Viewing angle switchable back light unit

A back light unit, comprising:

• a light guide film including a light entering part disposed at one side, a light changing part disposed at other side and a light guiding part connecting the light entering part and the light changing part;
• a light radiating part disposed on an upper surface of the light guide film;
• and
• a light source disposed at the light entering part of the light guide film,
• wherein the light entering part includes:
• a wedge light coupler converting expanded incident lights provide from the light source into vertical semi-collimated lights having a narrow expanding angle on a horizontal plane and being semi-collimated on a vertical plane;
• and
• a light deflection element converting the vertical semi-collimated lights received from the wedge light coupler into vertical collimated lights on the vertical plane satisfying a total reflection condition within the light guiding part,
• wherein the light changing part includes:
• a light reflecting pattern for coverting the vertical collimated lights received from the light guiding part into collimated lights being further collimated on the horizontal plane, breaking the total reflection condition within the light guiding part and satisfying a total reflection condition at the interface with the light radiating part, and for sending the collimated lights to the light guiding part,
• wherein the light radiating part includes:
• a light radiating film providing some of the collimated lights as back lights and reflecting others of the collimated lights into the light guiding part.