1
PLANAR LIGHT SOURCE DEVICE AND
LIQUID CRYSTAL DISPLAY DEVICE USING
THE SAME
BACKGROUND OF THE INVENTION 5
1. Field of the Invention
The present invention relates to a planar light source device and a liquid crystal display device, and more specifically to a liquid crystal display device with enhanced 10 viewing angle characteristics.
2. Description of Related Art
A liquid crystal display device has a liquid crystal panel comprising two substrates with a liquid crystal layer interposed therebetween. Aplanar light source device is mounted 15 on the back of the liquid crystal panel. A twisted nematic (TN) mode is widely used in the liquid crystal panel, where liquid crystal molecules are aligned substantially parallel to the substrates in a quiescent state with a twist angle of given value and twist at the substrates according to an applied 20 signal voltage.
A light source of the planar light source device is conventionally a linear cold-cathode tube. Recently, a light emitting diode (LED) also comes into use because of its longer life and good light emission. When using the LED as 25 a light source, three LEDs of red, green, and blue are provided, each in a plurality of numbers. The angles of light from the three colors of the LEDs are substantially the same. The light emitting angle of light sources affects the viewing angle of the planar light source; a wider emission angle of 30 light from the LED allows a wider viewing angle. Accordingly, the wavelength dependence of luminance at each angle of viewing direction in the conventional planar light source device using the LEDs is as shown in FIG. 11A. The luminance decreases as an angle of viewing direction 35 becomes larger but remains substantially constant for any wavelength.
FIG. 11B shows the wavelength dependence of luminance at each angle of viewing direction in a conventional TN mode liquid crystal display device having the planar light 40 source device using the LEDs. When viewing the liquid crystal display device from the same angle, the luminance of shorter wavelength light is lower relative to that of longer wavelength light, resulting in a reddish tone over all. Further, as the angle of viewing direction increases, the lumi- 45 nance differs more significantly depending on wavelength, causing color shift problem.
As described above, the conventional liquid crystal display device has a problem of color shift depending on angles of viewing direction. 50
SUMMARY OF THE INVENTION
The inventors have found that when a planar light source device using LEDs is applied to a TN mode liquid crystal 55 display device, the viewing angle characteristics of the liquid crystal display device are determined by the relation of the viewing angle characteristics of the planar light source device and a liquid crystal display panel. In the TN mode liquid crystal panel, a light transmittance varies depending 60 on a viewing direction and wavelength. FIG. 10 shows the wavelength dependence of transmittance at each viewing direction in the TN mode liquid crystal panel. In FIG. 10, the horizontal axis is a wavelength and the vertical axis is a transmittance of the liquid crystal panel. The transmittance 65 when viewing from a display normal (at the angle of 0 degree) is standardized as 1, and the transmittances at the
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angles of 20, 40, and 60 degrees with respect to the normal to the panel are shown. The larger is the angle of viewing direction, the smaller is the ratio of the transmittance at the shorter wavelength to at the longer wavelength. The viewing angle characteristics of the TN mode liquid crystal display device using the LEDs shown in FIG. 11B is expressed as the product of the luminance distribution of the planar light source device shown in FIG. 11A and the transmittance of the liquid crystal display panel shown in FIG. 10.
Accordingly, an object of the present invention is to provide a liquid crystal display device with less color shift when changing viewing directions, and a planar light source device used for the same.
A planar light source device according to the present invention has a plurality of light sources emitting different colors of light, and a light guide plate receiving light from the plurality of light sources at a side face to distribute the light over a surface thereof. A light emission angle differs among the plurality of light sources. So the planar light source device can have an angular dependency of each light. The planar light source device can be made to have an arbitrary angular dependency of color.
The planar light source device can have a refractor mounted on an emission surface of each of the plurality of light sources for changing a direction of light. The refractor has a different shape for the different light sources.
In the planar light source device, the amount of the emitted light from the light sources with longer wavelength can be concentrated in narrow range around the center of viewing angle as it is compared with that with shorter wavelength.
In the planar light source, the plurality of light sources can be red, green, and blue light emitting diodes.
A liquid crystal display device according to the present invention has the planar light source device, and a liquid crystal panel placed above an emission surface of the planar light source. The liquid crystal panel has two substrates with a liquid crystal layer interposed therebetween. The liquid crystal display device allows less color shift when changing viewing directions.
In the liquid crystal display device, the different light sources can have different light emission angles in order that wavelength dependence of transmittance at a viewing direction in the liquid crystal panel is canceled out by wavelength dependence of luminance at the viewing direction in the planar light source device.
Another planar light source device according to the present invention has a plurality of light sources emitting different colors of light, a light guide plate receiving light from the plurality of light sources at side face to distribute the light over a surface thereof, and a refractor refracting light from the plurality of light sources with different refraction angles for different colors. The planar light source device allows less color shift when changing viewing directions.
In the planar light source device, the refractor can be formed on a side face of the light guide plate facing the plurality of light sources. The refractor has a different shape for the different light sources.
The planar light source device can have a prism plate mounted between the plurality of light sources and the light guide plate. The refractor can be formed on a side face of the prism plate facing the plurality of light sources. The refractor has a different shape for the different light sources.
In the planar light source device, a refraction angle of longer wavelength light can be smaller than a refraction angle of shorter wavelength light.
