a) Flicker occurs when the refresh rate of the video is too low and the light level on the display begins to decrease before new information is written to the screen to maintain the light level. To prevent the human eye from seeing flicker, the screen refresh rate needs to be at least 24 frames per second. b) A rapid visible change in brightness, not part of the original scene. See also Flicker Frequency, Fusion Frequency, Judder, Large-Area Flicker, and Twitter.