Are LED aisle lights in theatres being pulsed?

Article By : John Dunn

In this blog post, John Dunn offers an answer as to why LEDs seem pulsing like automobile tail light LEDs when captured with a camera phone.

Christmas is fast-approaching and bright light displays are more incredible than ever. If you have photographed light displays with your camera phone and wondered why even stationary light displays appeared fuzzy or pulsing, you are not alone. I have found a possible explanation for that.

While I was in a theatre waiting for the start of a movie, I noticed two rows of LEDs lining the edges of the centre aisle. Just on a whim, I photographed one of the rows with my cell phone's camera and got this image.

[Movie theatre aisle lighting cr]
Figure 1: The rows of LEDs lining edges of the theatre's centre aisle caught my attention.

As I scanned my eyes from side to side, I noticed the LEDs were pulsing like those automobile tail light LEDs I once wrote about, so I swung my cell phone's camera up and down a little and got this next image.

[Movie theatre aisle lighting1 cr]
Figure 2: LEDs appeared to be pulsing in sync in the dark.

The LEDs were all being pulsed in sync with each other but the only number of pulses I could capture in any single image was six. It was not five, it was not seven, it was ONLY six.

What at first puzzled me was why were these aisle lights being pulsed at all? Those automobile tail lights had changeable brightness levels so it seemed plausible that they used pulse width modulation for brightness control, but the theatre lights were always run at a constant brightness so why were they being pulsed too?

It finally occurred to me that the pulsing was merely incidental and was at the power line frequency. I know that LEDs don't like reverse voltage so perhaps each LED was in series with another diode to absorb reverse voltage during one half of the AC cycle and maybe, with a couple of added resistors, each LED circuit might have looked something like this:

[LED circuit cr]
Figure 3: The LEDs are in a series with a separate diode, which absorbs reverse voltage halfway in a cycle.

The camera had captured six dots for each LED because with the LEDs turning on and off at line frequency, the time between turn-ons would be 1/60th of a second or 16.6ms. From that, the six visible dots in the picture would mean that my cell phone camera's effective shutter speed would be six times 16.6ms or 0.1s.

This was apparently the shutter speed of my cell phone's camera that was affecting each picture I took. That's rather slow as camera shutter speeds go, but it would explain why this other picture I once took of a spinning ceiling fan has fuzziness at both edges of the fan blades.

[Fuzzy photo of ceiling fan cr]
Figure 4: The fan blades appeared blurry while in motion due to my camera's shutter speed.

At both edges of each blade, the camera is averaging its received light over that 0.1s time interval which is partly bright from the ceiling tiles and partly dark from the fan blades. It also explains why getting sharp, non-fuzzy photographs of my two grandsons in action can be such a challenge.

First published on EDN.

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