A backlight unit comprising: a base layer; a plurality of substrates (210, 2900, 2910, 2920) disposed on the base layer and separated from each other; a reflection layer disposed on top of the base layer; and light sources (220, 2901, 2902, 2903) disposed on each of the substrates, wherein: the plurality of substrates includes a first substrate and a second substrate, and the light sources include a first light source disposed on the first substrate and a second light source disposed on the second substrate, wherein: the base layer includes a first part overlapping with the first substrate, a second part overlapping with the second substrate, and a third part (10B, IP, DP) positioned between the first part and the second part, the third part (10B, IP, DP) has a maximum thickness at a first position, said maximum thickness being thicker than the first part and the second part, and thickness of the third part (10B, IP, DP) is gradually increased from the first part to first position forming a first slope, and thickness of the base layer is gradually decreased from the first position to second part forming a second slope, wherein the light sources (220, 2901, 2902, 2903) each include a respective light emitting face configured to emit light, and characterized in that the light emitting face of the first light source is configured to face the first position, and the light emitting face of the second light source is configured to face opposite to the first position, and in that the first slope is less steep than the second slope. a base layer; a plurality of substrates (210, 2900, 2910, 2920) disposed on the base layer and separated from each other; a reflection layer disposed on top of the base layer; and light sources (220, 2901, 2902, 2903) disposed on each of the substrates, wherein: the plurality of substrates includes a first substrate and a second substrate, and the light sources include a first light source disposed on the first substrate and a second light source disposed on the second substrate, the plurality of substrates includes a first substrate and a second substrate, and the light sources include a first light source disposed on the first substrate and a second light source disposed on the second substrate, wherein: the base layer includes a first part overlapping with the first substrate, a second part overlapping with the second substrate, and a third part (10B, IP, DP) positioned between the first part and the second part, the third part (10B, IP, DP) has a maximum thickness at a first position, said maximum thickness being thicker than the first part and the second part, and thickness of the third part (10B, IP, DP) is gradually increased from the first part to first position forming a first slope, and thickness of the base layer is gradually decreased from the first position to second part forming a second slope, wherein the light sources (220, 2901, 2902, 2903) each include a respective light emitting face configured to emit light, and characterized in that the light emitting face of the first light source is configured to face the first position, and the light emitting face of the second light source is configured to face opposite to the first position, and in that the first slope is less steep than the second slope. the base layer includes a first part overlapping with the first substrate, a second part overlapping with the second substrate, and a third part (10B, IP, DP) positioned between the first part and the second part, the third part (10B, IP, DP) has a maximum thickness at a first position, said maximum thickness being thicker than the first part and the second part, and thickness of the third part (10B, IP, DP) is gradually increased from the first part to first position forming a first slope, and thickness of the base layer is gradually decreased from the first position to second part forming a second slope, wherein the light sources (220, 2901, 2902, 2903) each include a respective light emitting face configured to emit light, and characterized in that the light emitting face of the first light source is configured to face the first position, and the light emitting face of the second light source is configured to face opposite to the first position, and in that the first slope is less steep than the second slope.