Backlight device for liquid crystal display

1. A back light device for a liquid crystal display device, comprising:

a light source providing light;

a light waveguide plate guiding light from the light source, and said light waveguide plate having an emitting surface, a front surface and a bottom surface, the emitting surface being adjacent to the light source;

a reflector arranged under the bottom surface of the light waveguide plate, reflecting light; and

at least one single layer cholesteric liquid crystal (CLC) film arranged on the front surface of the light waveguide plate, collimating light, wherein the at least one single layer CLC film transmits a first circularly polarized vertically incident visible light and circularly polarized obliquely incident light having a wavelength greater than about 780 nm and wherein the at least one single layer CLC film reflects a first circularly polarized obliquely incident visible light, a second circularly polarized vertically incident visible light and a second circularly polarized obliquely incident visible light having a wavelength range from 380 nm to 780 nm and a circularly polarized vertically incident light having a wavelength greater than about 780 nm.

2. The back light device of claim 1 , wherein the single layer CLC film is one of either a right-handed or a left-handed CLC layer, the right-handed CLC layer selectively reflecting right-handed circularly polarized light and the left-handed CLC layer reflecting left-handed circularly polarized light.

3. The back light device of claim 1 , wherein the at least one single layer CLC film has a dual-layered structure having both a right-handed and a left-handed CLC layer, the right-handed and left-handed CLC layer selectively reflecting right-handed and left-handed circularly polarized light, respectively.

4. The back light device of claim 1 , further comprising a prism sheet arranged between the at least one single layer CLC film and the front surface of the light waveguide plate.

5. A back light device for a liquid crystal display device, comprising:

a light source providing light;

a light waveguide plate guiding light from the light source, and said light waveguide plate having an emitting surface, a front surface and a bottom surface, the emitting surface being adjacent to the light source;

a reflector arranged under the bottom surface of the light waveguide plate, reflecting light; and

at least one single layer cholesteric liquid crystal (CLC) film arranged over the front surface of the light waveguide plate, collimating light, wherein the at least one single layer CLC film transmits a first circularly polarized visible light incident within a predetermined angle and a circularly polarized light of a wavelength greater than about 780 nm incident outside the predetermined angle, and wherein the at least one single layer CLC film reflects the first circularly polarized visible light incident outside the predetermined angle, a second circularly polarized visible light incident within the predetermined angle and the second circularly polarized visible light incident outside the predetermined angle and having a wavelength range from 380 nm to 780 nm and the circularly polarized light of a wavelength greater than about 780 nm incident within the predetermined angle.

6. The back light device of claim 1 , wherein the at least one CLC film selectively reflects vertically incident light with a wavelength of more than 600 nm by controlling a helical pitch P of said CLC film according to the equation:

λ o =P ( n o +n e )/2,

where λ o is a wavelength of vertically incident light, P is a helical pitch, no is an ordinary refractive index, and ne is an extraordinary refractive index.

7. The back light device of claim 5 , wherein each single layer CLC film is one of either a right-handed or a left-handed CLC layer, each right-handed CLC layer selectively reflecting right-handed circularly polarized light and each left-handed CLC layer reflecting left-handed circularly polarized light.

8. The backlight device of claim 5 , wherein each single layer CLC film is formed by a dual-layered structure, each structure having both a right-handed and a left-handed CLC layer, the right-handed and left-handed CLC layer selectively reflecting right-handed and left-handed circularly polarized light, respectively.

9. The back light device of claim 5 , further comprising a prism sheet arranged between the at least one single layer CLC film and the front surface of the light waveguide plate.