Directional backlight unit, method for manufacturing the same, and 3D image display apparatus having the same

A directional backlight unit comprising:

• a light guide plate;
• a light source that irradiates light to the light guide plate;
• and
• a grating formed on a light-emitting surface of the light guide plate, wherein a shape of the grating is configured such that an intensity of one ray of light, of the light irradiated by the light source and diffracted and emitted by the grating is greater than a sum of intensities of all other rays of light, of the light irradiated by the light source and diffracted and emitted by the grating,
• wherein the grating comprises a lower grating formed into the light-emitting surface of the light guide plate, and an upper grating connected to the lower grating, and
• wherein the lower grating comprises a plurality of round furrows periodically formed in the light-emitting surface of the light guide plate, each of the plurality of round furrows is spaced apart by a flat area of the light-emitting surface of the light guide plate, and the upper grating comprises a plurality of round ridges alternatingly disposed with the plurality of round furrows, each of the plurality of round ridges is directly contacted to the flat area of the light-emitting surface of the light guide plate and protrudes from the light guide plate.