Abstract:
An illumination device including a first light source for emitting light of a first primary color, a second light source for emitting light of a second primary color, and a third light source for emitting light of a third primary color, wherein the first light source is a light emitting diode, the second light source is a fluorescent tube, and the third light source is either a light emitting diode or a fluorescent tube. The illumination device generates white light by mixing the light emitted by the first, second and third light sources. A liquid crystal panel is illuminated with the white light generated by the illumination device.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present invention contains subject matter related to Japanese Patent Application JP 2004-164785 filed in the Japanese Patent Office on Jun. 2, 2004, the entire contents of which being incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an illuminating device for illuminating an object to be illuminated by a white light generated by using a light emitting diode. The present invention also relates to a liquid crystal display device having a liquid crystal panel and an illuminating device for illuminating the liquid crystal panel, and more particularly to a liquid crystal display device having an illuminating device for illuminating a liquid crystal panel by a white light generated by using a light emitting diode. 
     2. Description of Related Art 
     As a display provided for a television, a personal computer, a portable electronic device, etc., a thin light weight liquid crystal display has been widely used. The liquid crystal display has a liquid crystal panel for displaying an image, but the liquid crystal panel is not self-luminous. Therefore, the liquid crystal display has an illuminating device for illuminating the liquid crystal panel, such as a backlight device for illuminating the liquid crystal panel from its back surface. 
     As a light source used frequently in the backlight device, there is a cold cathode fluorescent lamp (hereinafter referred to as “CCFL”). However, since the CCFL uses mercury, when the backlight device is, for example, broken, there is a possibility of giving an adverse influence to an environment, due to flowing out of the mercury or the like. 
     Then, a backlight device using a light emitting diode (hereinafter referred to as “LED”) is proposed (refer, for example, Patent Document 1: Jpn. U.M. Appln. Publication No. 7-36347 and Patent Document 2: PCT Appln. Laid-Open Publication No. 2000-540458). 
     SUMMARY OF THE INVENTION 
     There are, as the LED, a red LED for emitting a red light, a green LED for emitting a green light, and a blue LED for emitting a blue light. As shown by R, G and B in  FIG. 1 , a light emitted from each LED does not include a light having a wavelength except an object wavelength band. Therefore, a white light which contains a small amount of the wavelength except wavelength band showing red, green and blue colors can be obtained by mixing the red light emitted from the red LED, the green light emitted from the green LED, and the blue light emitted from the blue LED. 
     On the other hand, a white light emitted from the CCFL contains, as shown by C in  FIG. 1 , a large amount of the wavelength except the wavelength bands showing red, green and blue colors. 
     The less the amount of the wavelength except the wavelength bands showing red, green and blue colors included in the white light for illuminating the liquid crystal panel is, the higher a color reproducibility of the liquid crystal display device becomes. Therefore, as shown in  FIG. 2 , the liquid crystal panel is illuminated with the white light obtained by mixing the red light, the green light and the blue light emitted from the respective LEDs by using the red LED, the green LED and the blue LED as the light source of the backlight device. Accordingly, the color reproducibility of the liquid crystal display device is improved as compared with the case using a CCFL as a light source. Incidentally, a triangle T 11  in  FIG. 2  shows the color reproducibility of the liquid crystal display device when the red LED, green LED and blue LED are used as a light source of the backlight device, and a triangle T 12  shows the color reprodicibility of the liquid crystal display device when the CCFL is used as the light source of the backlight device. 
     However, it is assumed that the light-emitting efficiency of the red LED and green LED is about 20 lumens/W, and the light-emitting efficiency of the blue LED is about 5 lumens/W. On the other hand, it is assumed that the light-emitting efficiency of the CCFL is about 50 lumens/W. That is, the light-emitting efficiency of each LED is lower than that of the CCFL, and particularly, it is assumed that the light-emitting efficiency of the blue LED is about 1/10 of that of the CCFL. 
     Therefore, when the LED is used as the light source, it is difficult to obtain sufficient luminance, and if the size of the liquid crystal display device is increased to a certain degree, it is difficult to illuminate the liquid crystal panel wholly. 
     Also, since the size of the LED is small, for example, a bottom is a circular shape having a diameter of about 9.6 mm and a height is about 6.09 mm, a range that is illuminated by the emitted light is narrow. Since the range illuminated by the light emitted from the LED is narrow, when the LED is used as the light source of the backlight device, it is necessary to use a plurality of LEDs by aligning the LEDs in a row or in a planar state. 
