The contrast value is the ratio that defines the maximum brightness value (all white) divided by the minimum brightness value (all black). CRT displays typically have contrast ratios as high as 500:1, so that rendering a true black screen on a CRT monitor is easy. However, it is not easy for the LCD. The backlight formed by the cold cathode ray tube is difficult to perform fast switching operations. Therefore, the backlight is always on. In order to get a full black picture, the liquid crystal module must completely block the light from the backlight, but in terms of physical characteristics, these components can not fully meet such requirements, and there will always be some light leakage. In general, the acceptable value for human eyes is about 250:1.