Car headlight flicker - gopro

candytog

Hi there,

I'm a wedding videographer and lately I've taken to mounting a gopro on my car and following the wedding car.

Only issue is at the last wedding the wedding car was a BMW and the headlights flickered like crazy on the footage.

Granted it was a wet day so I probably won't use the footage but for future this could be an issue.

My question is does anyone know what frequency these lights are (50hz etc) and is it different for different cars?

If I know I can set my frame rate to suit.

Thanks in advance

vectra

Did the BMW have LED lights by any chance?

pa990

I see that all the time on top gear.

I presume that it isn't an easy fix, otherwise they would remedy it.
I mean after all they have a massive fx budget

Foxhole Norman

If they're LED lights then camera's can't really pick them up, they flicker like crazy.

galwaytt

Yep, took footage of some Audi's at a track event last year and all the rear LED's showed up as flickering.

Lucifer

This is due to the way they control the LED lights on the car. With the rear lights they will use PWM (Pulse Width Modulation) to lower the brightness of the rear light to be the normal rear light when the lights are turned on, and then put it on full brightness for the brake light.

Something like this http://provideocoalition.com/aadams/story/pulse_width_modulation_is_not_your_friend/

ironclaw

Lucifer wrote: »

This is due to the way they control the LED lights on the car. With the rear lights they will use PWM (Pulse Width Modulation) to lower the brightness of the rear light to be the normal rear light when the lights are turned on, and then put it on full brightness for the brake light.

Something like this http://provideocoalition.com/aadams/story/pulse_width_modulation_is_not_your_friend/

This. You will also see on slow-mo on pitch side banners. The only correction would be to adjust your frame rate to suit.

pete4130

It's to do with the shutter speed/frame rate the camera uses. If the camera is shooting with a high shutter speed then it captures frames that are dim, bright or brightest for example and this shows as flickers. If you lower the shutter speed of the camera (less frame per second) the single frame will capture all 3 and it will look more evenly lit.

I'm a photographer so I see this a lot with LED's. Slow your shutter speed down and try to even it out.

tossy

Did anyone actually read the OPs post ? He knows what causes it and knows he has to change his frame rate.

Special Circumstances

Searching for "automotive HID ballast frequency" gives this page as one result:
http://www.aa1car.com/library/hid_headlamps.htm

"The ballast adjusts the voltage and current frequency to operating requirements. The AC ballast frequency is usually in the 250 to 450 Hz range."

That's for HID lights.

I didn't have much luck finding a frequency for DRL or Brake lights in a quick google. The results I got for "automotive LED driver PWM frequency" seemed very high, and contradicted my own feeling that the PWM used on some LEDs seems to produce a slight flicker visible to the human eye.

candytog

Special Circumstances wrote: »

Searching for "automotive HID ballast frequency" gives this page as one result:
http://www.aa1car.com/library/hid_headlamps.htm

"The ballast adjusts the voltage and current frequency to operating requirements. The AC ballast frequency is usually in the 250 to 450 Hz range."

That's for HID lights.

I didn't have much luck finding a frequency for DRL or Brake lights in a quick google. The results I got for "automotive LED driver PWM frequency" seemed very high, and contradicted my own feeling that the PWM used on some LEDs seems to produce a slight flicker visible to the human eye.

Thanks for the all the replies! Just to clarify the car did have circular led lights and it was the headlights I had the issue with.

So looking at the article above it seems the car adjusts the voltage frequency to regulate the brightness, so really the only way to get around it is test beforehand and see can you eliminate it in with the camera settings.

However I think I may have found the issue in my case.

I had the gopro set to 30fps (my bad) which in fact records at 29.97 fps. Flicker results when your framerate/shutter are not exact multiples of the light frequency.

If I shoot at 25fps then it's more likely to be a multiple of the cars headlight freq (the freq range above seems to step in multiples of 50).

This may not be the case but it's more likely to work than 29.97

But I guess the only way to be sure is to test it before I set off (or visit a friendly dealer with my gopro..)

Thanks again!

Oral Surgeon

tossy wrote: »

Did anyone actually read the OPs post ? He knows what causes it and knows he has to change his frame rate.

Sure we did.
Op, only film older cars in future or kindly ask that they adjust their headlights in future.

Special Circumstances

candytog wrote: »

Flicker results when your framerate/shutter are not exact multiples of the light frequency.

If I shoot at 25fps then it's more likely to be a multiple of the cars headlight freq (the freq range above seems to step in multiples of 50).

This may not be the case but it's more likely to work than 29.97

But I guess the only way to be sure is to test it before I set off (or visit a friendly dealer with my gopro..)

Thanks again!

Sounds like 25fps is worth a try. Unfortunately, in my searches I also found reference to LED drivers that don't stay at a particular frequency - http://www.elektor.com/news/Spread-Spectrum-LED-driver/
I think someone with more expereince of videoing modern cars may have better advice for you than the motors forum.