     However, since the LED is expensive, when the backlight device is manufactured by using many LEDs, a cost for manufacturing the backlight device is increased. Therefore, the liquid crystal display having the backlight device using the LEDs becomes expensive. 
     The present invention is proposed in view of the conventional circumstances as described above, and it is desirable to provide an illuminating device which emits a white light containing a small amount of a wavelength except wavelength bands showing red, green and blue colors and in which an amount of emitting light is sufficient and a cost required to manufacture is low as well as a liquid crystal display device which has high color reproducibility, and in which the liquid crystal panel is illuminated with a sufficient amount of light and a cost required for manufacture is low. 
     According to the present invention, there is provided an illuminating device which generates a white light by mixing a light of a wavelength band showing a first primary color, a light of a wavelength band showing a second primary color and a light of a wavelength band showing a third primary color, and which illuminates a liquid crystal panel with the generated white light, comprising: a first light source for emitting the light of the wavelength band showing the first primary color; a second light source for emitting the light of the wavelength band showing the second primary color; and a third light source for emitting the light of the wavelength band showing the third primary color, wherein the first light source is a light emitting diode, the second light source is a fluorescent tube, and the third light source is either of a fluorescent tube or a light emitting diode. 
     According to the present invention, there is also provided a liquid crystal display device having a transmission type liquid crystal panel, and an illuminating device for illuminating the liquid crystal panel from one main surface side, wherein the illuminating device comprises: a first light source for emitting a light of a wavelength band showing a first primary color; a second light source for emitting a light of a wavelength band showing a second primary color; and a third light source for emitting a light of a wavelength band showing a third primary color, where the first light source is a light emitting diode, the second light source is a fluorescent tube, and the third light source is either of a fluorescent tube or a light emitting diode, and wherein the illuminating device generates a white light by mixing the light of the wavelength band showing the first primary color, the light of the wavelength band showing the second primary color and the light of the wavelength band showing the third primary color, and illuminates the liquid crystal panel with the generated white light. 
     In the illuminating device according to the present invention, one or two of the red light, the green light and the blue light as origins of the white light to be emitted are emitted from the fluorescent tube, and the residual two or one is emitted from the light emitting diode. The light emitted from the fluorescent tube contains a large amount of light, and the light emitted from the light emitting diode contains a small amount of light except the object wavelength band. 
     Therefore, the white light emitted from the backlight device according to the present invention contains a small amount of light of the wavelength except the wavelengths showing the red, green and blue colors and a large amount of light. That is, in accordance with the backlight device according to the present invention, by illuminating the liquid crystal panel with the white light which contains a small amount of the light of the wavelength except the wavelengths showing red, green and blue colors and a high luminance, the color reproducibility of the image displayed on the liquid crystal display device can be improved and an image displayed on the display device can be clearly shown. 
     Since the fluorescent tube is less expensive than the light-emitting diode necessary to emit the light of the same light amount, the backlight device according to the present invention can suppress the cost for its manufacture by using the fluorescent tube as the light source. 
     Also, since the fluorescent tube generates a little heat as compared with a light emitting diode, the backlight device according to the present invention does not need to place a cooler, such as a fan, etc., by using the fluorescent tube as the light source, which can simplify its construction. 
     The liquid crystal display device according to the present invention emits one or two of the red, green and blue colors, which are origins of the white light emitted from the backlight device, from the fluorescent tube, and the residual two or one is emitted from the light emitting diode. 
