Photoing Colour Lights

[Peter Kazmierczak]

These two pictures were taken seconds apart.

 

 

 

Why does one show the red aspect and the other zilch? The signal wasn't changing from one aspect to another either.

 

Thanks in anticipation.

[colin penfold]

I suspect it's the same as trying to photograph a TV screen. Although the eye doesn't pick it up the light is actually flicking on and off rapidly. The camera fires for such a short time it can fire during an off part of the flicker. The solution with TV was to use 1/30 sec or slower, which meant you inevitable caught pert of the "on" cycle. I suspect a signalling expert will tell you the science and give you the appropriate shutter speed for this application, or you could just experiment. Of course,  if your camera is digital you could just check the image after you take it, and re-do if necessary.

[russ p]

Has that signal got an LED cluster in it?

It may be like photographing 68s where the LED headlights appear to flash

 

In this picture the drivers side headlight appears to be out, but infact was actually set to main beam

[Peter Kazmierczak]

Thanks guys.

It's not one of the new-fangled LEDs, just one of BRs standard types - this one dating from Nov'74 I believe.

[St. Simon]

Hi Peter,

 

I presume that you could see that the signal was lit the whole time you were taking the photos?

 

If it was just a standard SL35 filament lamp, then I can't think of a reason for the light not to be lit in both photos, a filament lamp does not have a 'update rate' (frequency) to it, so it wouldn't of sync with that of the camera, if that makes sense.

 

Simon

[Peter Kazmierczak]

Thanks Simon,

I'm sure the signals at Strawberry Hill (where I took the photo) are still all the original type. Also I'm sure it was lit all the time - would have to be as it protected the level crossing and junction.

 

Think this one at Twickenham is LED though (put in when the new footbridge for the Rugby World Cup was installed), but all my pictures of it show it illuminated with no flicker.

 

 

[St. Simon]

Thanks Simon,

I'm sure the signals at Strawberry Hill (where I took the photo) are still all the original type. Also I'm sure it was lit all the time - would have to be as it protected the level crossing and junction.

 

Think this one at Twickenham is LED though (put in when the new footbridge for the Rugby World Cup was installed), but all my pictures of it show it illuminated with no flicker.

DSCN1833.JPG

Yeah, it looks like an filament lamp at Strawberry hill. All signals, unless they are approach lit but that's a new thing, have to be lit all the time, if the lamp goes out, it drops the lamp proving relay which turns the signal in rear to red and sets off a filament failure alarm in the signalling centre, which causes a bit of panic!

 

The ones at Twickenham are new(ish) Dorman LED heads, but almost all signals heads are DC powered, so there is no flicking, as DC current doesn't have a frequency, which causes the flickering.

 

Simon

[TheSignalEngineer]

............................... DC current doesn't have a frequency, which causes the flickering.

 

Simon

DC derived by rectifying AC will have a peak to peak frequency of twice the AC frequency. As the quality of the smoothing of the supply improves this becomes a ripple rather than a zero-peak-zero waveform. The degree of ripple can be varied by design to suit the application and can affect the life of the connected equipment. 

[royaloak]

It is probable that the signal head has had the normal SL35 lamp replaced with an LED equivalent hence the pulsing and your first picture has caught it when it was blank and the second one has caught it when it was lit.

 

A lot of 'ordinary' heads are having these LED packs fitted, they are easy to tell because the colours are more 'polar', ie red is redder etc.

 

Edit-

This thread post 63 shows the new LED lamp-

http://www.rmweb.co.uk/community/index.php?/topic/112706-a-new-signal-to-me/page-3&do=findComment&comment=2378267

[Southernman46]

I've had the same issue photographing bus with LED destination displays - depending on luck you get nothing / part or the full message

[jamesg]

I've noticed it mainly when photographing Voyagers and class 170s with LED destination displays, but also the 68s with LED headlights.  In information displays, not all parts of the display are lit at the same time; rows, columns or elements are multiplexed, relying on persistence of vision to make the whole pattern visible.  This technique makes the LED display easier to construct.

 

I don't know why the headlights and signal lights would appear to flicker.  It could be, as has been suggested, because they're being supplied from rectified AC power.  Another possibility is that there's a PWM circuit involved for setting the apparent brightness of the lamp by switching the LEDs on and off very rapidly, again relying on persistence of vision.