A signal processing device includes an elongation coefficient calculation circuit receiving a first image signal including a red pixel value, a green pixel value and a blue pixel value, and determining an elongation coefficient value with respect to one frame based on the first image signal, and an image signal processing circuit receiving the first image signal and the elongation coefficient value. The image signal processing circuit includes an image signal conversion circuit converting the first image signal into a second image signal including a red pixel value, a green pixel value, a blue pixel value and a white pixel value, an elongation processing circuit elongating the second image signal based on the elongation coefficient value, an image signal analysis circuit analyzing the elongated second image signal, and an adjustment circuit adjusting the elongated second image signal.