An electro emission device (EED) operated by a driver for modulating the pulse width of a driving pulse according to a pulse width modulation (PWM) clock signal to control the brightness of images. The EED calculates a brightness level of image data indicating the brightness of the whole image for each unit image of a video signal. The image data is stored for each unit image of the video signal. The reference brightness level stored in the look-up table is compared with a calculated brightness level to determine a signal of the PWM clock signal condition. A PWM clock signal is generated according to the signal of the PWM clock signal condition to drive the display panel based on the PWM clock signal.