Patent ID: 7517132

Claim:
A light-emitting and light-receiving backlight unit comprising: a light guiding plate including a first incident/exit surface and a second incident/exit surface which are capable of inputting and outputting 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, wherein the light guiding plate is configured to receive light emitted from the light emitting device through the first incident/exit surface, to transmit the light in the light guiding plate, and to emit the light through the second incident/exit surface; the light guiding plate is configured to receive light from outside said plate through the second incident/exit surface, to transmit the light from outside said plate in the light guiding plate and to emit the light from outside said plate through the first incident/exit surface; and the light receiving device is configured to receive the light from outside said plate through the first incident/exit surface; and wherein the light receiving device includes at least one light receiving element disposed to face the first incident/exit surface, an amplification circuit to amplify a light receiving signal from the light receiving element and a control circuit configured to receive as input an output signal from the amplification circuit.