Liquid crystal cells – elements and systems – Particular structure – Particular illumination
Reexamination Certificate
2002-03-05
2004-07-20
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Particular illumination
C349S061000, C349S071000
Reexamination Certificate
active
06765634
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a display device and a liquid crystal display device, particularly, to a display device and a liquid crystal display device that perform color display by emitting light from a light source through a color filter layer.
In recent years, liquid crystal display devices are utilized in a wide range of fields from medium-sized and large-sized displays used for computers, television sets and the like to small-sized displays used for car navigation systems and mobile telephones. A liquid crystal display device includes a backlight unit and a liquid crystal display panel. The liquid crystal display device performs image display by the liquid crystal display panel that controls transmission of light from the backlight unit.
Among them, active matrix liquid crystal display devices, which use active elements such as thin film transistors (TFTs) and metal-insulator-metal (MIM), are drawing attention because of excellent display characteristics. An active matrix liquid crystal display device normally includes a TFT array substrate having TFTs as active elements being arrayed in a matrix and an opposing substrate that opposes to the TFT array substrate, and liquid crystal is filled between the two substrates.
The liquid crystal display device includes a display region composed of a plurality of pixels, each of which having a display electrode and a TFT. Light transmittance is varied by application of an electric field to the liquid crystal with the display electrode, thus performing image display. In a color liquid crystal display device, a color filter layer for performing color display is normally provided on the opposing substrate. The color filter layer is composed of three layers of red (R), green (G) and blue (B) side by side, and a black matrix layer formed between these respective color filter layers. Each of the color filter layers transmits light of only a specified range of wavelengths, thus displaying a desired color. Each of the pixels performs color display of any one of R, G and B, whereby an entire display screen can display desired color images.
Regarding a color display device including the liquid crystal display device, two important factors are known from the view point of display quality. One is luminance of the display device, and the other is a reproducible range of colors. In order to perform high-definition display, high luminance and a wide reproducible range of colors are required. Particularly, in light of the reproducible range of colors, such color reproducibility is required to approximate assumed primary color coordinates of a National Television System Committee (NTSC) color TV system as closely as possible. That is, it has been deemed ideal to bring an NTSC ratio to 100%. Here, the NTSC ratio refers to an a real ratio of a triangle of a color reproduction region realized by a display device with respect to an area of a triangle formed by a color reproduction region of NTSC in a chromaticity coordinate system.
Nevertheless, it has been deemed impossible to achieve color reproducibility at an NTSC ratio of 100% with a conventional liquid crystal display device. It is attributed to the fact that widening the range of color reproducibility requires either considerable thickening of the color filter or considerable condensing of photosensitive pigments contained in the color filter layer. Thickening a film of the color filter layer incurs two problems. One of the problems is that optical transmittance is largely reduced by thickening the film of the color filter layer (or by condensing the pigments), and thus sufficient luminance cannot be secured. For this reason, in liquid crystal display devices used in general, an NTSC ratio of an LCD used for a note PC has been limited to about 45%, and an NTSC ratio of a stationary-type liquid crystal display monitor has been limited to about 70%.
Moreover, in order to achieve the NTSC ratio of 100% with a conventional fluorescent light tube and a color filter layer, experiments proved that a color filter layer required a thickness of about 8 micrometers. Such thickness outsteps a boundary of practically produceable color filter layers. Of course, the thickness can be reduced if a pigment density is increased. However, the pigment density also has a certain limitation attributable to curing of a base member (acrylic resin and the like) of the color filter layer. Furthermore, increasing energy supplies to a lamp may raise luminance of the lamp, however, such energy supplies are also limited because of problems concerning heat generation, durability of electrodes and the like.
Accordingly, it has been conceived that fluorescent light tubes with higher luminous efficiency were necessary for achieving a high NTSC ratio and securing sufficient luminance at the same time.
FIG. 10
is a graph showing a radiant energy spectrum from a cold cathode fluorescent light tube and spectral transmittance of a color filter layer in a conventional backlight unit. The graph in the drawing corresponds to a conventional liquid crystal display device of an NTSC ratio of 70%. In
FIG. 10
, the x axis indicates a wavelength of light. The y axis on the left corresponds to a radiant energy spectrum of a lamp, and its unit is an arbitrary unit. The y axis on the right indicates transmittance of a color filter layer. In the drawing, reference numerals
1001
,
1002
and
1003
respectively denote spectral transmittance of a blue filter layer, spectral transmittance of a green filter layer and spectral transmittance of a red filter layer.
In the conventional liquid crystal display device, a tri-phospher fluorescent light tube is used as a light source of the backlight unit. The inside of the fluorescent light tube is coated with three kinds of phosphors, each of which emits light corresponding to RGB, respectively. The backlight unit obtains the light from the light source (the lamp) by allowing the phosphors to emit light. The phosphor s conventionally used are as follows: BaMg
2
Al
16
O
27
:Eu for a blue phosphor; LaPO
4
:Ce,Tb for a green phosphor; and Y
2
O
3
:Eu for a red phosphor, and the like.
FIG. 10
shows radiant energy spectra of the three kinds of the phosphors, namely, BaMg
2
Al
16
O
27
:Eu, LaPO
4
:Ce,Tb and Y
2
O
3
:Eu. In
FIG. 10
, reference numerals
1004
,
1005
and
1006
are a radiant energy spectrum of the blue phosphor, a radiant energy spectrum of the green phosphor and a radiant energy spectrum of the red phosphor, respectively. The blue phosphor possesses a maximum peak of the spectrum in the vicinity of 450 nm. The peaks near 405 nm and 435 nm indicate light emission of Hg filled in the fluorescent light tube. The green phosphor possesses a maximum peak of the spectrum in the vicinity of 545 nm and sub peaks respectively in the vicinity of 490 nm, 590 nm and 620 nm. Note that, light emission of Hg is also observed in the vicinity of 580 nm. The red phosphor possesses a maximum peak of the spectrum in the vicinity of 610 nm.
The inventors of the present invention paid attention in particular to the radiant energy spectrum of the conventional green phosphor. The green phosphor possesses two sub peaks apart from the maximum peak. And the sub peak on the short-wave side is located approximately in the middle of a wavelength region where spectral transmittance curves of the blue color filter layer and the green color filter layer overlap. In addition, the sub peak on the long-wave side is located approximately in the middle of a wavelength region where spectral transmittance curves of the green color filter layer and the red color filter layer overlap.
Each of the light at the sub peaks is recognized as a major factor obstructive to color purity of the liquid crystal display device, because the light at the sub peaks is intensely emitted from both of the blue color filter layer and the green color filter layer, or from both of the green color filter layer and the red color filter layer.
Therefore, it is conceivable that a liquid crystal display device of a high NTSC ra
Fujita Takashi
Horibe Akihiro
Kushida Naoya
Suzuki Masaru
Caley Michael H
Chowdhury Tarifur R.
Pepper Margaret A.
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