A backlight unit comprising: a light guide plate; an input coupler configured to input light into the light guide plate; an output coupler configured to output the light that has travelled in the light guide plate; and a uniformity changing element (200, 200a) having a light-incident surface and a light-exiting surface, facing the light-incident surface, and configured to: receive the light output from the output coupler through the light-incident surface; increase uniformity of the received light from the output coupler; and, output the received light with the increased uniformity of the received light through the light-exiting surface facing the light-incident surface; a polarization conversion element configured to convert a plurality of portions of polarized light into single polarized light when the light output from the uniformity changing element (200, 200a) comprises the plurality of portions of polarized light; wherein the uniformity changing element (200, 200a) is further configured to output the light incident on a first location on the light-incident surface through a plurality of different second locations on the light-exiting surface; wherein the uniformity changing element (200, 200a) is further configured to propagate the light along a plurality of different light-traveling paths inside the uniformity changing element (200, 200a) according to polarization of the light; wherein the uniformity changing element (200, 200a) comprises: a first uniformity changing element (210, 230) configured to change uniformity of the light into a first level of uniformity and output light having the first level of uniformity; a second uniformity changing element (220, 240) configured to change the first level of uniformity of the light into a second level of uniformity and output light having the second level of uniformity, the first level of uniformity being different from the second level of uniformity. a light guide plate; an input coupler configured to input light into the light guide plate; an output coupler configured to output the light that has travelled in the light guide plate; and a uniformity changing element (200, 200a) having a light-incident surface and a light-exiting surface, facing the light-incident surface, and configured to: receive the light output from the output coupler through the light-incident surface; increase uniformity of the received light from the output coupler; and, output the received light with the increased uniformity of the received light through the light-exiting surface facing the light-incident surface; receive the light output from the output coupler through the light-incident surface; increase uniformity of the received light from the output coupler; and, output the received light with the increased uniformity of the received light through the light-exiting surface facing the light-incident surface; a polarization conversion element configured to convert a plurality of portions of polarized light into single polarized light when the light output from the uniformity changing element (200, 200a) comprises the plurality of portions of polarized light; wherein the uniformity changing element (200, 200a) is further configured to output the light incident on a first location on the light-incident surface through a plurality of different second locations on the light-exiting surface; wherein the uniformity changing element (200, 200a) is further configured to propagate the light along a plurality of different light-traveling paths inside the uniformity changing element (200, 200a) according to polarization of the light; wherein the uniformity changing element (200, 200a) comprises: a first uniformity changing element (210, 230) configured to change uniformity of the light into a first level of uniformity and output light having the first level of uniformity; a second uniformity changing element (220, 240) configured to change the first level of uniformity of the light into a second level of uniformity and output light having the second level of uniformity, the first level of uniformity being different from the second level of uniformity. a first uniformity changing element (210, 230) configured to change uniformity of the light into a first level of uniformity and output light having the first level of uniformity; a second uniformity changing element (220, 240) configured to change the first level of uniformity of the light into a second level of uniformity and output light having the second level of uniformity, the first level of uniformity being different from the second level of uniformity.