Why do some car headlights flash on and off when filmed in slowmo, eg on top gear?

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Why do some car headlights flash on and off when filmed in slowmo, eg on top gear?

In: Technology

3 Answers

Anonymous 0 Comments

LED lights on cars aren’t dimmed the traditional way.

LED’s like to have a specific amount of current to function. Basically, when you start to turn down the current to an LED (easy way to dim a standard incandescent), the light output gets less predictable and stable. It could wander around a bit getting brighter and darker, or flicker, or just be brighter one time vs the next without much explanation.

One of the benefits of LED lights are the dramatically faster response time vs incandescent lights. This is why LED brake lights seem to jump out at you when beside incandescent light, as they are basically instant on/instant off. You don’t have to wait for the light to warm up and cool down, a process which is so slow it’s visible to our eyes on incandescent lights. Anyways, to get around this dimming problem, LED lights on a car are dimmed by having the controller flash the lights on and off faster than the eye can see. On most cars, this rate is around 1000Hz or 1000times/second. If you want 90% brightness, 90% of the cycles the lights will be on while the other 10% the lights are off. Our wonderful brains just average the amount of light that is emitted and it’s perceived to be a constant beam of light.

The camera can’t do that. It just takes a bunch of pictures (frames) and plays them back fast. The reason the lights flicker on the video is that the camera happens to snap a few frames where the lights are off (due to the fast on/off cycle for dimming, it’s inevitable). The camera has no need to average out the light output as it won’t have a seizure if a light flashes 1000 times/second in its face. Because video is only played back at 60Hz on average, it’s a speed slow enough our brains can ‘see’ and we notice the flickering. If you take a slow-motion video this effect is amplified as more frames with the lights ‘off’ are recorded and exaggerated (1ms off played back at half speed becomes 2ms off).

If they were to film with the high beams on, or in a setting that the lights aren’t dimmed, there would be no flickering. If you have a car with LED lights, you can try. I have a newer Toyota Corolla and it’s running lights are just the LED’s dimmed. Video of the lights in this setting flicker. If I turn the full headlights on, the LED’s go to full power and then even in slow motion, the lights are still steady lit on video.

Anonymous 0 Comments

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Anonymous 0 Comments

Those are LED headlights, and LED’s aren’t just hooked up directly to a battery the way that incandescent bulbs are. They are driven by a small microchip which basically pulses power to them in order to control brightness and protect the LED from overheating. Incandescent headlights also use separate filaments or bulbs for high and low beams, where LED’s just change the frequency of the pulses to increase brightness.

Now when things get filmed it’s done in by taking a lot of pictures very quickly. When you film something that is blinking at a high rate, sometimes some of those pictures in the film catch the LED’s when they’re in the off state, hence the flickering.