     Therefore, in the liquid crystal display device according to the present invention, the backlight device emits the white light having a little light of a wavelength other than the wavelength showing the red, green and blue colors and having a high luminance and illuminates the liquid crystal panel. That is, according to the backlight device of the present invention, since the liquid crystal panel is illuminated by the white light having a little light of the wavelength other than the wavelength showing the red, green and blue colors and having high luminance, the color reproducibility of the displayed image is improved and the image is displayed clearly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing wavelength distribution of a red light, a green light and a blue light emitted from LEDs and wavelength distribution of a white light emitted from a CCFL for emitting a white light; 
         FIG. 2  is a view showing the color reproducibility of a conventional backlight device; 
         FIG. 3  is a schematic perspective view showing a structure of a liquid crystal display device according to the present invention; 
         FIG. 4  is a block diagram showing a drive circuit of the liquid crystal display device; 
         FIG. 5  is a plan view showing the backlight device according to the present invention; 
         FIG. 6  is a sectional view showing the backlight device; 
         FIG. 7  is a plan view showing a light source unit provided in the backlight device; 
         FIG. 8  is a sectional view showing an optical element provided between an optical wavelength and a color mixer; 
         FIG. 9  is a view showing wavelength of the red light emitted from the red LED and wavelength of the green light emitted from the green LED; 
         FIG. 10  is a view showing the color reproducibility of the liquid crystal display device having the backlight device; 
         FIG. 11  is a plan view showing another backlight device according to the present invention; 
         FIG. 12  is a partly cutout perspective view showing still another backlight device according to the present invention; 
         FIG. 13  is a plan view showing arrangement of red LED, green LED and blue CCFL disposed in the light source section of the backlight device; 
         FIG. 14  is a plan view showing another arrangement of the red LED, the green LED and the blue CCFL provided in the light source section; 
         FIG. 15  is a plan view showing still another arrangement of the red LED, the green LED and the blue CCFL provided in the light source section; and 
         FIG. 16  is a plan view showing still another arrangement of the red LED, the green LED and the blue CCFL provided in the light source section. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described in detail while referring to the drawings. 
     A best mode for carrying out the present invention will be described in detail while referring to the drawings. 
     The present invention is applied to a back light type liquid crystal display device  100  of a structure as shown, for example, in  FIG. 3 . In the embodiments of the present invention, the liquid crystal display device  100  has an aspect ratio of 9:16 and a size of 17 inches. 
     The liquid crystal display device  100  comprises a transmission type liquid crystal panel  10 , and a backlight device  20  provided on one main surface side (hereinafter referred to as “back surface side”) of the liquid crystal panel  10 . The user observes an image projected on the liquid crystal panel  10  from the other main surface side (hereinafter referred to as “front surface side”). 
     The liquid crystal panel  10  has a structure that a TFT substrate  11  and an opposed electrode substrate  12  which are two transparent substrates each made of glass or the like are disposed oppositely to each other, and a liquid crystal layer  13  in which a twisted nematic liquid crystal is sealed in the gap between the TFT substrate  11  and the electrode substrate  12 . 
     Signal lines  14  and scanning lines  15  disposed in a matrix state are formed on the TFT substrate  11 . Thin film transistors  16  as switching elements disposed at intersection points of the signal lines  14  and the scanning lines  15  and pixel electrodes  17  are formed on the TFT substrate  11 . The thin film transistors  16  are sequentially selected by the scanning lines  15 , and video signals supplied from the signal lines  14  are written in the corresponding pixel electrodes  17 . 
     On the other hand, the opposed electrodes  18  and color filters  19  are formed on an inner surface of the opposed electrode substrate  12 . Incidentally, in the liquid crystal panel  10 , the side disposed with the TFT substrate  11  is defined as the back surface side, and the side disposed with the opposed electrode substrate  12  is defined as the front surface side. 
     In this liquid crystal display device  100 , the liquid crystal panel  10  of the structure as described above is sandwiched between two polarizing plates  25  and  26 , and active matrix driven by the backlight device  20  in a state that the white light is illuminated from the back surface side, thereby obtaining a desired full-color video display. Incidentally, the backlight device  20  will be described in detail later. 
     This liquid crystal display device  100  is driven by a drive circuit  200  showing an electrical block structure shown, for example, in  FIG. 4 . 
     The drive circuit  200  comprises a power source  110  for supplying a drive power of the liquid crystal panel  10  and the backlight device  20 , an X driver circuit  120  and a Y driver circuit  130  for driving the liquid crystal panel  10 , an RGB processor  150  to which a video signal is supplied from the outside through an input terminal  140 , a video memory  160  and a controller  170  connected to this RGB processor  150 , a back light drive controller  180  for controlling to drive the backlight device  20 , etc. 
     In this drive circuit  200 , the video signal inputted through the input terminal  140  is signal processed by chroma processing, etc., by the RGB processor  150 , further converted into an RGB separate signal suitable for driving the liquid crystal panel  10  from a composite signal, and supplied to the controller  170 , and supplied to the X driver circuit  120  through the video memory  160 . Also, the controller  170  controls the X driver circuit  120  and the Y driver circuit  130  at a predetermined timing in response to the RGB separate signal, and drives the liquid crystal panel  10  according to the RGB separate signal supplied to the X driver circuit  120  through the video memory  160 , and thereby displaying the video in response to the RBG separate signal. 
