Viewing angle switchable back light unit

1. A back light unit, comprising:

a light guide film including;

a light entering part defined at one side;

a light guiding part extending from the one side to an opposite side of the one side; and

a light radiating part defined on one plane surface;

a light radiator disposed on the light radiating part;

a first light source disposed near to the light entering part; and

a light collimator disposed on the light entering part as facing to the first light source,

wherein the light collimator:

receives an expanding light from the first light source,

converts the expanding light into a collimated light, and

provides the collimated light to the light entering part, and

wherein the light collimator includes:

a horizontal taper part disposed near to the first light source, the horizontal taper part collimating the expanding light from the first light source on a horizontal plane; and

a vertical wedge part extending from the horizontal taper part, the vertical wedge part collimating the expanding light from the first light source on a vertical plane.

2. The back light unit of claim 1 , wherein the light guide film has a refractive index equal to or less than a refractive index of the light collimator.

3. The back light unit of claim 1 , further comprising:

a light absorber disposed on a bottom surface of the light guide film as facing with the light collimator,

wherein the horizontal taper part and the vertical wedge part are attached on the light entering part of the light guide film by an adhesive layer having a lower refractive index than the refractive index of the light collimator.

4. The back light unit of claim 1 , wherein:

only the vertical wedge part of the light collimator is attached on the light entering part of the light guide film by an adhesive layer having a lower refractive index than the refractive index of the light collimator; and

the horizontal taper part is disposed outside of the light guide film.

5. The back light unit of claim 1 , wherein:

the horizontal taper part includes:

a light incident surface closing to the first light source, and having a height and a width corresponding to a size of the first light source;

a top surface and a bottom surface extended to one direction from the light incident surface with a predetermined distance and with a predetermined expanding angle; and

two side surfaces defined as the height is extended from the light incident surface with the predetermined distance; and

the vertical wedge part includes an inclined surface extended along to the one direction from the horizontal taper part in which the height is linearly decreased as converging from the top surface to the bottom surface along to a length of the vertical wedge part.

6. The back light unit of claim 5 , wherein:

a plurality of the horizontal taper part are disposed along to a side of the vertical wedge part in a series, the side is perpendicular to the one direction; and

each of a plurality of the first light source is disposed at each of the light incident surface of the horizontal taper part.

7. The back light unit of claim 1 , wherein the light collimator controls an incident angle of the collimated light:

to be larger than a total internal reflection critical angle at an interface between the light guide film and air; and

to be smaller than a total internal reflection critical angle at an interface between the light guide film and the light radiator.

8. The back light unit of claim 1 , wherein:

the light guiding part propagates the collimated light from the one side to the opposite side;

as propagating the collimate light, the light guiding part totally reflects the collimated light at an opposite plane surface facing to the one plane surface, and

the light guiding part, at the one plane surface:

refracts some of the collimated light to the light radiator to output the some of the collimated light from the light guide film; and

reflects others of the collimated light to the opposite plane surface.

9. The back light unit of claim 1 , further comprising:

a light coupler disposed laterally near to the light collimator; and

a second light source disposed as facing to the light coupler and laterally near to the first light source.

10. The back light unit of claim 1 , further comprising:

a second light source disposed as facing an empty space between each of the horizontal taper part; and

a reflector disposed on at least one of a top surface and a bottom surface.