Contrast ratio promotion method

A display device comprising:

• a receiving element receiving a picture signal corresponding to a picture;
• a calculating element calculating a luminance ratio of a luminance of the picture to a maximum luminance of the display device, wherein the maximum luminance is determined by a total number of pixels of the display device and a pixel resolution related to each of the pixels, and the luminance of the picture is determined by grey values of each of the pixels in the picture;
• a back light module having a maximum brightness and a minimum brightness;
• and
• a control element adjusting a brightness of the back light module according to a quantitative relation between the luminance ratio and a predetermined value,
• wherein the predetermined value comprises a first and a second predetermined values, and the second predetermined value is greater than the first predetermined value,
• wherein the control element gradually and linearly decreases the brightness of the back light module from the maximum brightness to the minimum brightness when the luminance ratio is smaller than the first predetermined value,
• wherein the control element gradually and linearly increases the brightness of the back light module from the minimum brightness to the maximum brightness when the luminance ratio is larger than the second predetermined value.