     Then, the backlight device  20  will be described. 
     First Embodiment 
     First, a first embodiment of the backlight device  20  according to the present invention will be described. 
     As shown in  FIG. 5 , the backlight device  20  comprises a flat plate-like optical waveguide  31 , blue light emitting units  32   a ,  32   b  provided at both end faces  31   a ,  31   b  of a width direction of the optical waveguide  31 , and yellow light emitting units  33   a ,  33   b  provided at both end faces  31   c ,  31   d  of a longitudinal direction of the optical waveguide  31 . 
     As shown in  FIG. 6 , the backlight device  20  has a diffusing sheet  34 , a first lens sheet  35 , and a second lens sheet  36  sequentially disposed at a light emitting surface  31   e  side of a main surface of the liquid crystal panel  10  side of the optical waveguide  31 , and a reflecting sheet  37  disposed at the reflecting surface  31   f  side of an opposed surface to the light emitting surface  31   e . Incidentally,  FIG. 6  is a cross-sectional view taken along the line A-A′ in  FIG. 5 . 
     The optical waveguide  31  is a transparent plate having a predetermined thickness. The aspect ratio and the size of the optical waveguide  31  is the same as those of the liquid crystal panel  10 . In this embodiment, the aspect ratio of the liquid crystal panel  10  is 9:16 and the size thereof is 17 inches. Accordingly, the aspect ratio of the optical waveguide  31  is 9:16 and the size thereof is 17 inches. The aspect ratio and the size of the optical waveguide  31  do not define the present invention. 
     The optical waveguide  31  generates a white light while fully reflecting, guiding and mixing a yellow light incident from both the end faces  31   c ,  31   d  of a longitudinal direction and a blue light incident from both the end faces  31   c ,  31   d  of a width direction, and emits the white light generated from the light emitting surface  31   e  of one man surface of the optical waveguide  31 . 
     The optical waveguide  31  is formed by injection molding a transparent thermoplastic resin, such as an acrylic resin, a methacrylic resin, a styrene resin, and a polycarbonate resin, etc. Also, a fine uneven shape, such as a prism pattern, a dot pattern, etc., is formed on a light reflecting surface  31   f  of the other main surface of the optical waveguide  31 , and is processed to efficiently emit a light guided in the optical waveguide  31  in the direction of the light emitting surface  31   e . The light incident in the optical waveguide  31  is emitted as a uniform light from the light emitting surface  31   e  entirety by this prism pattern, the dot pattern, etc. 
     A diffusing sheet  34  diffuses the white light emitted from the light emitting surface  31   e  to a uniform light. Also, the first lens sheet  35  and the second lens sheet  36  control to orient the light emitted from the diffusing sheet  34  to condense to the liquid crystal panel  10  side, that is, a front surface side. 
     A reflecting sheet  37  reflects the light emitted from the light reflecting surface  31   f  of the light guided by the optical waveguide  31 , and returns the light to the inside of the optical waveguide  31 . In the backlight device  20 , the reflecting sheet  37  reflects the light emitted from the light reflecting surface  31   f  to return the light to the inside of the optical waveguide  31 , thereby suppressing the loss of the amount of the light emitted from the light emitting surface  31   e  due to the flying out of the light from the optical waveguide  31 . 
     The blue light emitting units  32   a ,  32   b  respectively have a cold cathode fluorescent lamp (hereinafter referred to as a “CCFL”)  41 B emitting the blue light. The blue CCFL  41 B has a length substantially equal to a longitudinal direction of the optical waveguide  31 , and is disposed substantially parallel to the longitudinal direction of the optical waveguide  31 . The blue lights emitted from the blue CCFLs  41 B provided in respective blue light emitting units  32   a ,  32   b  are incident from both end faces  31   a ,  31   b  of the width direction of the optical waveguide  31 , and guided by the optical waveguide  31 . The blue CCFL  41 B is coated on its inner surface, for example, with a phosphor which emits a blue color, such as, for example, BaMg 2 Al 16 O 27 :Eu, etc. 
     The light emitting efficiency of the blue CCFL  41 B is said to be about 10 lumens/W. On the other hand, the light emitting efficiency of the light emitting diode (hereinafter referred to as “LED”) for emitting a blue light is said to be about 5 lumens/W. That is, when the same power is consumed, the light emitted from the blue CCFL  41 B becomes about twice as large as the blue light emitted from the LED which emits the blue light. Therefore, by using the blue CCFL  41 B, while the power consumption remains set constant, an amount of the light emitted from the backlight device  20  can be increased. 
