Interesting LED light effect

Discussion in 'Bikes and Buying Advice - What Bike?' started by TimO, 15 Aug 2007.

  1. TimO

    TimO Veteran

    I noticed this on some footage I took on the way into work today. Some of the more modern (or updated) London buses have LED rear/brake lights and indicators. I suspect the brightness of the rear lights is probably adjusted by pulse width modulating the drive to the LEDs. The frequency this is done at is near, or is some near multiple of the frame rate of my ATC2K, since you get a flashing effect on the footage which clearly isn't visible to the naked eye (or wasn't to me on this occasion).

    This short video shows the effect on a Bus just in front of me at the traffic lights on the north side of Chelsea bridge.

  2. Arch

    Arch Married to Night Train

    Salford, UK
    Yes, I've noticed that. Some of the buses here have LED light displays for their destination boards, so the whole board flashes like that!

    Been thinking about using the ATC in winter, knowing it's not much good in the dark. Would mounting a small light alongside on the helmet help, or would the light beam just get swallowed up in the ambient light? (I suspect this would be so...) Of course, if it was an LED, this flicker might cause a problem anyway...
  3. Keith Oates

    Keith Oates Janner

    Penarth, Wales
    As you say, interesting. The commuter ahead of you didn't seem to have much space although the van did wait for him to move out and on!!!!!!!!!!!!!!!!!
  4. OP

    TimO Veteran

    I've got a USE Exposure Enduro Turbo light, aligned on the same boresight as the camera, which has dual Luxeon LEDs powered by a Li-Ion battery. It's probably the brightest entirely self contained bike light you can buy. Without street lighting you still can't see much, unless it's something like a wall a couple of foot in front of the camera, which isn't generally the case!

    Car lights show up OK, and very very very well lit junctions show up badly, as do some street signs, but in general no lights which you could sanely carry are going to be enough to illuminate the scene for this camera.

    If you really wanted to film at night I suspect you would have to buy one of the bullet cameras which are designed for low light usage.
  5. col

    col Veteran

    Interesting,but your lucky if the lights work on the buses where i work,they are old and falling to bits,and wont be renewed anytime soon.
  6. Arch

    Arch Married to Night Train

    Salford, UK

    Yeah. I guess in the winter months, the camera will have to be worn for effect only!:ohmy:
  7. domd1979

    domd1979 New Member

    Quite often if you take a photo of a bus with an LED destination display, you end up with it either appearing blank or a stripy effect going on! Something to do with the display refreshing however many times a second, which means its momentarily blank when you take a phot.

    On modern vehicles, the light clusters at the back use some kind of multiplex wiring, which means that the brake light isn't in any way (directly) connected to the brake pedal. Signals from some kind of CPU go down a wire to tell the light to light up, so guess there's a similar thing going on to the displays where it has to refresh several times per second... although I'm not an expert!! An LED bike lamp presumably is less complex and is either on or off, and isn't switching multiple times per second.

  8. Road Fiddler

    Road Fiddler New Member

    All LED lights have this effect if they have the proper circuitry, according to a number of articles i have read re emergency beacons and led torches, it has something to do with the efficient use of power drain so that the circuitry does not burn out or something. I am sure candlepower forum has a thread or two about this.

    I have an very small emergency beacon that is designed to be whirled around your head on a length of string to make it more noticeable to helos, When you whirl it around you can actually see the flashing effect. A camera frame rate is a lot slower than the frequency of the flashing but it catches some flashes whilst they are on and some when they are not on so you get the effect of a much slower flash.
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