Just wanted to share a little thing I and mostly parametrek have been doing to assess the visibility of flickering/pwm on lights. The basic idea came from derlichtpeter.de whose CFD index tries to convey in a single value how visible flickering is.
We just call it the snob index, but the technique is similar. This is still very much work in progress and uses a modified curve from the CFD. It takes into account the modulation/flicker frequency and its other fft harmonics, percent modulation and duty cycle. A snob index of greater than 20% is usually objectionable and clearly visible. At over 50% it is outright annoying. We are still working on the thresholds. I’m waiting for an arbitrary signal generator to try different factors and their visibility.
The workings of the process are simple. I take a measurement with a scope using a Thorlabs DET36A photodetector, transfer the scope memory via lan to the computer where parametrek’s script parses, performs an fft and other calculations to spit out a bunch of metrics. Then another script draws a graph which represents the output of the light source relative to time. This graph also has the metrics calculated previously.
The metrics are:
Frequency - the rate of flicker in hertz
Modulation - also known as percent flicker which is calculated by (max level - min level) / (max level + min level). 100% modulation means that the light is turning completely off during the cycles
Duty cycle: the relative time the light spends above the mid level during one cycle. 50% means that the light is above mid (“on”) 50% and below mid (“off”) 50% of the time.
Index: flicker index is calculated using the area under the curve. This integral is separated into area above and below the average output. The index is the area above that average divided by the total area. A pwm light with a duty cycle of 50% will have a flicker index of 0.5, but it is more useful with more complicated periodic waveforms, especially those whose modulation is less than 100%.
Snob index: a rating, which tries to convey a single number of flicker visibility
1-5% - Probably not visible to the naked eye (high frequency PWM, >10kHz or low modulation at lower frequencies)
5-10% – Might be visible to the most sensitive people who know what to look for and try hard enough (incandescent)
10-20% – Sensitive people will see it at least on bright reflections, probably not a dealbreaker yet in general use
20-50% – YMMV area. If you can usually spot mid frequency PWM, you’ll easily see the flicker
>50% – Visible to most people at least on some occasions (low frequency PWM, <200 Hz)
Here’s an example with two PWM lights that have very different visibility of flicker and an incandescent 60W bulb in a 50Hz AC grid.