Class 68 Alternating Marker Lights? Or just a frequency clash with LED vs Camera technology?

[Bob Sumsion]

Opinions are divided, but IF this is a clash of LED driver frequency vs Camera shutter frequency, then why is the top light not effected in both videos, and why are the alternating lights operating at the same frequency of changeover in both videos?

Thoughts?

Robert

 

[tumbledown]

Theory: The headlights need to be dimmable so are run from 'chopper' circuits with variable on-off proportions. The frequency of the chopper is close to a multiple of the (standard) video frame rate so you get 'beats'.
The pilot light at the top of the cab doesn't need to be dimmable so is run on constant current.

 

[Bob Sumsion]

Theory: The headlights need to be dimmable so are run from 'chopper' circuits with variable on-off proportions. The frequency of the chopper is close to a multiple of the (standard) video frame rate so you get 'beats'.
The pilot light at the top of the cab doesn't need to be dimmable so is run on constant current.

For LED lights in general, I can accept your view. However, the two videos are made by different YouTubers and it does not explain the changing of lights alternately at a slow, but regular frequency, this does not match what I have seen previously with LED vs Camera technology. I think there maybe more to this. Unless both video makers happened to use the same frame rate in which case your reply holds water.

 

[chrisjo]

Video I took of 68004 a couple of weeks ago shows the top light constantly lit and the two lower ones flashing on and off about three or four times per second, also alternating like yours - you can't see the alternation when the video is running, but if you freeze-frame it there's sometimes one lit but not the other.

 

[Bob Sumsion]

Video I took of 68004 a couple of weeks ago shows the top light constantly lit and the two lower ones flashing on and off about three or four times per second, also alternating like yours - you can't see the alternation when the video is running, but if you freeze-frame it there's sometimes one lit but not the other.

Did you manage to view the event with just your eyes rather than through the viewfinder? Beginning to wonder about the frame rates like tumbledown mentioned.

This video has the look you describe.

 

[Ashley Hill]

I asked this question on YouTube 7 years ago and never got an answer after posting a clip of a 68 pulling out of Marylebone. Here’s the clip,it shows the drivers side lamp starting to blink whilst pulling away.

 

[Bob Sumsion]

I asked this question on YouTube 7 years ago and never got an answer after posting a clip of a 68 pulling out of Marylebone. Here’s the clip,it shows the drivers side lamp starting to blink whilst pulling away.

I am now thinking that a combination of tumbledown's description coupled with varying frame rates used by different Mobile Phone and Digital cameras used for the videos explains the varying rates of alternating light patterns. This video on fidget spinners vs frame rates looks close to describing what is happening.

 

[chrisjo]

Did you manage to view the event with just your eyes rather than through the viewfinder?

No I have never noticed it happening when just looking. I've just been searching through all my still photos of 68's and I've found just one where one light appears to be unlit. Don't know what this means though!


(...and I just noticed I spelled Grayrigg wrong. Bulk re-name coming up.)

Also interesting are these two stills of an 88 taken in the same burst.

 

[Ashley Hill]

For the record Class 88s do it as well. Here’s a clip by zctrainspotting showing an 88 departing with flickering lights.

I personally think it’s digital technology that creates this illusion of flashing because the lights on the locos do not flash to the naked eye,plus it would not be allowed.

 

[Bevan Price]

Many LED & LCD lights are actually flickering, but at rates that cannot be detected by the human eye. If shutter speed is faster than the flicker speed, there is a chance that your exposure coincides with a time when the light is "off".
You can get similar effects when photographing bus or train destination screens.

With stationary vehicles, you can eliminate the problem for destination screens by using longer exposure times (typically 1/50th second or more), because that captures several cycles of the light source. With moving trains, it is pot luck as to whether the light is on or off at the same time as the shutter is open.

 

[norbitonflyer]

What is odd is that, in most of the videos, one light is flashing faster than the other, so that they are going in and out of phase. This suggests that the two lights' flicker rates are not the same. Why would that be? Are they set to different brightness levels?