Backlight unit and display device

A display device comprising:

• a display unit;
• and
• a backlight unit for applying light to the display unit, the backlight unit comprising:
• a light source;
• a light-guide plate provided at the light source for guiding light emitted by the light source to the display unit,
• an optical sensor arranged to face the light-guide plate and having a photo detector for receiving light;
• a light-shielding portion that shields light that is otherwise incident to the photo detector from the periphery of the optical sensor, the light-shielding portion being provided around the photo detector;
• and
• a board provided on the optical sensor,
• wherein the light-guide plate has an extended portion,
• wherein the board is held to face the extended portion, and
• wherein the extended portion is arranged to extend out of the display area of the display unit, the extended portion having a recess formed in a thickness direction thereof, and the optical sensor is arranged within the recess and the photo detector is arranged to face a bottom of the recess for receiving light emitted from the bottom of the recess.