JP2004294984A - Liquid crystal display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To widen a color reproduction range by using a cold-cathode tube and LEDs in combination, and using the cold-cathode tube for colors of high light emission efficiency of the cold-cathode tube and the LEDs for colors of high light emission efficiency of the LEDs. <P>SOLUTION: In a liquid crystal display device, a light source provided on a liquid crystal panel has the cold-cathode tube which emits light of green and blue selected out of the primary colors of colors and the light emitting diodes emitting lights of red. Red light emitting diodes have a wide color reproduction ranges wherein peaks of bright lines are positioned nearby a spectral track. Consequently, the liquid crystal display device eliminates the trouble that although a wide reproduction range is desired, the color reproduction range is less improved with the cold-cathode tube as a conventional light source and luminance is low when light emitting diodes is substituted for the light source. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a light source of a liquid crystal display device, and more particularly, to using a cold cathode tube and a light emitting diode together as a light source.
[0002]
[Prior art]
Conventional liquid crystal display devices are roughly classified into two types. A reflection type liquid crystal display device and a transmission type liquid crystal display device. The reflection type device is a device that reflects light incident from the surface of the liquid crystal panel on the bottom surface of the liquid crystal panel and recognizes an image by the reflected light. The transmission type device is a light source (back light) provided on the bottom surface of the liquid crystal panel. This is a device for recognizing an image by the emitted light of a light.
[0003]
In a reflection type liquid crystal display device, the visibility is not constant due to a change in environment because an image is visually recognized by external light. However, a transmissive liquid crystal display device is widely used in notebook personal computers, portable televisions, and the like, since the light emitted from a light source is constant and visibility is not easily affected by environmental changes. . Hereinafter, the description will be given on the premise of a transmissive liquid crystal display device, particularly a transmissive liquid crystal display device capable of multicolor or full-color display.
[0004]
In the current liquid crystal display device, the color reproduction range is almost approaching the technical standard of the color reproduction range defined by EBU (European Broadcasting Union: EBU). However, some plasma display panels, liquid crystal projectors, and the like approach the technical standard of the color reproduction range defined by NTSC (National Television Standard Committee: NTSC), which has a wider color reproduction range than the EBU standard. Also, it is desired to expand the color reproduction range.
[0005]
Here, the color reproduction range will be described. FIG. 6 is an xy chromaticity diagram. In the case of displaying a color, there is a method of representing the tristimulus value by the value of Y and chromaticity coordinates x, y. The brightness is represented by Y, and the tint is determined automatically by indicating two of the three ratio values when the mutual ratio of the tristimulus values is x, y, z. Therefore, two values of x and y are represented as chromaticity coordinates. A diagram in which the chromaticity coordinates xy are used as coordinate axes is called an xy chromaticity diagram.
[0006]
In the xy chromaticity diagram, the curve is a spectrum locus, and a straight line connecting both end points of the spectrum locus is called a pure purple locus. In this figure, the color does not appear outside the spectrum locus and the pure purple locus, and the closer the inner color approaches the spectrum locus, the brighter the color becomes. Therefore, a single wavelength is associated with the outermost.
[0007]
Here, when the technical standards defined by NTSC are plotted in this chromaticity diagram, they are as shown in FIG. 6 (2). In this figure, R represents Red, G represents Green, and B represents Blue. The area of the triangle plotted at these three points is the color reproduction range of the NTSC standard. The closer the R, G, and B points are to the outer edge of the spectrum locus, the brighter the color and the wider the color reproduction range.
[0008]
For example, when the point R in FIG. 6B becomes the wavelength of 770 nm in FIG. 6A, the color reproduction range has been expanded to the maximum as Red. Furthermore, the maximum color reproduction range of each color is 520 nm for G and 380 nm for B.
[0009]
As one method for expanding the color reproduction range, an attempt has been made to change the fluorescent material of the cold-cathode tube as a light source (see Non-Patent Document 1). According to this approach, as the phosphor of the CCFL, _Sr 3 _{(PO 4)} blue _{3 Cl: Eu + (Sr,} Ca, Ba) 5 (PO 4) 3 Cl: Eu + BaMg 2 Al 16 O 27 : It is said that the color reproduction range of blue can be improved by mixing and using Eu, using LaPO ₄ : Ce, Tb for green, and using Y ₂ O ₃ : Eu for red.
[0010]
In addition, it is necessary to adjust a color filter in order to expand a color reproduction range using a current cold cathode tube. Since the cold cathode tube has a plurality of extra bright lines in addition to the main bright line of RGB (Red Green Blue), it is necessary to increase the pigment concentration in the color filter so as not to transmit the extra bright lines. There is. However, as a result, there is a problem that the transmittance of the color filter is remarkably reduced and the brightness is lacking.
