Liquid crystal cells – elements and systems – Particular structure – Particular illumination
Reexamination Certificate
2001-03-29
2003-10-21
Niebling, John F. (Department: 2812)
Liquid crystal cells, elements and systems
Particular structure
Particular illumination
C362S035000
Reexamination Certificate
active
06636283
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a front light arranged in front of a display device to improve brightness and uniformity of brightness of the image plane on the display device, as well as to a reflective liquid crystal display device and a personal digital assistant.
2. Description of the Background Art
FIG. 57A
is a schematic cross section representing an example of a conventional front light (Japanese Patent Laying-Open No. 8-94844).
FIG. 57A
shows a cross section of the front light as a whole, and
FIG. 57B
is a partially enlarged illustration of an upper surface of an optical guide plate. The optical guide plate
101
of the front light has a surface
101
a
(hereinafter referred to as the “first surface”) opposing to the display device
130
and an upper surface
101
b
(hereinafter referred to as a “second surface”) opposite to the first surface, and on the second surface, a plurality of grooves each having a prism-shaped cross section are provided extending as ribs in the widthwise direction. The front light is placed in front of a reflective liquid crystal display device, that is, on the side of a viewer viewing the displayed image plane, and emits light toward the display device. In the front light
110
, a light beam entering from a light source
103
to the optical guide plate
101
proceeds through the optical guide plate
101
away from the light source, while the beam is reflected by a reflective surface
121
of a steep slope, which is one surface of the prism on the second surface
101
b
, and emitted to the side of the first surface, approximately perpendicular to the optical guide plate. Therefore, as shown in
FIG. 57B
, the reflective surface forms an angle of about 43° with a hypothetical bottom plane of the prism, and a gentle slope
122
, which is the other surface of the prism, forms an angle not larger than 10° with the bottom surface. The height of each prism is set to be 5 &mgr;m to 50 &mgr;m so as to reflect light of approximately the same intensity at various portions from end to end of the optical guide plate, to ensure uniform brightness in the plane. Along an end surface of the optical guide plate, light source
103
such as a cold cathode tube is arranged, and a light beam emitted from the light source enters the optical guide plate from that end surface and propagated through the optical guide plate. The light entering the optical guide plate propagates while repeating regular reflection at certain portions of the first and second surfaces
101
a
and
101
b
, and when the beam reaches the reflective surface
121
of the second surface, the beam is reflected and emitted approximately vertically, from the first surface
101
a
of the optical guide plate. Reflector plates
104
are arranged on an outer side of light source
103
and at the other end of the optical guide plate, so as to prevent any loss of light intensity. Further, the first and second surfaces are protected by a light transmitting layer
120
. The light emitted from the optical guide plate enters the reflective display device
130
, reflected by a reflecting member (not shown) arranged behind the liquid crystal, and returns to the optical guide plate
101
. For convenience of description, in the following, the term “display device” refers to the “display apparatus” with the front light excluded. During the progress, the light beam is modulated in the display device, and passes through the optical guide plate in the thickness direction to form an image on the screen, which is viewed by the viewer. When uniformity of light intensity in the plane is to be ensured by the front light, it is necessary to change area ratio of the reflective surface of the second surface
101
b
from portion to portion. It is impossible to change the area ratio of the reflective surface while keeping constant the angle of the prism structure. Therefore, in order to obtain high uniformity, it is necessary to change the angle of the prism structure. An optical guide plate having the prism angle varied in one same plane is difficult to process and to fabricate with high precision, resulting in higher cost.
FIG. 58A
is a schematic cross section representing another example of the conventional front light (Japanese Patent Laying-Open No. 10-326515).
FIG. 58A
is a cross section of the front light as a whole, and
FIG. 58B
is a partially enlarged illustration of the second surface
101
b
of optical guide plate
101
. The second surface
101
b
of the guide plate in the front light is stepwise, including a flat portion
123
that is parallel to the first surface, and a reflecting portion
121
forming an angle of about 45° with the flat portion. The light beam proceeding through the optical guide plate encounters the reflecting portion
121
and reflected approximately vertically to the optical guide plate, and emitted from the optical guide plate. In connection with the conventional front light described above, the following proposal has been made. More specifically, the ratio of the reflective surface inclined by 45° described above, which is the cause of loss of the light when the light is reflected by the liquid crystal panel or when the light passes again through the depth direction of the optical guide plate, or when ambient light passes through the optical guide plate, is set to at most ({fraction (1/20)}), so as to reduce the loss (Japanese Patent Laying-Open No. 11-202785).
Further, referring to
FIG. 59
, a proposal has been made in which prism-shaped grooves
112
are provided in optical guide plate
101
, with one inclined surface of each groove is made to have the inclination in the range of 35° to 55° and the other inclined surface is made to have the inclination in the range of 60° to 90° (Japanese Patent Laying-Open No. 11-242222). The light beam proceeding in the optical guide plate encounters the inclined surface in the range of 35° to 55° as in the conventional examples, reflected therefrom and emitted to the outside of the optical guide plate. Further, referring to
FIG. 60A
, a method has been proposed in which the depth of the grooves is made gradually deeper further away from the light source so that the intensity of the emitted light becomes uniform, or referring to
FIG. 60B
, a method of adjustment has been proposed in which the pitch between the grooves is made gradually narrower further away from the light source, so that the intensity of emitted light is made uniform regardless of the positions.
In order to prevent degradation of display quality by moire fringes resulting from interference of the pitch of the prism structure and the pixel pitch of the display device, a proposal has been made in which the period of the prism structure is limited to the range of 1/(1.3+N) to (1/1.6+N) of the pixel period (Japanese Patent Laying-Open No. 11-260128).
Further, a proposal has been made in which an anti reflection film is provided on the first surface of the optical guide plate, so as to prevent lowering of display contrast caused by reflection at the first surface when the light is emitted from the optical guide plate (Japanese Patent Laying-Open No. 11-242220).
The optical guide plates of the conventional front lights each have the prism-shaped structure. Therefore, in most cases, the rib-shaped or fringe-shaped prism structure is visually recognized by the viewer, lowering the display quality. Further, as the light beam is reflected by the liquid crystal panel and again passes through the optical guide plate, the light beam is refracted to different directions at surfaces of different angles, resulting in the problem that double images are generated. Further, the above described prism optical guide plates all utilize the method in which the direction of progress of the light is changed by the regular reflection at the reflective surface of the prism, so that the light is emitted from the optical guide plate. Therefore, characteristic of the emitted light significantly vary along the direction of progress of the l
Moroda Masahisa
Oda Kyoichiro
Sasagawa Tomohiro
Sugawara Naoto
Yokoi Masahiro
Leydig , Voit & Mayer, Ltd.
Luk Olivia T
Mitsubishi Denki & Kabushiki Kaisha
LandOfFree
Front light, reflective liquid crystal display device and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Front light, reflective liquid crystal display device and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Front light, reflective liquid crystal display device and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3169368