     The CCFL has a cheap cost as compared with the LED of the number needed to emit the light of the same amount. Therefore, the backlight device  20  can suppress its cost for manufacture by using the blue CCFL  41 B as a light source. 
     Furthermore, the CCFL generates a little heat as compared with the LED. Therefore, the backlight device  20  eliminates necessity of carrying a cooler, such as a fan by using the blue CCFL  41 B as a light source, which can simplify the structure. 
     Also, since the light emitted from the CCFL is small in directivity and good in spreading, the blue lights emitted from the blue light emitting units  32   a ,  32   b  spread over the optical waveguide  31  entirety, and is easily mixed with other light. 
     The yellow light emitting units  33   a ,  33   b  respectively have a plurality of light source units  42   a - 1  to  42   a - 10  aligned in one row along one end face of a width direction of the optical waveguide  31 , and light source units  42   b - 1  to  42   b - 10  aligned in one row along the other end face. Incidentally, in the foregoing description, if it is not necessary to distinguish, the light source units  42   a - 1  to  42   a - 10 , and the light source units  42   b - 1  to the light source units  42   b - 10  are totally termed as a light source unit  42 . 
     As shown in  FIG. 7 , the light source unit  42  has a red LED (Light Emitting Diode)  43 R, a green LED  43 G (hereinafter totally referred to an LED  43 ). The light source unit  42  has a red light collective lens  44 R disposed at the light emitting surface side of the red LED  43 R, and a green light collective lens  44 G disposed at the light emitting surface side of the green LED  43 G (hereinafter totally referred to a collective lens  44 ). The light source unit  42  has a color mixer  47  having a reflecting prism  45  disposed at the light emitting surface side of the red light collective lens  44 R, and a dichroic prism  46  disposed at the light emitting surface side of the green light collective lens  44 G The light source unit  42  has an optical member  48  disposed at the light emitting side of the color mixer  47 . 
     The red LED  43 R emits a light of a wavelength band showing a red color, the green LED  43 G emits a light of a wavelength band showing a green color. In this embodiment, as the red LED  43 R and the green LED  43 Q the LED having a light emitting efficiency of about 20 lumens/W is used. 
     The red light collective lens  44 R introduces a diffusing light included in the red light emitted from the red LED  43 R as a parallel light to the reflecting prism  45 . The green light collective lens  45 R introduces a diffusing light included in the green light emitted from the green LED  43 G as a parallel light to the dichroic prism  46 . The collective lens  44  sets the diffusing light included in the light emitted from the LED  43  to a parallel light to prevent the light emitted from the LED  43  from being leaked without being incident to the reflecting prism  45  and the dichroic prism  46 , and to introduce the light emitted from the LED  43  efficiently to the optical waveguide  31 . 
     The color mixer  47  mixes the red light incident to the reflecting prism  45  as the parallel light by the red light collective lens  44 R with the green light incident to the dichroic prism  46  as the parallel light by the green light collective lens  44 G, generates the light of the wavelength band showing a yellow color, and emits the light. 
     The reflecting prism  45  has a light incident surface  45   a , a light emitting surface  45   b  provided perpendicularly crossed with the light incident surface  45   a , and a light reflecting surface  45   c  provided at an angle of 45° to both the light incident surface  45   a  and the light emitting surface  45   b.    
     The light is incident on the light incident surface  45   a . The light reflecting surface  45   c  refracts a light incident from the light incident surface  45   a  at 90°, and advances the light in a light emitting surface  45   b  direction. The light emitting surface  45   b  emits the light reflected by the light reflecting surface  45   c.    
     The reflecting prism  45  is opposed to the red light collective lens  44 R at the light incident surface  45   a , and disposed to be opposed to the dichroic prism  46  at the light emitting surface  45   b . That is, in the reflecting prism  45 , the red light emitted from the red LED  43 R and then made as a parallel light by the red light collective lens  44 R is incident to the light incident surface  45   a , refracted at 90° by the light reflecting surface  45   c , and then emitted from the light emitting surface  45   b.    
     In this embodiment, as the reflecting prism  45 , a right-angle prism in which one of two surfaces for interposing a right angle is set as a light incident surface  45   a , and the other is set as a light emitting surface  45   b , and an oblique surface is set as a light reflecting surface  45   c , is used. 
