The numbers are "painted" one line or a group of lines across at a time. This happens very fast usually from top to bottom. If the cameras shutter speed is fast enough it will catch some of the lines in the "off" state. As long as the image or in this case the LEDs paint the image faster than approximately 1/25th of a second it will appear as a solid character. This is due to you eyes retaining the image of a brief period of time. This effect is known as "persistence of vision
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