Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Reexamination Certificate
2000-06-30
2002-11-12
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
C362S035000, C362S026000, C362S027000, C349S065000
Reexamination Certificate
active
06480307
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates generally to flat panel displays such as liquid crystal displays (LCDs), and more particularly, it relates to plane light source units for backlighting and a method for manufacturing holographic lightguides used for the same.
(b) Description of the Related Art
In recent years, flat panel displays have come into wide use as displays in electronic and electrical appliances, replacing conventional cathode ray tubes. Specifically, LCDs have been widely used in flat panel electronic displays.
LCDs comprise a plane light source unit as a backlighting source. Conventional plane light units for LCDs are shown in
FIGS. 1A-1C
. The conventional plane light source unit shown in
FIG. 1A
is adapted for an LCD having a relatively small or medium size, such as for a lap-top computer. Meanwhile, the plane light source unit shown in
FIG. 1B
or
1
C is adapted for an LCD having a relatively large size, such as a monitor for desk-top computers, or a wide television monitor which can be attached to a wall.
First, referring to
FIG. 1A
, the conventional plane light source unit will be explained for an LCD having a relatively small or medium size. The conventional plane light source unit comprises a tubular light source
1
, a light-guide plate
2
in which light from the tubular light source
1
propagates, and a reflecting plate
3
for reflecting light emitted from the tubular light source
1
to the LCD panel
8
. The plane light source unit further comprises first and second diffusers
4
,
5
and first and second prism plates
6
,
7
. Each prism plate has an array of microprisms formed on its upper surface. The edges of the microprisms are parallel to an x-axis in the first prism plate
6
, while the edges of the microprisms are parallel to a y axis in the second prism plate
7
(see
FIG. 2
(a)(b)).
FIG. 1B
shows a conventional plane light source unit for an LCD having a relatively large size. The conventional plane light source unit comprises a pair of tubular light sources
1
,
1
′, a light-guide plate
2
positioned between the pair of the light sources
1
,
1
′, in which light from the light sources
1
,
1
′ propagates, and a reflecting plate
3
for reflecting light emitted from the pair of tubular light source
1
,
1
′ to the LCD panel
8
. The unit further comprises two diffusers
4
,
5
.
Referring to
FIG. 1C
, another conventional plane light source unit for an LCD having a relatively large size is shown. The unit comprises a plurality of tubular light sources
1
, a light-guide plate
2
above the light sources
1
, and a reflecting plate
3
for reflecting light emitted from the plurality of tubular light sources
1
to the LCD panel
8
. The unit further comprises two or more diffusers
4
.
In the case of a conventional diffusion type light-guide plate used in the conventional plane light source units, scatterer patterns are formed on the reflecting surface on the light-guide plate in order to scatter light for uniform illumination. The scatterers are illustrated in FIG.
3
. The light-guide plate may have depressions like (a) prism type or (b) semi-sphere type. The light-guide plate may have an ink dot pattern in which ink including scattering material is printed ((c) ink printing type).
The operation of the conventional plane light source unit is as follows:
First referring to
FIG. 4
, the plane light source unit as shown in
FIG. 1A
will be described. The light emitted from the tubular light source
1
enters the light-guide plate
2
and reflects internally (ray R
1
) in the light-guide plate
2
. The light exits to the reflecting plate
3
when the light does not satisfy the total reflection condition or impinges on the scatterer of the light-guide plate
2
as shown in FIG.
3
. The light is then reflected on the reflecting plate
3
and may reenter the light-guide plate
2
(ray R
2
). On the other hand, when the light reflected internally in the light-guide plate
2
impinges on the upper surface of the light-guide plate
2
and does not satisfy the total reflection condition, the light exits from the light-guide plate
2
(ray R
3
) with an angle &thgr;
1
. The light then enter a first diffuser
4
and scatters with an angle &thgr;
2
(ray R
4
) which is larger than the angle &thgr;
1
. The light through the first diffuser
4
enters a first prism plate
6
. Since the first prism plate
6
has edges of the microprisms parallel to the x-axis, rays in the y-z plane are affected to decrease the scattering angle, but rays in the x-y plane are not affected (ray R
5
). The light though the first prism plate
6
enters a second prism plate
7
to make the main ray parallel to the y axis (ray R
6
) since the second prism plate
7
has edges of the microprisms parallel to the y-axis. Then the light is finally scattered by the second diffuser
5
to have a uniform light distribution for LCDs.
Referring to
FIG. 5
, the plane light source unit as shown in
FIG. 1B
will be described. The light emitted from the two tubular light sources
1
,
1
′ enters the light-guide plate
2
and reflects internally in the light-guide plate
2
. The light exits to the reflecting plate
3
when the light does not satisfy the total reflection condition or impinges on the scatterer of the light-guide plate
2
as shown in FIG.
3
. The light proceeding to the reflecting plate
3
is reflected from the reflecting plate
13
, and may then re-enter the light-guide plate
2
(ray R
2
). On the other hand, when the light reflected internally in the light-guide plate
2
impinges on the upper surface of the light-guide plate
2
and does not satisfy the total reflection condition, the light exits from the light-guide plate
2
. The light intensity distribution at the upper surface of the light-guide plate
2
is shown in (b) of the FIG.
5
. The lights then enter a first diffuser
4
and scatter to have the light intensity distribution shown in (c) of
FIG. 5
, which is more uniform than (b). The light that passes through the first diffuser
4
and enters a second diffuser
5
has a substantially uniform light intensity distribution as shown in (d) of
FIG. 5
, resulting in uniform illumination for LCDs.
As described above, since a light-guide plate of the conventional plane light source unit has scatterers having depressions like prisms or semi-spheres, it is required to have time-consuming and high cost manufacturing processes. Conventional injection molding methods or shaping methods with diamond cutters are not suitable for making light-guide plates which have fine depressions such as on the order of magnitude 10 &mgr;m.
On the other hand, it is also a time-consuming process to print dot patterns in the case of the ink printing type light-guide plate. Further, the ink printing type light-guide plate has drawbacks of overall low efficiency since both the ink and the scattering material may absorb light.
Now referring to
FIG. 6
, the plane light source unit as shown in
FIG. 1C
will be described. The light emitted from the tubular light sources
1
enters directly through the light-guide plate
2
(ray R
2
), or reflects from the reflecting plate
3
to the light-guide plate
2
(ray R
1
). When the light passes through the light-guide plate
3
, the light distribution intensity is high near the light sources
1
as shown in diagram (C). The light passes two or more diffusers and then has a substantially uniform light distribution as shown in diagram (b), resulting in uniform illumination for LCDs.
However, since light is absorbed by the scattering pattern, the utilization efficiency of light is poor, and the power consumption of the light source is large.
Further, since the conventional plane light source units need several diffusers or prism plates to obtain uniform illumination, the light is absorbed by the diffusers or prism plates, and this results in a degradation of the brightness of the LCDs.
Therefore, it is required to use a higher intensity light source or more
Alston & Bird LLP
Boutsikaris Leo
Spyrou Cassandra
LandOfFree
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