     The dichroic prism  46  has a first light incident surface  46   a , a second light incident surface  46   b  provided perpendicularly crossed with the first light incident surface  46   a , a bonding surface  46   c  provided at an angle of 45° to both the first light incident surface  46   a  and the second light incident surface  46   b , and a light emitting surface  46   d  provided at an angle of 45° to the bonding surface  46   c  perpendicularly crossed with the second light emitting surface. 
     The light is incident on the first light incident surface  46   a  and the second light incident surface  46   b . The bonding surface  46   c  selectively transmits a light of the wavelength band showing a green color of the light incident in the dichroic prism  46 , and selectively reflects the light of the wavelength band showing a red color of the light incident in the dichroic prism  46 . The light emitting surface  46   d  emits a red light reflected by the bonding surface  46   c  and a green light transmitted through the bonding surface  46   c.    
     The dichroic prism  46  is disposed so that the first light incident surface  46   a  is opposed to the green light collective lens  44 G and the second light incident surface  46   b  is opposed to the light emitting surface  45   b  of the reflecting prism  45 . 
     Therefore, to the dichroic prism  46 , the green light set to a parallel light by the green light collective lens  44 G after emitted from the green LED  43 G is incident from the first light incident surface  46   a , and the red light emitted from the reflecting prism  45  is incident from the second light incident surface  46   b . The bonding surface  46   c  transmits the green light incident from the first light incident surface  46   a , reflects the red light incident from the second light incident surface  46   b , and refracts the red light at 90°, thereby advancing the green light and the red light in the light emitting surface  46   c  direction. The green light transmitted through the bonding surface  46   c  and the red light reflected by the bonding surface  46   c  are mixed to a yellow light, which is emitted from the light emitting surface  46   c.    
     An optical element  48  guides the light emitted from the color mixer  47  to both end faces  31   c ,  31   d  of a longitudinal direction of the optical waveguide  31 . Since the optical element  48  is provided, if a length L 1  along a thickness direction of the optical waveguide  31  of the dichroic prism  46  is longer than a thickness L 2  of the optical waveguide  31 , as shown in  FIG. 8 , the fact that the yellow light emitted from the color mixer  47  is not completely introduced to the end faces  31   c ,  31   d  of the longitudinal direction of the optical waveguide  31  but leaked to the outside, can be reduced. 
     The yellow light emitted from the light emitting surface  46   c  of the dichroic prism  46  is incident to the outside of the optical waveguide  31  from both the end faces  31   c ,  31   d  of the longitudinal direction of the optical waveguide  31  as the yellow light emitted from the light source unit  42 . 
     Therefore, the blue lights emitted from the blue light emitting units  32   a ,  32   b  are incident from both the end faces  31   a ,  31   b  of the width direction, and the yellow lights emitted from the yellow light emitting units  33   a ,  33   b  are incident from both end faces  31   c ,  31   d  of the longitudinal direction. 
     The yellow light incident to the optical waveguide  31  is obtained by mixing the red light emitted from the red LED  43 R and the green light emitted from the green LED  43 G As shown by R in  FIG. 9 , the red light emitted from the red LED  43 R hardly includes a light of the wavelength except the wavelength band showing the red color, and has a high color purity. The green light emitted from the green LED  43 G hardly includes the light of the wavelength except the wavelength band showing the green color, as shown by G in  FIG. 9  and has a high color purity. 
     Therefore, to the optical waveguide  31 , the red light having a high color purity, and the green light having a high color purity are incident. That is, the lights incident from the yellow light emitting units  33   a ,  33   b  to the optical waveguide  31  contain less light of the wavelength except the wavelength band showing the red color and the green color. Therefore, the white light obtained by mixing in the optical waveguide  31  contains less light of the wavelength except the wavelength bands showing the red, green and blue colors as compared with the white light emitted from the CCFL. 
     When the liquid crystal display device  1  having the above-mentioned backlight device  20  and the conventional liquid crystal display device having the backlight device using the CCFL for emitting the white light as the light source are measured for the color reproducibility, and when the measured result is shown by an XYZ display color system proposed by a CIE, as shown in  FIG. 10 , in the liquid crystal display device  1 , as compared with the conventional liquid crystal display device, a value of Y coordinates of a chromaticity point showing the red color is remarkably raised, and a value of the X coordinates of the chromaticity point showing the green color are remarkably raised. Furthermore, an NTSC ratio is raised from 71% to 111%. Incidentally, a triangle T 1  in  FIG. 10  shows the color reproducibility of the liquid crystal display device  1  according to the present invention, and a triangle T 2  shows the color reproducibility of the conventional liquid crystal display device. 
