A multifunctional-type backlight unit, including a light guiding plate
having a first incident/exit surface and a second incident/exit surface
which are capable of receiving and emitting light, a light emitting
device disposed to face the first incident/exit surface and configured to
supply light to the first incident/exit surface, and a light receiving
device disposed to face the first incident/exit surface and configured to
receive light emitted from the first incident/exit surface, the light
guiding plate being configured so that the light emitted from the light
emitting device enters the light guiding plate, passes through the light
guiding plate, and is emitted from the second incident/exit surface, and
the light guiding plate being configured so that the light which enters
the second incident/exit surface, passes through the light guiding plate
emits from the first incident/exit surface is received by the light
receiving device.