I've noticed on cars with LED tail lights that they have a flicker to them – I presume because they're PWM controlled.
But I've seen it on vehicles where brake lights and tail lights are separate luminaires, therefore are just on or off and do not have varying brightness. I understand that LEDs need to have current through them limited, but I don't see how PWM would achieve this?
Edit: bad phrasing – sorry. Better phrasing: why would the LEDs be flickering even though there is no apparent feature to allow changing of the brightness? Perhaps the brightness is factory set, or maybe there is a feedback circuit to keep brightness constant. But could there be any other reason?
Thoughts: because a vehicle's battery has a significantly different voltage across it when the engine is off or on, (12v vs 14.5v) perhaps there is a controlling PWM circuit that keeps brightness constant over a range of voltages. Saying that, I'm convinced that smart phone screen back-lights have some flicker to them even on full brightness, yet their Li-ion batteries' voltages are near constant.
Best Answer
Contrary to initial intuition, it's actually to increase the brightness.
LEDs can be driven at a constant current, or they can be driven with a pulsed current.
With constant current you have to limit the current to a relatively low value - for instance many common small LEDs are limited to a constant current of say 20mA. That gives good brightness for indication purposes, but it's not that great.
LEDs, when driven with pulsed current, can be driven with a considerably higher current - maybe 5 to 10 times as much, or even more. That could be say 100mA for what would normally be a 20mA LED. However, there are restrictions on what the pulses can be - typically with limits on the frequency and duty cycle - maybe as little as 1% duty.
The end result is that the higher current increases the perceived brightness of the LEDs, since more photons are being emitted when they are on, but at the cost of some flicker, which is only really noticed when the LEDs are in motion.
So you get more perceived brightness from smaller and cheaper LEDs without using more current (often less current) on average than if they were on constant.
The report above goes on to explain the effect in more detail: