Liquid crystal display device including back light having improved light concentration efficiency

A liquid crystal display device comprising:

• a liquid crystal panel configured to represent an image;
• a lamp disposed below the liquid crystal panel to provide light to the liquid crystal panel;
• a light guide plate having the lamp disposed at least at one side thereof;
• and
• a birefringence optical sheet disposed between the liquid crystal panel and the light guide plate, and the birefringence optical sheet including a first base film, a second base film, a first prism pattern with a first refractive index, and a second prism pattern with a second refractive index, the first prism pattern being facing the liquid crystal panel and the second prism pattern being facing the light guide plate, the second refractive index of the second prism pattern being higher than the first refractive index of the first prism pattern,
• wherein the lower surface of the first prism pattern and the second prism pattern are respectively formed on the first base film and the second base film and the upper portions of the first prism pattern and the second prism pattern are respectively contacted with the second base film and the first base film,
• wherein the first base film is apart from the second base film, and the first and the second prism patterns are disposed between the first base film and the second base film,
• wherein a thickness of the first base film of the birefringence optical sheet is smaller than that of the second base film when a thermal expansion ratio of the first base film is greater than that of the second base film, and the thickness of the second base film of the birefringence optical sheet is smaller than that of the first base film when the thermal expansion ratio of the second base film is greater than that of the first base film.