     More particularly, the liquid crystal display device  1  has high color reproducibility as compared with the conventional liquid crystal display device having the backlight device using the CCFL for emitting the white light as the light source, and can clearly display the image. 
     As described above, the backlight device  20  emits the white light obtained by mixing the red light emitted from the red LED  43 R, the green light emitted from the green LED  43 G, and the blue light emitted from the blue CCFL  41 B. That is, the backlight device  20  can emit the white light having a light of small wavelength except the wavelength band showing the red, green and blue colors as compared with the backlight device using only the CCFL for emitting the white light as the light source and can illuminate the liquid crystal panel  10 . Therefore, the liquid crystal panel  10  is illustrated by the backlight device  20  according to the present invention, the color reproducibility of the liquid crystal display device  1  can be raised. 
     Also, the backlight device  20  uses the blue CCFL  41 B as the light source for emitting the blue light. Therefore, as compared with the case that the LED is used as the light source for emitting the blue light, an amount of the emitting light is large. 
     Since the blue CCFL  41 B has a lower cost than the LED which becomes necessary to emit the blue light of the same amount, the backlight device  20  can suppress a cost for its manufacture by using the blue CCFL  41 B as the light source. 
     Since the blue CCFL  41 B generates a little heat as compared with the LED, the backlight device  20  does not need to carry a cooler, such as a fan, etc., by using the blue CCFL  41 B as the light source, which can simplify its structure. 
     Since spread of the light emitted from the blue CCFL  41 B is good, the backlight device  20  can easily mix the yellow light with the blue light. 
     Since the LED for emitting the blue light has an unevenness in luminance of the emitted blue light, when the backlight device  20  using the LED for emitting the blue light is mounted, the color reproducibility of the liquid crystal display device  1  brings about an unevenness. In the backlight device  20 , the unevenness of the luminance of the blue light incident in the optical waveguide  31  can be reduced by using the blue CCFL  41 B as the light source for emitting the blue light. 
     Incidentally, in this embodiment, the color mixer  47  is constructed by combining the prisms. However, the color mixer  47  may be constructed except the prisms. For example, the color mixer  47  may be constructed by combining, for example, a reflector or a beam splitter. 
     Second Embodiment 
     Incidentally, the liquid crystal display device  1  may have a backlight device  50  shown in  FIG. 11 , instead of the backlight device  20 . In the description below, as a second embodiment of the present invention, the backlight device  50  according to the present invention will be described. In the following description, the same component parts as the backlight device  20  described in the first embodiment are given by the same reference numerals as those of the backlight device  20 , and the description will be omitted. 
     As shown in  FIG. 11 , the backlight device  50  comprises a flat plate-like optical waveguide  31 , blue green light emitting units  51   a ,  51   b  provided at both ends  31   a ,  31   b  of a width direction of the optical waveguide  31 , and red light emitting units  52   a ,  52   b  provided at both ends  31   c ,  31   d  of a longitudinal direction of the optical waveguide  31 . 
     The backlight device  50  has a diffusing sheet  34 , a first lens sheet  35 , and a second lens sheet  36  sequentially disposed at a light emitting surface  31   e  side of a main surface of the liquid crystal panel  10  side of the optical waveguide  31 , and a reflecting sheet  37  is disposed at a reflecting surface  31   f  side of an opposed surface to the light emitting surface  31   e.    
     The blue green light emitting units  51   a ,  51   b  have a blue CCFL  41 B for emitting a blue light, and a green CCFL  41 G for emitting a green light. The blue green light emitting units  51   a ,  51   b  emit blue green lights obtained by mixing a blue light emitted from the blue CCFL  41 B and a green light emitted from a green CCFL  41 G The blue CCFL  41 B and the green CCFL  41 G are set to substantially the same length as a longitudinal direction of the optical waveguide  31 , and disposed substantially parallel to the longitudinal direction of the optical waveguide  31 . The green CCFL  41 G is coated on its inner surface with a phosphor for emitting a green color, for example, an LaPO 4 :Ce, Th, etc. 
     The light emitted from the CCFL has small directivity and good spread. Therefore, the blue green light emitted from the blue green light emitting unit  41 G spreads over the entirety in the optical waveguide  31 . 
