According to an embodiment, the representative brightness value of image data is determined through an analysis of the image data, the dimming value of a backlight is adjusted according to the representative brightness value, and the image data is converted so as to correspond to the adjusted dimming value such that pixels maintain original brightness. In order to simplify the image data conversion, a characteristic value may be generated according to the adjusted dimming value, and a gamma curve may be generated on the basis of the characteristic value, thereby converting the image data.