A backlight unit includes a substrate, a plurality of light sources arranged on the substrate in a spaced-apart relationship with one another for irradiating lights, a total reflection layer for upwardly reflecting the lights irradiated from the light sources, and an optical layer disposed above the light sources and having an oxide compound layer laminated thereon. The optical layer is adapted to allow a part of the lights to pass through the optical layer but reflect the remaining part of the lights downwardly toward the substrate, to thereby induce the lights from the light sources to optically resonate. The optical resonance caused by the oxide compound-laminated optical layer helps to reduce the half amplitude of a color-based spectrum histogram in a light-emitting spectrum, thereby enhancing the color purity.