Liquid crystal cells – elements and systems – Particular structure – Particular illumination
Reexamination Certificate
2000-05-17
2002-02-05
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Particular illumination
C349S095000, C349S096000
Reexamination Certificate
active
06344886
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light guide unit for use in a liquid crystal display device in which a polarized component of light is enhanced and a liquid crystal display device which is provided with such light guide unit. Particularly, this invention relates to a light guide unit for efficiently converting the light from a light source to a polarized light and a liquid crystal display device having means for efficiently directing the polarized light emitted from such light guide unit to a liquid crystal cell.
2. Description of the Related Art
A liquid crystal display device is conventionally observed by directing polarized light to a liquid crystal cell to cause the polarization plane to be rotated depending on the condition of the cell for passage through a polarizer plate. A light source of the polarized light is placed in the back of the liquid crystal plate and thus is called a “back light”. For obtaining such polarized light wave, a non-polarized light was conventionally incident to a polarizer plate and either one of the polarized components; i.e., S component and P component, was absorbed.
Assuming that a plane defined by a light incident to a point of incidence on a surface is an incident plane, a polarized component parallel to the incident plane is called a P component while a component perpendicular to the incident plane is called an S component. Therefore, more than 50-percent of an incident light was not effectively utilized in principle and an actual measurement shows that about 58-percent of the incident light is absorbed.
Further, a light dispersing sheet having printed dots was typically used in addition to a polarization device for obtaining polarized light by absorbing a polarized component in a conventional Liquid Crystal Display (LCD) device, and this makes an additional 20-percent of the light unavailable.
In
FIG. 1
, a LCD module
100
of a conventional LCD device is shown. The light emanating from a light source
101
transmits through a light guide plate
102
having 96% transmittance, a dispersion sheet
103
having 80% transmittance, a lower polarizer plate
104
having 42% transmittance, a glass substrate
105
having a numerical aperture of 40%, a color filter
106
having 30% transmittance, and an upper polarizer plate
107
having 90% transmittance, resulting in an actually available light intensity which is 3.5% of the light generated in the light source
101
. This greatly prevents the energy from being utilized efficiently.
A back light system of a high intensity for use in a low power consumption LCD device is especially desired because it is an important objective in a portable personal computer to assure a longer usable time with a given capacity of a battery and the power consumption of a back light
108
is a major percentage of total power consumption.
Also, the light energy absorbed in the lower polarizer plate
104
, etc., is converted to heat energy which contributes to degradation of parts of the LCD device. Particularly for a liquid crystal material of STN (Super Twisted Nematic) type in which the display quality is degraded by heat, it is an important objective to reduce such heat generation. As seen from
FIG. 1
, 66.4% of the light energy is converted to heat energy by the light absorption in the lower polarizer plate
104
and the dispersion sheet
103
(this is 69% of heat generation by the light energy).
In order to solve such problems, the applicant of this application filed Japanese patent application no. 9-249139 relating to a method of improving the efficiency of light utilization in obtaining a polarized light by making available for use at least a part of a polarized component which had not been utilized. The principle of this method is shown in FIG.
3
.
Light from a fluorescent lump CFL which is a light source is incident to the end surface of a laminated light guide plate unit via a reflecting mirror and a collimator. It propagates through the layers of the light guide plates, and arrives at the other end surface which is cut in an angle. The incident light is partly reflected at the other end surface with the rest being transmitted therethrough. The polarization plane of the light transmitting through the end surface is rotated by a quarter wave length plate placed thereunder and reflected by a reflecting plate placed under the quarter wave length plate for reentrance to layers of the light guide plate again through the quarter wave length plate as a P component.
The P component reentering the light guide plates is incident to the interface with an adjacent light guide plate layer. The angle of incidence of the light on the interface is the Brewster angle (to be described later in detail). Therefore, all the P component and a part of the S component of the light incident to the interface transmit through the interface with the rest of the S component reflected back to the quarter wave length plate and the reflecting plate. The light reflected again by the reflecting plate is again directed to the interface after being converted to a P component by the quarter wave length plate where all the P component and a part of the S component, if any, transmit with the rest being reflected.
The light reflected here is reflected repeatedly in a similar manner and a light converted to a P component for each reflection transmits through the interface. As such, the light guide unit ultimately emits a large portion of the light from the light source as a P component. The polarized light is emitted in the direction largely deviated from the normal to the front. A prism sheet for redirecting the light to the front toward the liquid crystal cell is used. The polarization can be further improved by placing a further polarization plate on the prism sheet.
Because the reflectance and transmission characteristics are different between the S component and the P component, the light transmitting through the interface and the light reflected by the interface have different polarization components. To explain the principle of operation of this invention, a change of polarization components of the light in transmitting through or reflecting from the interface between materials of different indices of refraction is described with reference to
FIGS. 4
,
5
and
6
.
In
FIG. 4
, when light
204
reaches an interface
203
between two materials
201
and
202
having different indices of refraction n
1
and n
2
, respectively, a part of the light
205
is reflected when the angle of incidence &phgr;
1
is less than a critical angle while a part of the light
206
transmits through the interface. Assuming that a plane defined by a light incident to a point of incidence on a surface is an incident plane, the incident light
204
is divided into a P component parallel to the incident plane and an S component perpendicular to the incident plane.
Modifying Maxwell equation for a dielectric material, the transmittance of the polarized components P and S are given by;
Tp
=sin (2&phgr;
1
)×sin (2&phgr;
2
)/(sin
2
(&phgr;
1
+&phgr;
2
)×cos
2
(&phgr;
1
−&phgr;
2
))
Ts
=sin (2&phgr;
1
)×sin (2&phgr;
2
)/sin
2
(&phgr;
1
+&phgr;
2
)
dn
1
×in (&phgr;
1
)=
n
2
×sin (&phgr;
2
)
where Tp: transmittance of P component (1−reflectance Rp)
Ts: transmittance of S component (1−reflectance Rs)
&phgr;
1
: incident angle of light
&phgr;
2
: exit angle of light
n
1
: index of refraction of material
201
n
2
: index of refraction of material
202
or it is known that;
Rp=
((
n
1
/cos &phgr;
1
−n
2
/cos &phgr;
2
)/(
n
1
/cos ∠
1
+n
2/cos &phgr;
2
))
2
Rs=
((
n
1
×cos &phgr;
1
−n
2
×cos &phgr;
2
)/(
n
1
×cos &phgr;
1
+n
2
×cos &phgr;
2
))
2
where
Rp: reflectance of P component (1−transmittance Tp)
Ts: reflectance of S component (1−transmittance Ts)
The reflectance of the P polarized component and S polarized component vary depending on the incident angle &phgr
Kawada Koji
Oki Yoji
Suzuki Masaru
Hancock Earl C.
Holland & Hart LLP
Stanley Electric Co. Ltd.
Ton Toan
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