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why do LED lights look like they're flickering when viewed through digital camera/camcorder/dash cam?

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  • why do LED lights look like they're flickering when viewed through digital camera/camcorder/dash cam?

    why do LED lights look like they're flickering when viewed through digital camera/camcorder/dash cam?

    I never noticed this before. When I stare at them with my bare eyes, they just appear as constant red light. I was in the passenger seat most cars with LED brake lights look like they're flickering. (including a couple of small Cadillac SUVs, an Infinity G35, and one of those new Dodge Charger(?) with the crazy taillight) When I looked away from the camcorder, digital camera, as well as my cell phone camera, their lights were rock solid steady. When I look into the viewfinder, they seemed to be flickering. This also appears when I playback these videos.

    Is this normal?

  • #2
    Chances are those LED's are being driven by a switching power supply that sends pulses of voltage to light up the LED's, the pulses are fast enough that the human eye can't detect them, but camcorders but are visible to recording devices.

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    • #3
      ^^this.
      ***MH1 Retro***
      "America has three cities, New York, San Francisco and New Orleans. Everywhere else is Cleveland."
      - Tennessee Williams

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      • #4
        Originally posted by axipher View Post
        Chances are those LED's are being driven by a switching power supply that sends pulses of voltage to light up the LED's, the pulses are fast enough that the human eye can't detect them, but camcorders but are visible to recording devices.
        I wonder if this is the reason that my cat freaked out last year when I started using big ass all-LED Christmas lights. Perhaps cat eyes could see the "flicker" in LED lights?

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        • #5
          i assume you mean when the tails are in tail lamp mode and not brake mode. right? because that would be the pulse width modulation which flashes the led's at a certain duty cycle to make them appear dimmer. and most use a switching frequency of around 60hz, which aggravates me.. i can't stand to be stuck behind one of those cars..
          The time you enjoy wasting, is not wasted time

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          • #6
            Even professional photographers/videographers can't avoid this effect (around 1:37): http://www.youtube.com/watch?v=4mRbWh6HiaA
            Last edited by soulsea; October 31st, 2013, 11:14 PM.

            2015.5 RR V8SC
            2015 Z06/07

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            • #7
              Originally posted by twophonecases View Post
              I wonder if this is the reason that my cat freaked out last year when I started using big ass all-LED Christmas lights. Perhaps cat eyes could see the "flicker" in LED lights?
              Well I can see those flickering as well, it's definitely a weird effect. The tail lights are indeed the same way, dimmed via PWM rather than under driving them.

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              • #8
                Im pretty sure all LEDs flicker regardless of how they are driven. When seen from the human eye, its not noticeable, but when seen at 30fps as most off the shelf camcorders do, you see the pulse.

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                • #9
                  No, it's dependant on the power source. Most of the AC powered LED Christmas lights and Halloween lights are non rectified AC power and have a 60 hertz flicker. So 60 times a second, pretty easy to see. The better ones use rectification decreasing that flicker to 120HZ, much harder to see.
                  For automotive use they may be using switch mode power supplies as mentioned, more likely with high powered leds. So yes, those are turning on and off as well, the frequency can be anywhere from a hundred hertz or several k-hertz.
                  Or as also mentioned dual brightness LED setups may run straight DC for full brightness but pulse width modulation for dimming. Again, periods of ons and offs.
                  To further complicate matters standard video cameras only capture ~30 frames per second. Imagine a still camera taking a photo 30 times a second. Well some of those times it will catch the LED in it's off cycle. At 29.97 frames per second it may actually ONLY catch the "off" cycles several times in a row, emphasizing them and making them appear longer than they actually are.

                  If you have an led powered directly from a DC source (such as a battery) try taking a video of it with your phone or whatever, and you won't notice any flicker.

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                  • #10
                    Originally posted by VegasF6 View Post
                    No, it's dependant on the power source. Most of the AC powered LED Christmas lights and Halloween lights are non rectified AC power and have a 60 hertz flicker. So 60 times a second, pretty easy to see. The better ones use rectification decreasing that flicker to 120HZ, much harder to see.
                    For automotive use they may be using switch mode power supplies as mentioned, more likely with high powered leds. So yes, those are turning on and off as well, the frequency can be anywhere from a hundred hertz or several k-hertz.
                    Or as also mentioned dual brightness LED setups may run straight DC for full brightness but pulse width modulation for dimming. Again, periods of ons and offs.
                    To further complicate matters standard video cameras only capture ~30 frames per second. Imagine a still camera taking a photo 30 times a second. Well some of those times it will catch the LED in it's off cycle. At 29.97 frames per second it may actually ONLY catch the "off" cycles several times in a row, emphasizing them and making them appear longer than they actually are.

                    If you have an led powered directly from a DC source (such as a battery) try taking a video of it with your phone or whatever, and you won't notice any flicker.
                    I don't have a video because my phone wouldn't record video last night, but I set up some high power LED's (~150 mA 10mm round dome LED's) with an Arduino and a current limiting resistor. At anything less than 80% duty cycle, the LED's whould show some flicker when looking at them through my phone screen.

                    Alternatively, the LED fog lights in my car, when driven by a constant current source, don't flicker at all on my phones screen.

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                    • #11
                      Yes interesting. As you say "80% duty cycle" is the key word here, that's PWM. The CC power source in your car may not be SMPS, with linear reg you don't have this effect. Or could
                      be that it's such a high freq switcher
                      just can't see it.

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                      • #12
                        Originally posted by VegasF6 View Post
                        Yes interesting. As you say "80% duty cycle" is the key word here, that's PWM. The CC power source in your car may not be SMPS, with linear reg you don't have this effect. Or could
                        be that it's such a high freq switcher
                        just can't see it.
                        Good call, with Arduino PWM duty cycle at 80%, flicker is eliminated when using a basic N-MOSFET and a resistor to control the current at 100 mA.

                        In my car, I'm using a 12V > 24V Boost Controller then going to a capacitor bank and a LM317T as a constant current regulator to drive the string of LED's at 100 mA.

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                        • #13
                          Originally posted by chawski View Post
                          Im pretty sure all LEDs flicker regardless of how they are driven. When seen from the human eye, its not noticeable, but when seen at 30fps as most off the shelf camcorders do, you see the pulse.
                          no, led's do not flicker unless the power supply is either pwm controlled or has a smps unit driving it.
                          The time you enjoy wasting, is not wasted time

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                          • #14
                            Ive notice it to with older led drl systems that still run led diodes but not with the newer "xB styled" led drls such as the new 2014 Corvette

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