Illumination – Revolving
Reexamination Certificate
1999-02-26
2001-03-13
Husar, Stephen (Department: 2875)
Illumination
Revolving
C362S026000
Reexamination Certificate
active
06199995
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light guide plate capable of forming a bright and high-contrast reflection-type liquid-crystal display device, and a planer light source unit that uses the light guide plate to permit efficient utilization of light.
The present application is based on Japanese Patent Application No. Hei. 10-64311, which is incorporated herein by reference.
2. Description of the Related Art
To meet the current need for illumination equipment that enabled the viewing of a reflection-type liquid-crystal display device in the dark or other situations, the present inventors attempted the application of a front light system in which the back light used in a transmission-type liquid-crystal display device was provided on the viewing side of the liquid-crystal cell. The back light uses a light guide plate that receives incident light on a side and that allows it to emerge from either the top or the bottom surface through a light issuing means. The front light system allows the display to be viewed via the light guide plate.
However, the back light using the conventional light guide has involved difficulty in attempts for commercialization since it suffers from various problems such as low contrast in ON state, low brightness in OFF state and disturbed display.
It should also be mentioned that a back light using a light guide plate in which diffusing dots or fine asperities form a light issuing means provides only low contrast during viewing in On state and also provides low contrast and insufficient display brightness during viewing in OFF state with extraneous light such as indoor illumination; in addition, the display image that can be viewed through the light guide plate is so extensively disturbed that it lacks clarity.
A back light is known that uses a light guide plate in which a stepped prism structure consisting of inclined surfaces having a slope of 45 degrees and flat surfaces (zero degrees of inclination) form a light issuing means (as described in Japanese Patent Publication No. Sho. 62-73206). Even this back light has the disadvantage of giving low contrast and insufficient display brightness during viewing in ON state.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to develop a reflection-type liquid-crystal display device that gives high contrast during viewing in both OFF state and ON state, that provides a highly bright display and that creates a display image through a light guide plate that is less disturbed and features high clarity.
Another object of the invention is to develop a light guide plate and a planer light source unit that are capable of forming the reflection-type liquid-crystal display device.
The present invention provides a light guide plate in which the light incident on an entrance end surface is allowed to emerge from a bottom surface via a light issuing means formed on a top surface, with the direction of maximum intensity of the emerging light as measured in a plane perpendicular to the two reference planes defined by said entrance end surface and said bottom surface being within 30 degrees from the line normal to said bottom surface and wherein when the direction toward the entrance end surface away from said normal line taken as the reference is assumed to be negative, the intensity of the light leaking from the top surface at the same angle as the direction of said maximum intensity is no more than a tenth of said maximum intensity, with the incident light from the bottom surface being transmitted through the top surface.
The present invention also provides a planer light source unit having a light source adjacent the entrance end surface of said light guide plate, as well as a reflection-type liquid-crystal display device having a reflective layer furnished, light-crystal cell provided beneath the bottom surface of the planer light source unit.
According to the invention, one can produce a light guide plate in which the light emerging from the bottom surface has high directivity in a vertical direction-and in which the leakage light from the top surface is less likely to overlap a display image. Using this light guide plate, one can produce a planer light source unit capable of efficient utilization of light and can hence provide a reflection-type liquid-crystal display device that gives high contrast during viewing in both OFF and ON states, that provides a highly bright display and that creates a display image through the light guide plate that is less disturbed and features high clarity and display quality.
These advantages of the invention are based on the characteristics imparted to the light guide plate. While making intensive studies in order to solve the aforementioned problems, the present inventors found the following facts about light guide plates using the above-mentioned diffusing dots or fine asperities as light issuing means: as shown in
FIGS. 9 and 10
, transmitted light incident on a side of the light guide plate
18
is divergent in almost all directions on account of the scattering by the light issuing means and due to its scattering characteristics, both the light &agr;
1
emerging from the bottom surface and the leakage light &bgr;
3
from the top surface have maximum intensity (B or b) in a direction that forms an angle of about 60 degrees (&thgr;
4
) with the line H dropped normal to the bottom surface (looked into the front surface) and their maximum intensities are substantially the same; hence, the quantity of light in the directions effective for viewing, particularly, in the viewing angle range of from about 15 degrees up to about 30 degrees down in longitudinal direction with reference to said normal line and in the range of about 30 degrees both to the right and the left in a transverse direction, is small enough to provide an insufficiently bright display; in addition, the light &agr;
2
emerging from the bottom surface to form the display image overlaps the leakage light &bgr;
4
from the top surface, thereby producing only low contrast; in OFF state, the scattered light &ggr;
2
washes out the display image to reduce its contrast; what is more, due to the scattering caused by the light guide plate, rays of display light &ggr;
1
and &ggr;
3
mix together and the display image is extensively disturbed.
It has been found that the same problem occurs in the light guide plate having the prism-like light issuing means according to Japanese Patent Publication No. Sho. 62-73206. So much light leaks from the top surface of the light guide plate that it overlaps the display image forming light emerging from the bottom surface, thereby lowering the contrast of the image. In addition, more light emerges at great angles that the light issuing in the directions effective for viewing and, as a result, the brightness of the display decreases to deteriorate its quality.
Therefore, in order for the light guide plate to form a bright and clear display image, it is required that the light incident on a side emerge from the bottom surface with high directivity, particularly at an angle of &thgr;
3
which is the nearest possible to the normal H to the bottom surface as illustrated by the arrow A in
FIG. 4
, preferably with a high degree of convergence within the above-stated viewing angle range. Reflection-type liquid-crystal display devices usually intend to produce uniform and clear displays via a roughened reflective layer having an average diffusing angle of about 5 to 15 degrees. Therefore, if more of the incident light (B in FIG.
9
and &agr;
1
in
FIG. 10
) forms a large angle with the reflective layer, less of the light emerges in the directions effective for viewing and this introduces difficulty in providing a bright display. In addition, large-angle viewing increases the chance of display inversion and massive color charges tend to occur in displays of a field birefringence type.
For higher contrast, it is required that the leakage light from the top surface (as indicated by a in
FIG. 4
) make the least overlap wi
Umemoto Seiji
Yano Shuji
Husar Stephen
Nitto Denko Corporation
Sughrue Mion Zinn Macpeak & Seas, PLLC
LandOfFree
Light guide plate, planer light source unit 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 Light guide plate, planer light source unit and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Light guide plate, planer light source unit and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2530050