[0011]
Further, there has been an example in which the color reproduction range is expanded by balancing the spectral distribution, luminous efficiency, and transmission characteristics of a color filter of a cold cathode tube (see Patent Document 1).
[0012]
Therefore, recently, it has been studied to use an RGB three-color light emitting diode (hereinafter, LED) having three main bright lines of RGB as a light source instead of a cold cathode tube (see Patent Document 2). In this example, all the three primary colors of RGB are emitted by the LED, and the light source is used. However, since the luminance is insufficient, white is grayish. Therefore, it is disclosed that the white display cycle is time-divided to emit each color of RGB.
[0013]
[Non-Patent Document 1] IEICE Technical Report EID99-112 (2000-01)
[0014]
[Patent Document 1] Japanese Patent Application Laid-Open No. 2002-277875
[Patent Document 2] Japanese Patent Application Laid-Open No. 11-52327
[Problems to be solved by the invention]
However, although the above IEICE technical report EID99-112 (2000-01) discloses that three types of phosphors are used for blue, in actuality, total phosphors taking into account the life of the phosphors, luminous efficiency, and the like are taken into account. It is best to use BaMg ₂ Al ₁₆ O ₂₇ : Eu alone as the blue phosphor, and the mixing of other blue phosphors is not performed at the current commercial level.
[0017]
Japanese Patent Application Laid-Open No. 2002-277875 only describes that a cold cathode tube is used to approach the NTSC standard, and there is no description about using an LED.
[0018]
Furthermore, Japanese Patent Application Laid-Open No. 11-52327 discloses that all RGB light is emitted by LEDs, but the problem of insufficient brightness of the LEDs themselves has not been solved yet.
[0019]
In particular, the blue and green LEDs have a significantly lower luminous efficiency than the cold cathode fluorescent lamps and poor brightness. Further, among the blue LEDs, those having a peak at a wavelength of 470 nm or less have a problem that productivity is low and cost is high.
[0020]
Therefore, it cannot be said that the Blue and Green LEDs are still in a practical stage.
[0021]
Therefore, an object of the present invention is to use a cold-cathode tube and an LED together, and use a cold-cathode tube for a color having a high luminous efficiency of the cold-cathode tube among the three primary colors, and use an LED for a color having a high luminous efficiency of the LED. An object of the present invention is to widen a color reproduction range by using the color reproduction device.
[0022]
[Means for Solving the Problems]
In order to achieve the above object, according to one aspect of the present invention, in a liquid crystal display device, a light source provided in a liquid crystal panel includes a cold cathode tube that emits two colors selected from three primary colors; And a light-emitting diode for emitting one color.
[0023]
With such a configuration, the cold cathode fluorescent lamp has higher luminous efficiency, and the two colors of green and blue use the cold cathode fluorescent lamp, and the remaining red has the same level of luminous efficiency and improves color purity. By using the LED that can be used, the color reproduction range can be widened.
[0024]
Further, another aspect of the present invention is that the light emitted from the cold-cathode tube is arranged on the first side of the light guide plate for guiding the light to the entire liquid crystal panel, and the light emitting diode is arranged on the second side of the light guide plate. The liquid crystal display device is characterized in that:
[0025]
With such a configuration, it is possible to widen the color reproduction range while maintaining the conventional light source size.
[0026]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the scope of protection of the present invention is not limited to the following embodiments, but extends to the inventions described in the claims and their equivalents.
[0027]
(First Embodiment of the Present Invention)
FIG. 1 is a comparison diagram of the emission spectrum distribution showing the first embodiment of the present invention. FIG. 1A is an emission spectrum distribution diagram when three color phosphors of RGB are used in a conventional cold cathode tube. In this cold cathode tube, LaPO ₄ : Ce, Tb is used as a green phosphor, BaMg ₂ Al ₁₆ O ₂₇ : Eu is used as a blue phosphor, and Y ₂ O ₃ is used as a red phosphor. Eu is enclosed.
[0028]
When this cold cathode tube emits light, a broad blue peak is observed at around 450 nm. At around 430 nm, a peak of mercury that excites the phosphor is observed. Further, a peak of a green emission line is observed at around 540 nm. Then, a subpeak is observed around 580 nm. The Red peak is observed around 615 nm.
[0029]
FIG. 1 (2) is an emission spectrum distribution diagram of a conventional cold cathode tube in which only the blue and green phosphors are sealed. Since the red phosphor is not sealed in the cold cathode tube, a broad blue peak is observed around 450 nm. The peak around 430 nm is the peak of mercury that excites the phosphor. Further, a Green peak is observed around 540 nm.
[0030]
FIG. 1 (3) shows the emission spectrum of only the Red LED. As the Red LED, a light source in which LEDs made of AlGaInP are arranged in an array is used. A peak is observed around 640 nm, and no sub-peak is observed. Compared with the peak of Red by the cold cathode tube in FIG. 1A, it is observed that the position of the peak shifts from around 615 nm to around 640 nm.
