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I wish modern vehicles that use LEDs as tail lights would use a capacitor or a higher PWM frequency to prevent those awful dots.
Am I lucky that a tiger has never attacked me
From real world experiments on my bench, the human eye tends to perceive an flickering image of more than 20FPS (Frames Per Second) -- as an still image. I should know since I have coded many video games. Pacman. Donkey Kong. Space Invaders. So an LED flashing at 20Hz would be perceive to the human eye as constantly lit. There would be no flicker at 20Hz or more. I have gotten away with doing moving message displays with thereabouts 20FPS with no flicker. 50Hz / 50FPS or more is optimal however, though. The images move smoothly across the screen. Most of my video games run at 50Hz. This is the same as the vertical scan rate on an television, although more modern screens tend to run at 100Hz or more for the vertical scan rate.If the PWM frequency is very low at up to 100Hz then you will see the LEDs flickering all the time. When the frequency is about 200Hz to 500Hz then you will see dots as you or the LEDs move. If the PWM is thousands or tens of thousand Hz then you will see no flickering.
You need to go back and do the test. The 20 FPS you are talking about is for movies. There are more than one format for film. Often the picture is updated 20 times/second but the same picture is flashed 3 times. So the light is at 60hz.There would be no flicker at 20Hz