular, it is difficult to fabricate the polarized light splitter layer on the inclined sides of the columnar prism array. This type of backlight system is therefore not suitable for mass production.
As for the backlight system of (3), if the parallel light generating device is constructed of a certain light scattering conductor having a wedge-shaped profile, superior efficiency in light usage can be obtained. However, it is difficult to manufacture such a light scattering conductor with a desirable light scattering performance. Accordingly, this system is not suitable for actual use.
In addition, since all the systems discussed in (1) to (4) utilize the Brewster law (or Brewster's angle), it is necessary to form multiple layers on an inclined face. Therefore, these systems require a complicated structure and are not suitable for mass production.
Furthermore, to produce a sufficient polarization light splitting property using the conventional multi-l
Arakawa Fumihiro
Kashima Keiji
Dai Nippon Printing Co. Ltd.
Kim Robert H.
Qi Mike
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