[0031]
FIG. 1D is an emission spectrum distribution diagram when an LED array consisting of only AlGaInP is used for Red, and the remaining Blue and Green are displayed by a cold cathode tube. The spectrum distribution is obtained by superposing (2) and (3) in FIG. Compared to FIG. 1A, Blue and Green are the same as in FIG. 1A, but only Red is observed to be shifted to around 640 nm. Therefore, by using the Red LED, the wavelength of 615 nm of Red by the cold-cathode tube approaches 770 nm, which is the maximum wavelength of the color reproduction range of Red.
[0032]
FIG. 2 is a diagram in which only the vicinity of Red in FIG. 6A is cut out and enlarged. The dotted line in this figure is the spectrum locus, and the large arrow is the direction in which the color coordinates improve. According to the present embodiment, since the wavelength of Red is 640 nm, if it is limited to Red, it can be said that it has a color reproduction range that surpasses EBU Standard Red and NTSC Standard Red plotted similarly.
[0033]
As described above, the color reproduction range can be widened by using the Red LED whose wavelength peak shifts to the longer wavelength side and the conventional cold-cathode tube in which the Blue and Green phosphors are sealed.
[0034]
Further, the emission light of the liquid crystal display device which is actually visible is light transmitted through the color filter. The color filter displays a desired color by transmitting only light in a predetermined wavelength range. Even if the color reproduction range of the emitted light is wide, it is meaningless if the color filter absorbs the bright line. Ideally, it is desirable that the peak of the transmittance of the color filter coincides with the peak of the emission line of the emitted light.
FIG. 3 shows the transmittance when the cold cathode fluorescent lamp is illuminated using a conventional color filter to express RGB, and when only Red is illuminated by an LED and Blue and Green express RGB when the cold cathode fluorescent lamp is used. FIG. The conventional color filter referred to here is a color filter used when expressing three colors of RGB with a cold cathode tube.
[0035]
FIG. 3A shows the transmittance and the emission spectrum distribution of the color filter when RGB is expressed by a conventional color filter and a conventional cold cathode tube. The vertical axis is transmittance, and the horizontal axis is wavelength.
[0036]
FIG. 3B shows the transmittance and the emission spectrum distribution when only Red is represented by an LED using a conventional color filter and Blue and Green are represented by a cold-cathode tube, similarly to (1). .
[0037]
Comparing the two, there is no difference between Blue and Green because they are the same cold-cathode tube, but the transmittance of Red by LED shown in (2) is almost the same as that of (1), and the emission of 585 nm in (2). A decrease in the amount is observed. Since the color of this wavelength is an intermediate color between Green and Red, it is an extra sub-peak for both colors. The decrease in the light emission amount of the sub-peak also contributes to widening the color reproduction range.
[0038]
In addition, since there is no particular problem with the transmittance of Red, there is also an advantage that a conventionally used color filter can be continuously used.
(Second embodiment of the present invention)
FIG. 4 is a diagram illustrating an example in which a cold cathode tube and an LED array are arranged on a liquid crystal panel. FIG. 4A is a diagram showing a case where only the conventional cold cathode fluorescent lamp (RGB) 3 is used as a light source for comparison. The liquid crystal panel 1 shows an example of a 17-inch size. Therefore, the long side is 380 mm and the short side is 240 mm. The light emitted from the cold cathode tubes (RGB) 3 is guided by the light guide plate 2 over the entire surface, and is displayed almost uniformly.
[0039]
FIG. 4B shows a preferred embodiment of the present embodiment. The configuration is such that a cold cathode tube (GB) 4 that emits only two colors of green and blue is arranged on the long side of the rectangular light guide plate 2, and a red LED array 5 is arranged on the short side. In this case, since the LED array of Red is extremely small, the size of the light guide plate is 380 mm on the long side and 240 mm on the short side as in the example of FIG. Nothing.
[0040]
However, as shown in FIG. 4C, when the cold cathode tubes (GB) 4 and the red LED array 5 are arranged on the same side of the light guide plate 2, the light guide plate 6 for the cold cathode tubes (GB) 4 is formed. Since the light guide plate 7 for the LED array 5 of Red and Red is required, two light guide plates having a thickness of about 6 mm are required. Moreover, a space for mixing the light of the LED array 5 of Red and the cold cathode tube (GB) 4 is required to be about 5 mm, so that the size of the light guide plate is further increased. Thus, FIG. 4 (3) is not a preferable example, as compared with FIGS. 4 (1) and 4 (2), in that the size is large and the weight is heavy. When the size is compared only for the light source (backlight), FIG. 4 (3) is twice as large as the conventional example (1) and the preferred embodiment (2).