     The red light emitting units  52   a ,  52   b  have a plurality of red LEDs  43 R. The red LED  43 R are aligned in a row along the both end faces  31   c ,  31   d  of the width direction of the optical waveguide  31 . The eight red LEDs  43 R are disposed at the respective red light emitting units  52   a ,  52   b  in this embodiment. 
     Therefore, to the optical waveguide  31 , blue green lights emitted from the blue green light emitting units  51   a ,  51   b  are incident from both ends faces  31   a ,  31   b  of the width direction, and red lights emitted from the red light emitting units  33   a ,  33   b  are incident from both end faces  31   c ,  31   d  of the longitudinal direction. 
     Since the red light incident to the optical waveguide  31  is a red light emitted from the red LED  43 R, the red light hardly contains the light of the wavelength except the wavelength band showing the red color, and has a high color purity. 
     Therefore, since the optical waveguide  31  hardly contains the light of the wavelength except the wavelength band showing the red color and the red light having the high color purity is incident on the optical waveguide  31 , the white light obtained by mixing in the optical waveguide  31  contains less light of the wavelength except the wavelength band showing the red, green and blue colors as compared with the white light emitted from the backlight device using the CCFL for emitting the white light as a light source. That is, the color reproducibility of the liquid crystal display device  1  can be raised by illuminating the liquid crystal panel  10  by the backlight device  50  according to the present invention. 
     Third Embodiment 
     The liquid crystal display device  1  may also have a backlight device  70  shown in  FIG. 12 , instead of the backlight devices  20  and  50 . In the description below, as a third embodiment of the present invention, the backlight device  70  according to the present invention will be described. Incidentally, in the following description, the same component parts as in the backlight device  20  described in the first embodiment are given by the same reference numerals as the backlight device  20 , and the description will be omitted. 
     As shown in  FIG. 12 , the backlight device  70  comprises a housing  71  of a substantially rectangular parallelepiped shape, and a light source  72  provided at a bottom  71   a  of the housing  71 . The backlight device  70  is of so-called a direct backlight, and the light emitted from the light source  72  is emitted from the entire upper surface of the housing  40  to perform surface emission to illuminate the liquid crystal panel  10 . 
     In this embodiment, the bottom  71   a  and four side faces of the housing  71  are formed by a reflecting plate  73 , and an upper surface of the housing  71  is formed by a diffusing plate  74 . 
     When the light emitted from the light source  72  is incident to the reflecting plate  73 , the reflecting plate  73  reflects the incident light to propagate the light in a direction where the diffusing plate  74  is provided. 
     The light emitted from the light source  72  or the light reflected by the reflecting plate  73  is incident on the diffusing plate  74  to diffuse the incident light and to emit the light from the entire main surface. The liquid crystal panel  10  is illuminated by the light emitted from the entire main surface of the diffusing plate  74 . 
     The light source  72  has a blue CCFL  41 B disposed in parallel with the longitudinal direction of the housing  71 , a red LED array  80 R having a plurality of red LEDs  43 R disposed and aligned in one row in parallel with the blue CCFL  41 B, and a green LED array  80 G having a plurality of green LEDs  43 G disposed and aligned in one row in parallel with the row of the red LEDs  43 R. The blue CCFL  41 B, the red LED array  80 R and the green LED array  80 G are disposed alternately at the bottom  71   a  of the housing  71 . 
     Incidentally, the arrangement of the blue CCFL  41 B, the red LED  43 R and the green LED  43 G are not limited to the above-mentioned arrangement. For example, as shown in  FIG. 14 , blue CCFLs  41 B arranged parallel to the longitudinal direction of the housing  71  and LED arrays  80 RG in which the red LEDs  43 R and green LEDs  43 G are arranged alternately in a row may be alternately aligned. 
     Also, as shown in  FIG. 15 , one row of the red LED array  80 R and the two rows of green LED array  80 G may be disposed parallel to the blue CCFL  41 B disposed parallel to the longitudinal direction of the housing  71 . Also, as shown in  FIG. 16 , blue CCFLs  41 B arranged parallel to the longitudinal direction of the housing  71  and LED arrays  80 RGG in which the two green LEDs  43 G and one red LED  43 R are arranged alternately in a row may be alternately aligned. 
     The number of the green LEDs  43 G are doubled, thereby increasing a ratio of the green light included in the white light emitted from the backlight device  70 , and an image displayed on the liquid crystal display device  1  becomes clear. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.