# AC frequency in the UK

James2020
When I record slow motion video of my filament bulb lamp connected to mains, I can be completely sure that it flashes 25 times per second. This is half of the 50 hertz AC frequency. I would expect it to flash 100 times per second as the current reaches a node twice per cycle in it’s AC wave. Why is this not the case.

Welcome to PF.
Does the filament lamp have a dimmer?
What is the frame rate of your camera?

James2020
No the lamp doesn’t have a dimmer.
The frame rate of the camera is 120fps whilst recording in slow motion. I can observe the lamp visibility dimming & brightening so it certainly seems that the frame rate is high enough.
Thank you.

You are probably observing the beat between the 120 FPS and the 100 Hz energy emission you expected. Based on those numbers, the difference frequency is 20 Hz.

James2020 and sophiecentaur
Gold Member
The frame rate of the camera is 120fps whilst recording in slow motion.
The camera is recording 'samples' (frames) of the scene. Nyquist's Sampling theorem says that, to get an accurate version of a changing scene, you need to sample at twice the rate that the scene is changing. If you are looking to record 100Hz variations correctly then you would need (significantly more than) 200Hz sampling rate. What you are seeing is referred to as an Alias. @Baluncore correctly describes it as a "beat". It's just like the wagon wheels going backwards on old films.

To avoid this problem, use DC lighting!

It's easier to describe the same effect in audio. Digital sound ADCs always use a low pass filter at half the sampling frequency to prevent aliases of high input frequencies turning up as low frequency aliases. Your digital camera has a spatial filter (fuzzy layer over the sensor) to avoid jazzy alias patterns. And we all (?) remember the problem of fine patterns on clothing producing distracting aliases when displayed on analogue colour TV. (Yes- sampling problems come in on analogue systems too)

James2020
My guess is that the camera was designed to operate at 120 fps so as to have a zero flicker rate when operated in countries like the USA that have 60 Hz power.
It is possible that it can be switched to 100 fps for use in countries where 50 Hz power is used.

sophiecentaur