1. Field of the Invention
The invention relates to a liquid crystal display device, and more particularly, to a multi-functional optical sheet for a liquid crystal display device.
2. Discussion of the Related Art
With the rapid development of information technology, flat panel display (FPD) devices, such as liquid crystal display (LCD) devices, plasma display panels (PDPs), electroluminescent display (ELD) devices and field emission display (FED) devices, have been developed and have replaced cathode ray tubes (CRTs).
Among these devices, liquid crystal display (LCD) devices widely used for monitors of notebook computers, monitors of personal computers and as displays for televisions because the LCD devices are excellent at displaying moving images and have a high contrast ratio.
LCD devices are not self-luminescent and require an additional light source. By disposing a backlight unit at the rear side of a liquid crystal (LC) panel to emit light into the LC panel, discernible images can be displayed. Backlight units may include cold cathode fluorescent lamps (CCFLs), external electrode fluorescent lamps (EEFLs) or light-emitting diodes (LEDs) as a light source.
Backlight units are classified as edge type or direct type according to the position of the light source with respect to the display panel. In edge-type backlight units, one or a pair of lamps are disposed at one side or at each of two sides of a light guide panel of a backlight unit. In direct-type backlight units a plurality of lamps are disposed directly under a display panel.
FIG. 1 is a cross-sectional view illustrating a liquid crystal display (LCD) device including a direct-type backlight unit according to the related art. The related art LCD device includes a liquid crystal panel 10, a back light unit 50, a support main 60, a top cover 70 and a cover bottom 80. The liquid crystal panel 10 includes upper and lower substrates 13 and 15. A printed circuit board (not shown) is attached at a side of the liquid crystal panel 10. The support main 60 surrounds side surfaces of the liquid crystal panel 10 and the backlight unit 50. The top cover 70 covers edges of a front surface of the liquid crystal panel 10, and the cover bottom 80 covers a rear surface of the backlight unit 50. The top cover 70 and the cover bottom 80 are combined with the support main 60 to thereby constitute one-united body.
The backlight unit 50 includes a reflection sheet 40, a plurality of lamps 30, a diffuser plate 22, and a plurality of optical sheets 20. The plurality of lamps 30 are disposed over the reflection sheet 40, and the diffuser plate 22 and the plurality of optical sheets 20 are sequentially disposed over the plurality of lamps 30.
The plurality of optical sheets 20 includes a diffuser sheet and at least one light-concentrating sheet. The plurality of optical sheets 20 will be described in more detail with reference to FIG. 2.
FIG. 2 is a view schematically illustrating the diffuser plate and the plurality of optical sheets of FIG. 1. As shown in FIG. 2, the plurality of optical sheets 20 includes a diffuser sheet 24 and a light-concentrating sheet 26 sequentially disposed over the diffuser plate 22.
The diffuser plate 22 may be formed of transparent acrylic resin, polymethylmethacrylate (PMMA) or thermoplastic polyethylene terephthalate (PET) including irregular particles therein. The diffuser plate 22 may have various haze properties. The diffuser plate 22 directs light emitted from the plurality of lamps 30 of FIG. 1 towards the liquid crystal panel 10 of FIG. 1 and diffuses light such that light is incident on the liquid crystal panel 10 of FIG. from a relatively wide range of angles.
The diffuser sheet 24 positioned over the diffuser plate 22 is generally composed of a base film of PET and acrylic resin layers on both sides of a base film that includes a light diffuser such as beads. The diffuser sheet 24 diffuses light from the diffuser plate 22 so that there may be no patched portion due to partially centralized light, and the diffuser sheet 24 directs light towards the light-concentrating sheet 26.
The light-concentrating sheet 26 includes a base film of PET and prism patterns 28 regularly arranged on an upper surface of the base film. The prism patterns 28 generally reflect and concentrate light.
FIG. 3 is a view illustrating paths of light passing through the light-concentrating sheet of FIG. 2.
As shown in FIG. 3, light incident on the light-concentrating sheet 26 has an angle of about 5 degrees with respect to the normal to a lower surface of the light-concentrating sheet 26. Light incident on a light-concentrating region A is refracted by the prism patterns 28 and is concentrated toward the liquid crystal panel 10 of FIG. 1 to thereby cause an increase of brightness. Light incident on a total reflection region B is reflected downward at a surface of the prism patterns 28.
The reflected light is re-reflected by the reflection sheet 40 to be re-supplied to the light-concentrating sheet 26. Therefore, a loss of light is minimized.
However, in addition to the concentration and reflection of light in the light-concentrating sheet 26, there is a loss of light due to side lobes. This loss decreases light efficiency and adversely affects viewing angle properties of the display.
More particularly, in the figure, a region designated by C means a region where there occur side lobes, wherein the region C is referred to as a side lobe region. Light incident on the side lobe region C is refracted at the surface of the prism patterns 28 in an undesired direction and not toward the liquid crystal panel 10 of FIG. 1, and is wasted. In other words, there is light leakage in the side lobe regions C.
FIGS. 4 and 5 are graphs of illustrating distributions of brightness and distribution of light versus viewing angles for a backlight unit that includes the related art light-concentrating sheet of FIG. 3. As may be appreciated from FIGS. 4 and 5, there are bright areas at both sides with respect to a center region O, and there are side lobe regions C.
To solve the problems described above, a reflective polarizing sheet or a diffuser sheet for recycling light may be further disposed over the light-concentrating sheet to decreased or removed the side lobes to thereby improve a light efficiency and viewing angle properties. This solution increases the number of optical sheets used in a display and increases the manufacturing time for producing a liquid crystal display module. Accordingly, manufacturing process efficiencies are lowered and costs are increased.