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Information displays using a multiplexed array of LED elements to display text or graphics can be made to appear up to 5 times brighter than an element would be it run at maximum constant current but the mark space ratio of the light from an individual element ( pixel ) would be as low as 1 ( ON ) to 10 ( OFF ) or even lower.
5x brighter? You're talking nonsense. I've worked on display systems and the only reason we multiplex is to simplify the drive circuitry, not for any perceived brightness changes. JohnW2 is correct - as soon as you have multiple elements multiplexed there is no perceived brightness change anyway. The effect you describe is only for a single PWM source where the eye is fooled into thinking something is brighter than it actually is. It also only applies to a relatively low (50-78Hz) frequencies which is several times lower than the PWM rate for a typical LED display.