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I have seen this many times that the daytime running LED headlamps or taillamps of cars blink in slow motion videos, why is that?

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    \$\begingroup\$ PWM. Because software engineers have designed the hardware. Should be banned. \$\endgroup\$ – winny Apr 6 '17 at 5:36
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It's how they make the lights brighter or dimmer depending on whether they are acting as daytime lights (dimmer) or as headlamps at night (brighter). They quickly turn them on and off fast enough that we can't see the flickering, but they change how long the lights are on each time they flash so the apparent brightness goes up or down. If you want your lights to be 50% dimmer, only turn them on half the time. They are actually flashing very fast, likely >1000 times per second.

This technique is called pulse-width modulation (PWM): https://en.wikipedia.org/wiki/Pulse-width_modulation

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    \$\begingroup\$ I can see the flickering just fine -- every time my glance sweeps past such a car. I find it quite annoying. LED taillights at night were bad enough, but now the LED DRLs are doing it too. :-( \$\endgroup\$ – Dave Tweed Apr 6 '17 at 11:59
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    \$\begingroup\$ Yep you're seeing the pwm. They choose a frequency not quite fast enough to get the complete effect. You can see it when you sweep your eyes because as the image travels across your cornea, the light is on at one side and is off when it reaches the other. Very noticeable. \$\endgroup\$ – Los Frijoles Apr 6 '17 at 12:59

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