Need Help w/Issue recording LED lights...

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Hi guys, I've been dealing with an annoying issue when it comes to the way my camera records certain flashing LED lights at railroad crossings. Some types of LED lights that I've recorded in my videos will appear to intermittently "black out", as seen in this video, or otherwise exhibit a "strobing" effect in the video recording. I need to see if anyone knows a.) what causes this weird phenomenon in the recording and b.) is there a setting on my camera that could be adjusted that would resolve this issue, or is this a problem inherent to my type of camcorder? The video below was recorded with a Canon VIXIA HF R21, using the MXP recording mode, and the PF30 frame rate setting. Other available recording modes on this camcorder are FXP, XP+, LP and SP; and other available frame rate settings are PF24 and 60i.

Here is the video that exhibits what I'm talking about (notice how the flashing lights at the crossing appear to "black out" intermittently every few seconds).


I know this is a fairly common problem, as I've seen it on other videos with LED crossing lights. Please respond with any insights and solutions you have concerning this issue.

Thanks!
 

CGW101c

CGW Fan
The problem comes from 2 things. One, your camera records frames at whatever rate you have set. The human eye puts the frames together into a moving picture essentially not seing the blank spots between frames, The LED crossing signal light is pulsed(flashed on an off) at a peak current to make the lights appear brighter without burning out the LEDs. When the pulse rate of the LEDs is roughly the same as the frame rate of the camera, what you are recording is the bright flashes and sometimes you are recording the off pulses. If you could make the camera frame rate the same as the LED pulse, it would not go on and off, but if you started at wrong time you would record the off pulse and not see the LEDs at all. Tom
 
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Hi Tom,

Thanks for your reply and analysis. Your explanation makes perfect sense, and I had never considered the fact that LED lights "pulse" before. I had been wondering if it might have something to do with the frame rate, and I'm glad somebody was able to confirm that for me. This video was recorded using the 30p frame rate setting; I wonder if switching to 24p might help with this problem? I guess it would probably just depend on the type of LED light and the "pulse frequency" of the LED vs. the frame rate my camera is recording in.

At any rate (pardon the pun), I wonder how the professionals in Hollywood deal with the issue. LED lights have become the industry standard in the 21st century, and I'm guessing all LED lights have this "pulsing" effect to avoid burning out the LEDs, so it seems this issue would extend beyond just railroad crossing lights - i.e. I could see it being a problem when recording scenes with LED street lights and traffic lights, LED car lights, LED store front signs, even inside buildings with LED lighting - basically any video shots where LED lights are being filmed; I would think this will become more of a problem going into the future as traditional incandescent lights are replaced with the new LED light technology.

It appears that modern cameras going forward will need to be designed to properly record LED lighting without the artifacting.
 
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UltraJoosh

Josh's Train Videos
Interestingly enough, it only seems to happen with WC Hayes LEDs. Not sure if that helps at all, but that's the only time it happens to me.
 

Eng8492

New Member
The problem comes from 2 things. One, your camera records frames at whatever rate you have set. The human eye puts the frames together into a moving picture essentially not seing the blank spots between frames, The LED crossing signal light is pulsed(flashed on an off) at a peak current to make the lights appear brighter without burning out the LEDs. When the pulse rate of the LEDs is roughly the same as the frame rate of the camera, what you are recording is the bright flashes and sometimes you are recording the off pulses. If you could make the camera frame rate the same as the LED pulse, it would not go on and off, but if you started at wrong time you would record the off pulse and not see the LEDs at all. Tom
I think you will need a higher end video camera to deal with this. First of all you have to take the camera out of automatic mode and put into shutter priority mode. Then you want to adjust the shutter to be as slow as possible without overexposing the shot. My video camera (Canon HF200) has a histogram display so you can see if anything if overexposed.

Basically in shutter priority mode, the camera will be adjusting aperture and/or gain of the image sensor (depending on how its designed and whether it has a lens with an iris) to keep things from becoming overexposed and underexposed--but it will NOT adjust BOTH shutter speed and aperture---- so there is a greater risk of the shot being too bright or too dark.

Finally, if the shutter is set too slow, the cars of a fast moving train may appear to become very slanted in your video, ruining your shot.

You have to experiment, a lot.
 




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