Liquid crystal cells – elements and systems – Particular structure – Particular illumination
Reexamination Certificate
2001-06-22
2003-07-15
Dudek, James (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Particular illumination
Reexamination Certificate
active
06593980
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a backlight assembly for a liquid crystal display device and a liquid crystal display device having the same. More particularly, the present invention relates to a liquid crystal display device having a light guiding plate with a novel structure, in which dispersing particles are distributed in the light guiding plate and a light diffusing pattern is formed on a rear portion of the light guiding plate, thereby improving the brightness and the light efficiency of the liquid crystal display device.
2. Description of the Related Arts
A cathode ray tube is generally used as a monitor in a television set, a measuring instrument and an information appliance. Since the cathode ray tube is relatively heavy and voluminous, it does not match with the trend of the miniaturization and light weight requirements of the electronic appliance.
In order to solve the above problem, a liquid crystal display device, which displays information by using the electrical and optical features of liquid crystal filled in a liquid crystal display panel, is developed. The liquid crystal display device not only can match with the miniaturization and light weight requirements of the electronic appliance, but also can reduce the waste of the electric power. Recently, the liquid crystal display device is used in a flat display panel device.
In the liquid crystal display device, a voltage is applied to a predetermined molecule alignment of the liquid crystal so that the molecule alignment of the liquid crystal is changed. By changing the molecule alignment of the liquid crystal, the optical features of liquid crystal cells, such as, birefringence, circumpolarization, dichroism and light scattering features are changed so that an image is variously displayed. That is, the liquid crystal display device modulates the light by using the liquid crystal cells. The liquid crystal display device is classified into a twisted nematic type and a super twisted nematic type. In addition, according to the driving method, the liquid crystal display device classified into an active matrix display type which uses a switching device and a twisted nematic liquid crystal and a passive matrix display type which uses a super twisted nematic liquid crystal.
The liquid crystal filled between a TFT (thin film transistor) substrate and a color filter substrate is a light receiving material which displays information in a screen by adjusting the amount of light incident from an exterior, so the liquid crystal display device requires a separate light source, such as a back light assembly for radiating the light into the liquid crystal display panel.
Since the back light assembly uses a great amount of energy in the liquid crystal display device, the size and the light efficiency of the back light assembly are closely related to mechanical/optical features of the liquid crystal display device. Recently, various kinds of back light assemblies are developed to improve the brightness and the light efficiency of the back light assembly and to reduce the waste of the electric power. The back light assembly is divided into a vertical type and an edge type according to the position of a light source and is divided into a flat type and an inclined type according to the shape of a light guiding plate. Preferably, a back light assembly with the edge type having an inclined light guiding plate is used to reduce the weight, to improve the brightness and to uniformly radiate the light.
Examples of the liquid crystal display devices are disclosed in U.S. Pat. No. 5,502,582 entitled “Light source cooler for LCD monitor”, U.S. Pat. No. 5,791,770 entitled “Light source cooler for LCD monitor” and U.S. Pat. No. 5,825,614 entitled “Compact personal computer with a LCD monitor”.
The amount of the light inputted into the light guiding plate of the back light assembly is different depending on the distance from the light source. In order to solve the above problem, U.S. Pat. No. 5,178,447 discloses the light guiding plate that can improve the brightness of the light and can uniformly distribute the light proceeding in the direction of a LCD panel.
FIG. 1
is a perspective view showing conventional back light assembly for a liquid crystal display device and
FIG. 2
is a sectional view of the liquid crystal display device cut along a line “A
1
-A
2
” portion shown in FIG.
1
.
Referring to
FIGS. 1 and 2
, the back light assembly for a liquid crystal display device consists of a lamp unit
100
and a light guiding unit
200
. The lamp unit
100
includes a lamp
110
as light source and a lamp reflector
120
surrounding the lamp
110
.
A cold cathode tube is preferably used as the lamp
110
. The light generated from the lamp
110
is inputted into a light guiding plate
210
.
At this time, since the light from the lamp
110
is omni-directionally radiated, the light radiated to a direction reverse to the light guiding plate
210
have to be reflected towards the light guiding plate
210
by the lamp reflector
120
so as to improve the light efficiency.
The light guiding unit
200
includes a reflection plate
220
, the light guiding plate
210
, a diffusing sheet and a plurality of light focusing sheets
240
.
The light guiding plate
210
is comprised of a transparent plastic material, such as an acryl resin. The light guiding plate
210
is in the form of a panel having an inclined rear surface, a horizontal light output surface, a light incident surface and a side surface. The light from the lamp
110
proceeds towards a liquid crystal display device (not shown) via the light output surface of the light guiding plate
210
.
At this time, in order to convert the direction of the light towards the liquid crystal display device, a diffusing ink
212
in the form of a dot pattern is printed on the rear surface of the light guiding plate
210
. The diffusing ink pattern
212
turbulently reflects, the light inputted into the light guiding plate
210
. Accordingly, the light inputted into the light output surface has an incident angle less than a critical angle of the light guiding plate
210
, so that the light is easily guided into a liquid crystal display panel (not shown).
In order to improve the uniformity of the light going to the liquid crystal display panel, the density of the diffusing ink pattern
212
varies depending on where it is formed on the rear surface of the light guiding plate. That is, the density of the diffusing ink pattern
212
is higher as it is more remote from the light source.
The reflection plate
220
is formed on the rear surface of the light guiding plate
210
. The diffusing sheet
230
and the focusing sheets
240
are sequentially stacked on the light guiding plate
210
.
The reflection plate
220
reflects the light, which is not reflected by the diffusing ink
212
, towards the light output surface of the light guiding plate, so the loss of the light can be reduced when it is inputted into the liquid crystal display panel and the uniformity of the light passing through the light output surface of the light guiding plate
210
can be improved.
The diffusing sheet
230
positioned between the light guiding plate
210
and the light focusing sheets
240
refracts the output light, which has a predetermined inclination with respect to the light output surface, towards the liquid crystal display panel thereby improving the front brightness.
The light focusing sheets
240
is positioned between the diffusing plate
230
and the liquid crystal display panel. The light focusing sheets
240
has a plurality of sheets in the shape of a triangular prism having a predetermined arrangement. The light focusing sheets
240
are offset from each other by a predetermined angle. The light focusing sheets
240
reduce the visual angle of the light incident from the diffusing sheets
230
so that the front brightness of the light inputted into the liquid crystal display panel can be improved. Accordingly, the electric power can be saved. Generally, the light
Lee Keun-Woo
Lee Kyoung-Don
Dudek James
McGuireWoods LLP
Park Hae-Chan
Samsung Electronics Co,. Ltd.
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