Illumination – Revolving
Reexamination Certificate
2001-08-06
2003-08-26
Quach-Lee, Y. My (Department: 2875)
Illumination
Revolving
C362S561000, C362S290000, C362S339000
Reexamination Certificate
active
06609807
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a backlight for illuminating a transmissive liquid crystal panel and to a liquid crystal display device having such a backlight. The present invention relates particularly to a backlight and a liquid crystal display device that permit the viewing of a displayed image only within a particular range of viewing angles.
2. Description of the Prior Art
A liquid crystal display device having a transmissive liquid crystal panel is so structured as to produce a desired image by illuminating the liquid crystal panel from behind with a backlight so that light is transmitted through particular pixels of the liquid crystal panel. This permits the viewing of a displayed image even, for example, at night when no ambient light is available.
A liquid crystal display device designed for use in a navigation system mounted on a car, an automated teller machine (ATM) installed at a banking institution, or the like is so configured as to permit the viewing of a displayed image only from a viewpoint within a particular range of viewing angles so that the displayed image cannot be viewed from a viewpoint outside that range of viewing angles. This is because, with a liquid crystal display device for car-mounted use, the range of angles of emergence of the light emerging therefrom is limited because, if an image displayed thereon is projected onto the windshield of the car, it may disturb the field of vision of the driver and lead to a car accident. On the other hand, with a liquid crystal display device for use in an ATM, the range of angles of emergence of the light emerging therefrom is limited so that no one can view the information displayed on the liquid crystal panel other than the one who is operating the ATM.
FIG. 26
shows the structure of a conventional liquid crystal display device that can control the range of angles of emergence of the light emerging therefrom. The liquid crystal display device
1
is composed essentially of a liquid crystal panel
6
and a backlight
7
. The backlight
7
has a light guide plate
3
covered with a reflector sheet
4
made of polyethylene terephthalate (PET) foam. Along opposite sides of the light guide plate
3
, light sources
2
are supported by a supporting member (not shown). The back surface
3
b
of the light guide plate
3
is formed as a non-glossy surface, so that the light emitted from the light sources
2
is incident on the light guide plate
3
and then emerges therefrom through its exit surface
3
a
as scattered light.
Above the light guide plate
3
are arranged a diffuser sheet
20
for producing diffused light and a light shield louver
5
for shielding the light incident within a predetermined range of angles. As shown in
FIG. 27
, the light shield louver
5
has light-transmitting layers
5
a
, which transmit light, and light-absorbing layers
5
b
, which absorb light, arranged, for example, at 50 &mgr;m intervals and sandwiched between transparent base plates
5
c
. Thus, of the light incident on the light shield louver
5
, the portion traveling within a range &thgr; of viewing angles is transmitted, but the portion traveling outside the range &thgr; of viewing angles is shielded.
Above the light shield louver
5
, a transmissive liquid crystal panel
6
is disposed. The light traveling within the range &thgr; of viewing angles (see
FIG. 27
) and thus transmitted through the light shield louver
5
illuminates the liquid crystal panel
6
, and is transmitted through particular pixels thereof so as to form an image. As a result, the image can be viewed from a viewpoint within a predetermined range of directions.
However, the conventional liquid crystal display device
1
described above, for example when it offers a range &thgr; of viewing angles of 90°, exhibits transmittance as shown in FIG.
29
. In this figure, the angle of incidence (°) with respect to the light shield louver
5
is taken along the horizontal axis, and the transmittance (%) is taken along the vertical axis. As this figure shows, the transmittance is highest at an angle of incidence of 0°, and the transmittance linearly decreases as the absolute value of the angle of incidence increases.
Owing to the characteristics of the backlight
7
shown in
FIG. 26
, the light emerging from the light guide plate
3
exhibits a brightness distribution as shown in
FIG. 28
, offering the highest brightness around an angle of incidence of 0° with respect to the light shield louver
5
. As a result, the light transmitted through the light shield louver
5
and then incident on the liquid crystal panel
6
exhibits a brightness distribution as shown in FIG.
30
.
In
FIG. 28
, the angle of incidence (°) with respect to the light shield louver
5
is taken along the horizontal axis, and the relative brightness (%) relative to the brightness (100%) at an angle of incidence of 0° is taken along the vertical axis. In
FIG. 30
, the angle of emergence (°) with respect to the light shield louver
5
is taken along the horizontal axis, and the relative brightness (%) relative to the brightness (100%) at an angle of emergence of 0° is taken along the vertical axis.
As
FIG. 30
shows, the brightness is highest at an angle of emergence of 0°, and the brightness falls sharply as the absolute value of the angle of emergence increases. This is the reason that, as the viewpoint of the viewer varies according to his or her height, sitting height, or the like, the viewability of the liquid crystal display device
1
degrades markedly.
Moreover, in the production of the light shield louver
5
, errors are inevitable in the intervals at which the light-transmitting layers Sa and the light-absorbing layers
5
b
are arranged. As a result, as shown in
FIG. 31
, when the liquid crystal display device
1
is viewed from the directions indicated by arrows P
1
and P
2
, part of the viewer's lines of sight are shielded by the light-absorbing layers
5
b
as arrow P
2
indicates. This causes black stripes to be observed by the viewer, and thus degrades the viewability of the liquid crystal display device
1
.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a backlight and a liquid crystal display device that permit the viewing of a displayed image within a desired range of viewing angles with reduced degradation of viewability.
To achieve the above object, according to one aspect of the present invention, a backlight is provided with: a light source; a flat-plate-shaped light guide plate for guiding the light emitted from the light source in a predetermined direction; a light shield louver, disposed so as to face the light guide plate, for shielding part of the light emerging from the light guide plate according to angles of incidence; and a converter for converting the brightness distribution of the light incident on the light shield louver into a predetermined brightness distribution. Here, the brightness distribution of the light incident on the light shield louver is converted in such a way that the brightness at a predetermined angle of incidence within the range from 0° to +90° and the brightness at a predetermined angle of incidence within the range from 0° to −90° are higher than the brightness at an angle of incidence of 0°.
In this configuration, the light emitted from the light source is guided to the light shield louver by the light guide plate. The light incident on the light shield louver is converted so as to have a predetermined brightness distribution by the converter before or after the light emerges from the light guide plate. The light thus converted exhibits the highest brightness in both the positive and negative directions relative to an angle of incidence of 0°, at which the brightness is lower than the highest brightness.
In the backlight configured as described above, the converter may be composed of prisms arranged at predetermined intervals.
According to another aspect of the present invention, a backlight is provided with: a light source; a f
Takahashi Nobuyuki
Torihara Hiroshi
Ukai Kenichi
Neils Peggy A.
Nixon & Vanderhye P.C.
Quach-Lee Y. My
Sharp Kabushiki Kaisha
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