Liquid crystal cells – elements and systems – Particular structure – Particular illumination
Reexamination Certificate
2000-07-18
2003-07-08
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Particular illumination
C349S059000, C349S065000, C349S067000, C362S035000
Reexamination Certificate
active
06590626
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a liquid crystal display and a backlight unit for use in a liquid crystal display panel or the like and more particularly to a liquid crystal display and a backlight unit which increase a rate of maintenance of luminance and are effective in a reduction in deterioration of luminance.
2. Prior Art
Recently, a liquid crystal display has been widely adopted as a display device for an image display for a personal computer, a television or the like and for various types of monitors. In general, this type of liquid crystal display comprises a backlight placed at the back of a liquid crystal display panel, the backlight being a planar light source for lighting, wherein a liquid crystal surface having a predetermined spread is wholly illuminated with a light of uniform brightness, whereby an image formed on the liquid crystal surface is formed into a visible image.
In this backlight, a hot cathode or cold cathode fluorescent lamp is adopted as the light source. It is necessary to illuminate the whole surface of the liquid crystal display panel with the light from a so-called linear light source by such a fluorescent tube. Thus, two types, i.e., a directly-under-light type and a side-light type (an edge-light type) have been heretofore adopted. The directly-under-light type comprises the fluorescent tube placed directly under the liquid crystal display panel, and a dimmer and a diffuser disposed on the fluorescent tube. On the other hand, the side-light type comprises the fluorescent tube disposed along two sides or one side of a light guide made of a transparent resin, wherein the light entering into the light guide is directed toward the liquid crystal display panel surface by a reflector worked on the rear surface of the light guide, whereby a uniform and planar light is obtained by using light diffusion.
A hot cathode fluorescent tube is the same type as a general fluorescent tube. That is, the hot cathode fluorescent tube comprises a filament composed of a tungsten wire wound into a coil, the filament being coated with an emitter having a high electron emission coefficient. Therefore, the hot cathode fluorescent tube has a high current density and can thus reduce power consumption in electrodes. Accordingly, the hot cathode fluorescent tube is superior in high efficiency and high luminance. On the other hand, a cold cathode fluorescent tube has the same structure as the hot cathode fluorescent tube except for the electrodes. A mechanism for emitting electrons from the electrodes works not by the action of heat but by secondary electron emission. Although the cold cathode fluorescent tube has low efficiency and low luminance, the cold cathode fluorescent tube is therefore superior in long life.
To use these fluorescent tubes as the backlight for the liquid crystal display, the high luminance and high definition are required. Thus, a technique of disposing a plurality of fluorescent tubes adjacent and parallel to one another has been heretofore adopted. For example, the side-light type comprises the backlight composed of six fluorescent tubes in total, three fluorescent tubes being placed at each side of the backlight, each fluorescent tube having an outer diameter of about
3
mm. A plurality of fluorescent tubes are used as the backlight, whereby the liquid crystal display can be adopted not only into a portable personal computer and word processor but also into desktop information equipment and a television receiver which need the brightness. The CRT-level luminance can be therefore ensured even if transmittance of liquid crystal is taken into account.
The prior art using a plurality of fluorescent tubes as the backlight is disclosed in Japanese Patent Laid-Open Sho 62(1987)-234185, Hei 5(1993)-2165, Hei 10(1998)-177170 and so on. In these publications, it is borne in mind that the reflector is disposed in a housing for holding a plurality of fluorescent tubes and the reflector condenses the lights from a plurality of fluorescent tubes, thereby increasing the luminance. Furthermore, Japanese Patent Laid-Open Hei 10(1998)-177170 discloses the technique in which an electrostatic shielding component made of a conductive material such as aluminum is disposed between the cold cathode fluorescent tubes in order that a plurality of cold cathode fluorescent tubes glow with stability, thereby reducing an electrostatic capacity between the cold cathode fluorescent tubes.
A plurality of fluorescent tubes are thus used, whereby the luminance of a fluorescent tube unit comprising a plurality of fluorescent tubes can be increased. However, it became clear that the life of the fluorescent tube unit could not achieve a development goal. That is, although the lighting life of a single fluorescent tube can achieve a design goal, the deterioration of luminance of the whole fluorescent tube unit is considerable and thus the life cannot be ensured from the viewpoint of the deterioration of luminance.
FIG. 9
is a graph of a result of a continuous lighting life test, showing measured data of the side-light type backlight (an LCD module) using a plurality of cold cathode fluorescent tubes and measured data of a single cold cathode fluorescent tube (a single lamp). In the drawing, a vertical axis indicates a rate of maintenance of luminance and a horizontal axis indicates a lighting time. Besides a measured value of the single lamp, the measured values of four LCD modules, each LCD module using a plurality of cold cathode fluorescent tubes, are shown in the drawing. Generally, the life of the backlight is defined as the lighting time that elapses before the luminance drops to 50% of initial luminance. The greater deterioration of luminance means the shorter lighting time that elapses before the luminance drops to 50%. In the drawing, by comparison of the luminance after an elapse of the lighting time of about 2000 hours, it turned out that the luminance of the LCD module severely deteriorates to about 65% with few exceptions when the single lamp has the rate of maintenance of luminance of about 90%. That is, the extent of the deterioration of luminance is considerable in the case of the LCD module of the backlight using a plurality of fluorescent tubes. This becomes a major problem from the viewpoint of an insurance of the life.
The present invention is made in order to solve the above technical problem. It is an object of the present invention to reduce the deterioration of luminance that can occur in the backlight for the liquid crystal display panel.
SUMMARY OF THE INVENTION
As shown in
FIG. 1
, a liquid crystal display of the present invention comprises a light guide
2
placed at the back of a liquid crystal display panel
1
; a first fluorescent tube
3
disposed along at least one side of the light guide
2
; a second fluorescent tube
4
disposed adjacent to the first fluorescent tube
3
; and a shielding component
5
for shielding an incoming light directly entering from the second fluorescent tube
4
into the first fluorescent tube
3
.
The liquid crystal display further comprises a housing
6
having an inner surface
7
forming a reflecting surface, the housing
6
being opened to the light guide
2
, the housing
6
being capable of disposing therein the first fluorescent tube
3
and the second fluorescent tube
4
substantially parallel to each other, wherein the shielding component
5
includes a shield
8
located between the first fluorescent tube
3
and the second fluorescent tube
4
and formed of a part of the housing
6
extending toward the light guide
2
. The liquid crystal display is superior in that a rate of maintenance of luminance of a plurality of fluorescent tubes included in the housing
6
can be brought close to the rate of maintenance of luminance of a single fluorescent tube.
Preferably, the shield
8
extends to substantially the same position as a position d
1
at which the first fluorescent tube
3
and the second fluorescent tube
4
are adjacent to the light guide
2
. Thus, the
Hanzawa Fumihisa
Suzuki Masaru
Kim Robert H.
Schechter Andrew
Scully Scott Murphy & Presser
Trepp Robert M.
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