I paid a visit to the Frankfurt Dippemesse on the 5th of May to get some photos of the fireworks and the fair at night. Generally I got some decent long exposures using a tripod and settings with low ISO and 2 to 4 second shutter. There are a lot of bright lights so f stop can be f8 to f11 and even f16 for more starry effects in the bright spots.
One thing that I noticed when looking at the photos is that most of the lights on them did not produce constant blur lines in the photo. Most of the lines of light are composed of dots when you look at them closely. I have noticed this phenomenon more over the years and finally realised the cause after blaming my technique, camera and other superstitious reasons.
In the olden days, lights were almost all incandescent bulbs or halogen. Nowadays, everything is LED. LEDs are cheap, efficient, don't produce heat like halogens and last longer, so they are now find everywhere in torches, home and automobile lighting and even fun fairs. The downside to LED lights is that they pulse (for cheap ones) at a much slower rate than incandescent lights or florescent tubes. As far as I can tell from reading on the internets an old style light bulb pulses at twice the frequency of the power supply current, while a cheap LED pulses at half of the frequency. This is measured in Hertz. Let's pretend the power supply is 50 Hertz, a florescent light would pulse at 100 Hertz, an LED at 25. Apparently, all lights on AC flicker but the effect is made more prominent with LEDs as photon are not emitted when they are flickering on and off. While a regular light bulb is in an off state it still produces heat and some light. If the LEDs are dimmable, this is achieved not by lowering the amount of voltage as in the case of a light bulb, but by increasing the time the LED is off per flicker cycle.
Does any of this matter?
Health-wise, lights that flicker at a slower cycle can cause seizures in some people. For long exposures, lights with slower flickers on fast moving objects are easy to spot as a series of dots rather than a single line as in the examples here. Photographers have long been aware of the effect of florescent lighting and shutter speed causing issues at certain rates. The LED issue is a newer version of this but I also found recently that if a room is predominantly lit by LED, my Sekonic flash meter doesn't function properly as it thinks a flash has been fired when it is actually the room lighting flickering. It was unable to get a reading at all while the LEDs were on. This even occurred when a regular light was the dominant light source. The only solution was to turn off the offending lights while reading for flash exposures. Seeing as LEDs are not going away any time soon, I guess we will just have to get used to seeing lines of Morse code instead of smooth clean light trails during long exposures in future.