Patent ID: 8870399

Claim:
A backlight unit comprising: a light source; an optical member configured to uniformly distribute light emitted from the light source; and at least two condensing films configured to condense light from the optical member, wherein hetero-conic lenses are two-dimensionally arranged on a surface of each of the condensing films, and each of the hetero-conic lenses is formed by combining a first conic lens expressed by Formula (1) below and a second conic lens expressed by Formula (2) below: y = H 1 - x 2 / r 1 1 + 1 - ( 1 + k 1 ) ⁢ ( 1 / r 1 ) 2 ⁢ x 2 ⁢ ⁢ ( 0 ≤ y ≤ y 0 ) Formula ⁢ ⁢ ( 1 ) where r 1 denotes a radius of curvature at an imaginary apex of the first conic lens, k 1 denotes a conic constant of the first conic lens, and H 1 denotes a height from a bottom surface to the imaginary apex of the first conic lens, y = H 2 - x 2 / r 2 1 + 1 - ( 1 - k 2 ) ⁢ ( 1 / r 2 ) 2 ⁢ x 2 + y 0 ⁢ ⁢ ( y 0 ≤ y ≤ H 2 ) Formula ⁢ ⁢ ( 2 ) where r 2 denotes a radius of curvature at an apex of the second conic lens, k 2 denotes a conic constant of the second conic lens, H 2 denotes a height from a bottom surface to the apex of the second conic lens, and y 0 denotes a height measured from the bottom surface of the first conic lens to a position at which a diameter of a cross section of the first conic lens is equal to a diameter of the bottom surface of the second conic lens.