Persistence of vision and flicker When the eye sees light, it continuous to see it for about 60ms after the light source is removed. This property of the eye is called persistence of vision. Time of persistence of vision is more for darkness than for bright light. This result in a phenomenon called flicker. It means dark interval between bright pictures becomes visible for short time and appear as flicker.
Relationship between flicker and brightness is given by Ferry Poster law Where , Fc is the critical frequency, at which the flicker will not seen. F is a constant related to viewing conditions[it is 37 for normal conditions] B is the luminance of the picture.
Total Gradation and Contrast While a tv picture may have overall average level of brightness, the illumination of any individual pixels on the screen may vary from one pixel to the other. Variation from pixel to pixel is called total gradation. A tv picture may have overall average level of brightness, the illumination value of an individual pixels on the screen will vary above and below the average value. Thus illumination of the pixel will appear differently with different brightness of the background. contrast