1. Field of the Invention
The present invention relates to a backlight unit suitably used in a liquid crystal display (LCD) device, and more particularly to a backlight unit having an improved structure.
2. Description of the Related Art
In a LCD device, there is provided a LCD panel on the front side of the light-emitting surface of a backlight unit. The LCD panel displays an image by performing optical switching for each of pixels arranged in a matrix. There are two types of backlight units including a central-light-source type in which the light source is provided at the rear side of the light-emitting surface and the light emitted from the light source passes directly through the light-emitting surface, and an edge-light-source type in which the light source is provided at the edge of an optical guide plate provided at the rear side of the light-emitting surface and the light emitted from the light source is guided by the optical guide plate to pass through the light-emitting surface.
The edge-light-source backlight unit has the advantage that the light source can be disposed at the side edge of the optical guide plate to provide a small thickness for the backlight unit. Examples of the light source of the edge-light-source backlight unit include a linear cold-cathode fluorescent lamp and a chip-type light emitting diode (LED). In recent years, the chip-type LED is the mainstream of the light source due to smaller dimensions and longer lifetime of the light source. Patent Publication JP-2004-186004A describes an edge-light-source backlight unit using such a chip-type LED as the light source.
In the backlight unit using a LED as the light source, in order to emit the light as white light, three color LEDS emitting three colors of red, green and blue are provided without using a single white LED, because the typical white LED has an insufficient emission power. In the backlight unit described in the above patent publication, if the optical path from the color LEDs to the light-emitting surface of the backlight unit has an insufficient length, there arises a problem in that the lights of three colors emitted from the LEDs are not well mixed to reach the light-emitting surface. In this case, the backlight unit cannot provide an even-color backlight at the light-emitting surface, especially at the edge portion thereof.
In order to solve the above problem involved with the conventional backlight unit including color LEDs, Patent Publication JP-2006-18175A describes provision of an additional light-mixing optical guide plate provided on the rear side of the main optical guide plate. FIG. 4 is a sectional view showing the configuration of the LCD device described in this patent publication. The LCD device, generally designated by 100, includes a backlight unit 11, a LCD panel 12 provided on the front side of the light-emitting surface 11a of the backlight unit 11, and a shield front 13 configuring a housing and defining the outer edge of the screen of the LCD panel 12.
Color LEDs 21 are provided at the rear side of a rear cover 38 configuring the rear housing of the backlight unit 11. The color lights emitted from the color LEDs 21 are guided by a first reflector 31, a light-mixing optical guide plate 32, a second reflector 33, and a main optical guide plate 34, thereby passing through an optical sheet 36 configuring the light-emitting surface 11a of the backlight unit to reach the LCD panel 12. In this configuration of the LCD device 100, the color lights emitted from the LEDs 21 are mixed sufficiently by the light-mixing optical guide plate 32, thereby achieving an even color light at the light-emitting surface 11a of the backlight unit.
For a color LCD device, it is important to maintain the color reproducibility of the display image for a long period of service. In order to maintain the color reproducibility of the display image during the lifetime of the LCD device, the change of color tones due to aged deterioration should be suppressed on the light-emitting surface of the backlight unit. For this purpose, it is important to suppress the rise of temperature of the LED devices due to the heat generated in the LEDs. If the temperature of the color LEDs rises excessively, the color LEDs will be deteriorated, lowering the luminance of the color LEDs. The reduction in the luminance occurs in an amount depending on the characteristic of the respective LEDs, resulting in a change of the color tone of the backlight.
In the LCD device 100 described in JP-2006-18175A, in order to suppress the temperature rise of the color LEDs 21, there is provided a heat sink 23 on the rear surface of a wiring board 22 on which the LEDs 21 are mounted, thereby diffusing the heat generated in the color LEDs 21 via the heat sink 23. In order to further enhance the heat dissipation capability of the LCD device, a structure is employed to enlarge the surface area thereof by providing fins 25 on the heat sink 23, as depicted in FIG. 4.
However, for a LCD device for use in the field of photograph, printing etc., which are required of a higher color reproducibility, the temperature rise should be suppressed as much as possible, and thus the heat dissipation capability should be further improved. In this improvement, a larger surface of the heat sink 23 should be avoided because the larger surface inevitably increases the dimensions of the backlight unit 11. Therefore, it is desirable to improve the heat dissipation capability of the backlight unit without increasing the dimensions of the backlight unit 11.