Liquid crystal cells – elements and systems – Particular structure – Particular illumination
Reexamination Certificate
1998-03-13
2001-09-11
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Particular illumination
C362S035000, C349S064000
Reexamination Certificate
active
06288760
ABSTRACT:
FIELD OF THE INVENTION
The present invention concerns a front illumination device to be used by mounting between an illuminated object and a viewer, and which is structured so as to project light onto the illuminated object and to transmit light reflected from the illuminated object so that the reflected light will be visible to the viewer, and concerns a reflection-type liquid crystal display device incorporating this front illumination device as an auxiliary light source.
BACKGROUND OF THE INVENTION
Unlike other displays such as the CRT (Cathode Ray Tube), PDP (Plasma Display Panel), or EL (Electro Luminescence), in the LCD (Liquid Crystal Display), the liquid crystal itself does not emit light, but displays letters or images by regulating the quantity of light transmitted from a specific light source.
Conventional liquid crystal display devices can be roughly divided into transmission-type LCDs and reflection-type LCDs. Transmission-type LCDs include a fluorescent tube or surface luminescent light source such as an EL provided on the back of the liquid crystal cell as a light source (back-light).
On the other hand, reflection-type LCDs, since they perform display using surrounding light, do not require a back-light, and thus have the advantage of low power consumption. Further, in very bright areas such as in direct sunlight, whereas the display of light-emitting displays and transmission-type LCDs becomes nearly impossible to see, that of reflection-type LCDs becomes more clearly visible. For this reason, reflection-type LCDs are applied in devices such as portable information terminals and mobile computers, for which demand has grown in recent years.
However, reflection-type LCDs have the following problems. Namely, since reflection-type LCDs use surrounding light, the brightness of display is highly dependent on the surrounding environment, and, in darkness, such as at night, there are cases when the display is not visible at all. This problem is particularly serious with reflection-type LCDs which use a color filter for color display or which use a polarizing plate, and auxiliary illumination is needed to provide against cases when surrounding light is insufficient.
However, since reflection-type LCDs are provided with a reflective plate on the back of the liquid crystal cells, they cannot use a back-light like that of transmission-type LCDs. A device called a “semi-transmission-type LCD” has been proposed, but since its display characteristics, being midway between transmission-type and reflection-type, are neither here nor there, practical application of this device is expected to be difficult.
Therefore, as auxiliary illumination for reflection-type LCDs when surrounding light is insufficient, a front-light system, for mounting on the front of the liquid crystal cell, has been proposed. Generally, such front-light systems have been made up of a light-conducting body and a light source provided at the side of the light-conducting body. Light projected by the light source from the side of the light-conducting body travels through the interior of the light-conducting body, and is reflected toward the liquid crystal cell by forms provided on the surface of the light-conducting body. As it passes through the liquid crystal cell, the projected light is modulated in accordance with the display information, and, being reflected by the reflective plate provided on the back of the liquid crystal cell, passes again through the light-conducting body toward the viewer. By this means, the viewer is enabled to see the display even when the surrounding light is insufficient.
Front-light systems of this type are disclosed, for example, in Japanese Unexamined Patent Publication No. 5-158034/1993 (Tokukaihei 5-158034) and in SID DIGEST (1995), p. 375.
The following will explain in brief the driving principle of the front-light system disclosed in SID DIGEST (1995), p. 375 with reference to FIG.
20
. This front light system is provided with a light-conducting body
104
, which has an interface
101
made up of flat portions
101
a
and inclined portions
101
b,
one side of the light-conducting body
104
being a light-entry surface
105
, through which light from a light source
106
enters the light-conducting body
104
. In other words, the light source
106
is provided in a position opposite the light-entry surface
105
of the light-conducting body
104
.
Some of the light from the light source
106
entering the light-conducting body
104
from the light-entry surface
105
travels straight, and some of it is projected onto interfaces
101
and
108
between the light-conducting body
104
and the surrounding medium. At this time, if the medium surrounding the light-conducting body
104
is air, and if the refractive index of the light-conducting body
104
is around 1.5, then, according to Snell's law (Equation 1), light with an angle of incidence at the interfaces
101
and
108
of approximately 41.8° or more will be totally reflected.
n
1
·sin&thgr;=
n
2
·sin&thgr;
2
&thgr;
c
=arc sin(
n
2
1
) (Equation 1)
Here,
n
1
is the refractive index of the first medium (here, the light-conducting body
104
);
n
2
is the refractive index of the second medium (here, air);
&thgr;
1
is the angle of incidence from the light-conducting body
104
at the interface
101
;
&thgr;
2
is the angle of light exiting from the interface
101
to the second medium; and
&thgr;
c
is the critical angle.
Of the light projected onto the interfaces
101
and
108
, light which is totally reflected from the inclined portions
101
b
(which are reflective surfaces) and light which, after being reflected from the interface
108
, is reflected from the inclined portions
101
b,
are projected into a liquid crystal cell
110
. Light projected into the liquid crystal cell
110
, after being modulated by a liquid crystal layer (not shown), is reflected from a reflective plate
111
provided on the back of the liquid crystal cell
110
, is projected once again into the light-conducting body
104
, and passes through the flat portions
101
a
toward the viewer
109
.
Light from the light source
106
entering through the light-entry surface
105
which is not projected onto the inclined portions
101
b,
but onto the flat portions
101
a
continues being transmitted and reflected between the interfaces
101
and
108
until it reaches an inclined portion
101
b
Incidentally, the inclined portions
101
b
are provided so that their area, in comparison with the area of the flat portions
101
a,
is sufficiently small when viewed by the viewer.
The foregoing front-light system has the following problems.
(1) As shown in
FIG. 21
, light which does not reach an inclined portion
101
b
even after repeated reflections, and light which enters the light-entry surface
105
substantially perpendicularly, exit the light-conducting body
104
from a surface
107
opposite the light-entry surface
105
as light
114
, and cannot be used in display. This problem is more marked the smaller the panel is, and with the sizes typically used in portable information terminals (5 in. to 6 in. diagonal), most of the light from the light source exits the light-conducting body, and thus the efficiency of light use is very poor.
(2) The form of the interface
101
, which is made up of inclined portions
101
b
and flat portions
101
a,
is similar to that of a prism sheet with the peaks of the prisms flattened. Thus, as shown in
FIG. 21
, surrounding light
115
is easily reflected back toward the viewer
109
, which leads to impairment of display quality.
Since most conventional front-light systems share these problems, improvement of the efficiency of use of the light from the light-source is needed.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a front illumination device comprising a light source and a light-conducting body which guides light from the light source to an illuminated object, which is to be used by mounting in front of the illuminated object, and which
Conlin David G.
Daley, Jr. William J.
Dike, Bronstein, Roberts and Cushman, Intellectual Pratice Group
Parker Kenneth
Sharp Kabushiki Kaisha
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