Liquid crystal cells – elements and systems – Particular structure – Particular illumination
Reexamination Certificate
2002-05-02
2004-10-19
Nguyen, Dung (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Particular illumination
C349S112000
Reexamination Certificate
active
06806923
ABSTRACT:
This application claims the benefit of Korean Patent Application No. 2001-24170, filed on May 3, 2001 in Korea, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a transmissive liquid crystal display device having a hologram diffuser.
2. Discussion of the Related Art
Liquid crystal display devices, which have properties of small thickness, low weight and low power consumption, are becoming increasingly popular as the information age rapidly evolves. Liquid crystal display devices generally have an array substrate and a color filter substrate, which are spaced apart and face each other. The array substrate includes a plurality of thin film transistors (TFTs), and the color filter substrate includes a color filter layer. Each of the substrates includes an electrode, and the electrodes of substrates face each other. A liquid crystal layer is interposed between the array substrate and the color filter substrate. When a voltage is applied to the liquid crystal layer through the electrodes of the substrates, an alignment of the liquid crystal molecules is changed in accordance with the applied voltage to display image. Because the liquid crystal display devices themselves cannot emit light, they need an additional light source to display images.
The liquid crystal display devices may be classified into three types depending on whether the device has a light source device or not. One type is a transmissive liquid crystal display device that has a backlight device. Another type is a reflective liquid crystal display device that uses ambient light without the backlight device. Still another type is a transflective liquid crystal display device that not only has the backlight device but also uses the ambient light. Among these liquid crystal display device types, the transmissive liquid crystal display device is most common because it can provide invariable brightness when it is placed in dark surroundings.
The liquid crystal display devices need a color filter layer having red (R), green (G) and blue (B) so that the color images can be displayed. The color filter layer contains a dye or pigment to produce the color of red (R), green (G) and blue (B). The thickness of the color filter layer has a proportional relationship with color purity, but an inversely proportional relationship with brightness. Thus, it is difficult to satisfy needs for both color purity and brightness when forming the color filter layer containing the dye or pigment to provide red (R), green (G) and blue (B). To overcome this problem, cholesteric liquid crystal (CLC) has been widely researched and developed in the LCD field to be used as a color filter layer.
The CLC color filter uses a selective transmission or reflection property of the cholesteric liquid crystal. Namely, the CLC color filter does not transmit or reflect all incident light, but selectively transmits or reflects the incident light of a particular wavelength in accordance with a helical pitch of the cholesteric liquid crystal. The transmitted or reflected light may display red (R), green (G) or blue (B) by controlling the helical pitch in accordance with each region of the CLC color filter.
In general, the color that an observer sees when looking at an object can be represented by the wavelength of the light reflected from or transmitted through the object. The wavelength range of visible light is from about 400 nm to about 700 nm. The wavelength of the red light region is about 650 nm, that of green is about 550 nm, and that of blue is about 450 nm. The pitch of the cholesteric liquid crystal is controllable, and therefore, the CLC color filter can selectively transmit or reflect light having the intrinsic wavelength of the color corresponding to a pixel. This enables a pixel to display red (R), green (G) or blue (B) with a high purity. The cholesteric liquid crystal color filter also determines a polarization state of the transmitted or reflected light. The rotational direction of the cholesteric liquid crystal helix is an important factor to a polarization phenomenon. For example, the left-handed cholesteric liquid crystal reflects a left circular polarization that has a wavelength corresponding to the pitch of the left-handed cholesteric liquid crystal. That is, a direction of a circular polarization of the reflected light depends on whether the helix structure of the cholesteric liquid crystal is right-handed or left-handed. Therefore, the CLC color filter has an excellent color purity and contrast ratio, as compared with color filter layers containing the dye or pigment, i.e., absorptive color filters.
As mentioned above, the CLC color filter can selectively transmit the light except reflected portions of the light, which correspond to the helical pitch of the CLC color filter. Therefore, when the CLC color filter is applied to the transmissive liquid crystal display devices, it can display color by way of transmitting the light having the corresponding wavelength. At this point, the transmission wavelength of the CLC color filter can be expressed as follow:
&Dgr;
n=n
e
−n
o
&Dgr;&lgr;=&Dgr;
n•P
&lgr;
peak
=n
avg
•P,
where, n
e
is a refractive index for extraordinary light, n
o
is a refractive index for ordinary light, &Dgr;n is a refractive index anisotropy value of the cholesteric liquid crystal material, P is a helical pitch of the cholesteric liquid crystal, &Dgr;&lgr; is a width of refraction wavelength of the cholesteric liquid crystal (CLC) color filter, &lgr;
peak
is a peak wavelength of the reflected light, and n
avg
is an average refractive index of the cholesteric liquid crystal.
Although the helical pitch of the cholesteric liquid crystal is adjusted for the predetermined color, the displayed color images can be affected by the artificial light from the backlight device. If the incident angle of the light generated from the backlight device is not fixed at a regular value, the refraction wavelength of the CLC color filter can be varied, thereby deteriorating the displayed images. Therefore, a collimated type backlight device, which collects artificial light and directs it in a determined direction, is required. However, such a collimated backlight device can cause a decline in viewing angle due to the fixed emission direction. Thus, a diffuser is applied to the substrate having the CLC color filter to obtain a wide viewing angle. Namely, the narrow viewing angle is compensated by the diffuser.
FIG. 1
is a schematic cross-sectional view of a conventional transmissive liquid crystal display device. As shown in
FIG. 1
, the conventional transmissive liquid crystal display device includes a liquid crystal panel
55
and a backlight device
60
beneath the liquid crystal panel
55
. The liquid crystal panel
55
includes an upper substrate
10
, a lower substrate
40
, and a liquid crystal layer
50
therebetween. On a front surface of a transparent substrate
1
of the lower substrate
40
, a cholesteric liquid crystal (CLC) color filter
42
is disposed. On a rear surface of the transparent substrate
1
of the upper substrate
10
, a hologram diffuser
12
that diffuses artificial light generated from the backlight device
60
is disposed. Array elements “A”, such as thin film transistors, which act as switching elements, are arranged beneath the hologram diffuser
12
. An overcoat layer
14
, which is generally formed of organic substance, is interposed between the hologram diffuser
12
and the array elements “A”. The overcoat layer
14
helps hologram diffuser
12
achieve better diffusion effects and serves as a planarizing layer for flattening the surface of hologram diffuser
12
for optimum performance.
The hologram diffuser
12
has a jagged surface (FIG.
2
). The jagged pattern is formed using laser beams that include first and second beams, for example. When the first and second laser beams are interfered with each other, an interferen
Lee Joun-Ho
Moon Jong-Weon
LandOfFree
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