A backlight unit is presented that contains lamp support structures uniquely fitted to facilitate installation of lamps in a particular orientation. The backlight unit has tube light-emitting lamps, each of which is fitted to an asymmetric lamp holder. Lamp supports fixedly engage the asymmetric lamp holders fitted to the tube type light-emitting lamps. At least one of the lamp holders has an asymmetric hole. An outer case includes the supports to fix and support each lamp. At least one lamp has a treated surface that includes an optical sheet having a light transmittance lower than 100%.