Display devices have an emission type display device that can self-emit, such as an organic emission display device and plasma display device, but a light receiving type display device, that cannot self-emit and requires another light source, such as a liquid crystal display device.
A general liquid crystal display device has two display panels having electric field generation electrodes and a liquid crystal layer having dielectric constant anisotropy. In such a liquid crystal display device, an electric field is generated in the liquid crystal layer by applying voltage to the electric field generation electrodes, and the intensity of this electric field is adjusted by changing the voltage so as to generate light valve, and a desired image is obtained by adjusting the transmittance of light that passes through the liquid crystal layer. The light to be used for this purpose is normally light from an artificial backlight light source, which is equipped separately.
For the backlight light source used for a liquid crystal display device, a fluorescent lamp, such as a cold cathode fluorescent lamp (CCFL), is often used as a light source that irradiates light from the rear face of the liquid crystal panel onto the entire liquid crystal panel evenly. Generally known backlight light sources are an edge light type backlight light source which irradiates onto the liquid crystal panel from the rear face side using nearly uniform light, that is a CCFL light which enters from the side face of a light guiding plate and is emitted from the front face of the light guiding plate, or a direct type backlight light source which has several CCFLs arrayed on the rear face of the liquid crystal panel, and which irradiates onto the liquid crystal panel via a diffusion plate.
Recently the development of image display devices using light emitting diodes (LED) and lasers as light sources, that do not use mercury and conserves power consumption, is ongoing, addressing environmental problems and energy saving issues. Laser in particular is an optimum light source for image display devices, because not only is power consumption low, but in terms of image quality, includes a wide color reproduction range.
On the other hand, a light box and light guiding plate are normally used to simplify liquid crystal TVs and to make the construction slim thereof. As a backlight device using this light box and light guiding plate, the configurations disclosed in Patent Literature 1 and Patent Literature 2 have been proposed.
Patent Literature 1, for example, proposes a system which allows laser lights to enter the light box by linearly reflecting laser light using a hologram mirror, based on mono-chromaticity, polarization and rectilinear propagation characteristics of laser light, and irradiates two-dimensionally via a half mirror array. Patent Literature 2 proposes a system which has arrayed light sources on the left and right of a light guiding plate, and irradiates light two-dimensionally via a triangular groove formed at the bottom face of the light guiding plate. An advantage of these systems is that if laser light, of which polarizing directions are the same, are used for the light source, a backlight with high light utilization efficiency can be constructed by maintaining polarization, compared with the case of non-polarization.
However in the case of a system where laser light are reflected using a hologram mirror, as disclosed in Patent Literature 1, if three colors: R (red), G (green) and B (blue) are used as the laser light source, the reflection direction by the hologram mirror is different depending on the color, which may generate uneven color in the distribution of light finally emitted from the light guiding plate. Also the light quantity distribution of laser light reflected by the plane hologram mirror reflects the profile of the laser light. For example, if the laser light has a Gaussian profile light quantity distribution, the brightness in the center area of the light guiding plate increases, which means that only uneven brightness can be implemented, which is not acceptable.
Generally the reflectance of a hologram mirror has poor diffraction efficiency depending on the wavelength, and generates a light quantity loss. Furthermore the half mirrors are used for the configuration to two-dimensionally reflect the laser light, which means that the reflectance errors of half mirrors are reflected in the brightness distribution. The light quantity distribution of the light source is also an influence on the distribution of the laser lights reflected by each half mirror, and uneven brightness is generated in the light source profile with the pitch of the half mirrors. In order to irradiate a large area panel with slim construction, many half mirrors, of which respective transmittance characteristics have subtle differences, are required, which makes manufacturing very difficult and increases cost, and generates many other problems, so it is therefore very impractical.
In the case of the system in which light sources are disposed in arrays on the side faces of the light guiding plate, as disclosed in Patent Literature 2 as well, the profiles of the light sources are directly reflected within the light guiding plate, and uneven brightness, when the profiles of the plurality of light sources are combined, is generated. Also in the case of using a laser for the light source of Patent Literature 2, the laser light that entered the light guiding plate directly reaches the side face of the light guiding plate at the opposite side of the side face where the laser light entered, because of the rectilinear characteristics thereof, and is transmitted through this side face, which generates loss. Even if this light is reflected using a reflection element or the like, the laser light that propagates horizontally continuously propagates for a long distance while being reflected, and generates light quantity loss due to the absorption of the light guiding plate. If the laser light spreads and enters the entrance of the light guiding plate, brightness around the entrance increases and generates uneven brightness, so this system also has many problems.
Patent Literature 1: Japanese Patent Application Laid-Open No. 2002-169480
Patent Literature 2: Japanese Patent Application Laid-Open No. 2006-202703
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