A display device capable of detecting the light intensity of ambient light with high accuracy is disclosed. The display device has a backlight light source and a photo sensing unit. The display device also includes a voltage supply unit for providing a voltage, which makes the photo sensing unit output a certain amount of photocurrent to the photo sensing unit; and a supply voltage control unit for modulating the voltage supplied to the photo sensing unit based on the operation status of the backlight light source. The supply voltage control unit controls the voltage supply unit by the way of outputting a voltage of a first voltage to the photo sensing unit while the backlight light source is operating; and outputting a voltage of a second voltage, which is different from the first voltage, to the photo sensing unit while the backlight light source is not operating.