Re: LED Headlight Flicker

Manufacturers “replace” bulbs too, not just owners. I think Google just badly worded it. You’ll see this with nearly all LED bulbs.

Tim

This is a standard problem with LEDs. In order to regulate the brightness, they are turned on and off very rapidly, with the proportion of on-time per cycle controlling the perceived brightness.

The dashcam will be running at a frame rate of either 25, 29.97 (*) or

You can see the pulsed nature of the lights even with the naked eye, especially on a car with LED tail-lights. If you move you hand very rapidly across the lights and make your eyes follow your hand, you see a series of still images of the red light, spaced a short distance apart. It can be quite distracting with pedestrian-crossing lights - the red or green man "repeater image" that there is on the box where a pedestrian presses a button to cross, if you drive past and catch a glimpse of the red man out of the corner of your eye.

(*) US TV used to be 30 frames per second. Then when colour was introduced, it was found that the colour information in the video signal would interfere with the sound signal in normal VHF or UHF broadcasts of the TV station. They resolved it by reducing the frame rate very slightly (I think by a factor of 1000/1001) which prevented the signals "colliding". Hence the 29.97 frames per second that is still used in broadcast TV to this day, even though it is no longer a problem with digital TV. Dashcams and security cameras and GoPros sometimes use 30 rather than 29.97.

There are two ways to control the brightness of LEDs.

2) Leave LED current at 300mA. Turn LED on 200 times a second. Leave LED ON for 30%, off for 70%. The human eye cannot (normally) detect the flicker, but there are certain situations where you can see the modulation out of the corner of your eye.

Now, given the two choices, simple current flow regulation, or PWM, now you can understand why certain braking red LEDs have been burning a hole in your retina :-/ Lots and lots of PWM out there.

And LEDs can be modulated at relatively high frequencies, such that you won't see the flicker, and it is unlikely to alias with a camera. The MOSFET might get warm if run at a higher frequency, so the modulation technique runs at a lower rate.

Cameras have two (electronics-based) shutter types.

Rolling shutter (most common, cheep) Global shutter (less common, avoids artifacts from flashing lights)

"Progressive Scan (Global Shutter) Versus Rolling Shutter"

Paul

Many thanks for the replies :-)

Is the light output controlled by the "on" time, i.e. are all my bulbs at home made with the same parts but some are on for longer so appear brighter?

You see the same with traffic signal lights if they are LED.