[0041]
Therefore, in the second embodiment, it is preferable to adopt the mode of FIG. In this embodiment, the red LED array 5 may be arranged on the long side, and the cold cathode tubes (GB) 4 may be arranged on the short side.
[0042]
FIG. 5 is a modification of the embodiment shown in FIG. When the size of the liquid crystal panel increases, the luminance may decrease even if the luminous efficiency is high. In such a case, as shown in FIG. 5, a cold cathode tube may be arranged on a first opposite side, and an LED array may be arranged on a second opposite side. In this case as well, only one light guide plate is required, so that even if the liquid crystal panel exceeds the 17-inch size, a reduction in luminance can be prevented and a compact light source can be achieved.
[0043]
Furthermore, in the case of the 17-inch size, the brightness is approximately doubled as compared with the mode of FIG. 4B, so that a bright screen display is possible.
[0044]
As described above, according to the arrangement of the light sources as shown in FIG. 4B or FIG. 5, it is possible to realize a liquid crystal display device in which the size of the light source is the same as the conventional one, and which is bright and has a wide color reproduction range. .
[0045]
As described above, the embodiments are summarized as follows.
[0046]
(Supplementary Note 1) In the liquid crystal display device,
A liquid crystal display device, wherein a light source provided in a liquid crystal panel includes a cold cathode tube that emits two colors selected from three primary colors and a light emitting diode that emits one remaining color.
[0047]
(Supplementary note 2) The liquid crystal display device according to supplementary note 1, wherein the light emitting diode is a red light emitting diode.
[0048]
(Supplementary Note 3) In Supplementary note 2,
The red light emitting diode has a peak wavelength of 630 nm or more.
[0049]
(Supplementary Note 4) In the liquid crystal display device,
A liquid crystal display device, wherein a light source provided on a liquid crystal panel includes a cold cathode tube that emits one of three primary colors and a light emitting diode that emits the remaining two colors.
[0050]
(Supplementary Note 5) In any one of Supplementary Notes 1 to 4,
A liquid crystal display device, wherein the light emitted from the cold-cathode tube is arranged on a first side of a light guide plate for guiding the light to the entire liquid crystal panel, and the light emitting diodes are arranged on a second side of the light guide plate.
[0051]
(Supplementary Note 6) In any one of Supplementary notes 1 to 4,
The light emitted from the cold-cathode tube is arranged on first opposed two sides of a light guide plate for guiding the light to the entire liquid crystal panel, and the light emitting diodes are arranged on second opposed two sides of the light guide plate. Liquid crystal display device.
[0052]
【The invention's effect】
As described above, according to the present invention, a cold-cathode tube and an LED are used together, and a cold-cathode tube is used for a color having a high luminous efficiency of the cold-cathode tube among the three primary colors, and an LED is used for a color having a high luminous efficiency of the LED. By using this, the color reproduction range can be widened.
[Brief description of the drawings]
FIG. 1 is a comparison diagram of an emission spectrum distribution showing a first embodiment of the present invention.
FIG. 2 is a view in which only a portion of a red point in FIG. 6A is cut out and enlarged.
FIG. 3 shows the transmittance between a case where a conventional color filter is used to emit light from a cold-cathode tube to express RGB, and the case where only Red is emitted by an LED and Blue and Green represent RGB when using a cold-cathode tube. FIG.
FIG. 4 is a diagram showing an example in which a cold cathode tube and an LED array are arranged on a liquid crystal panel.
FIG. 5 is a modification of the embodiment shown in FIG. 4 (2).
FIG. 6 is an xy chromaticity diagram.
[Explanation of symbols]
1 liquid crystal panel 2 light guide plate 3 cold cathode tube (RGB)
4 Cold cathode tubes (GB)
5 Red LED array 6 Light guide plate for cold cathode tube (GB) 7 Light guide plate for Red LED array

Claims (5)

In a liquid crystal display device,
A liquid crystal display device, wherein a light source provided in a liquid crystal panel includes a cold cathode tube that emits two colors selected from three primary colors and a light emitting diode that emits one remaining color. 2. The liquid crystal display device according to claim 1, wherein the light emitting diode is a red light emitting diode. In claim 2,
A liquid crystal display device, wherein the red light emitting diode has a peak wavelength of 630 nm or more. In a liquid crystal display device,
A liquid crystal display device, wherein a light source provided on a liquid crystal panel includes a cold cathode tube that emits one of three primary colors and a light emitting diode that emits the remaining two colors. In any one of claims 1 to 4,
A liquid crystal display device, wherein the light emitted from the cold-cathode tube is arranged on a first side of a light guide plate for guiding the light to the entire liquid crystal panel, and the light emitting diodes are arranged on a second side of the light guide plate.

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