Percent flicker
Percent flicker (modulation depth)
Definition
Percent flicker is the relative variation in light output of a source over one full modulation cycle, expressed as a percentage. 0% means no flicker; 100% means the light fully turns off and back on each cycle. It is the simplest and most widely-reported flicker metric.
Source overview
Percent flicker is computed as 100 × (Lmax − Lmin) / (Lmax + Lmin), where Lmax and Lmin are the maximum and minimum luminance values within one modulation cycle of the waveform. The metric is also called modulation depth or peak-to-peak modulation. It captures the amplitude of the modulation but not its waveform shape — for shape sensitivity, see flicker index.
The underlying standard is paywalled or otherwise not directly reproducible here. The text above is a plain-language summary written by Slick Engineering — not a verbatim quote. Authoritative source linked below.
Technical detail
By convention, percent flicker is computed at the dominant frequency component of the waveform. Multi-frequency waveforms (common with phase-cut dimmers and switching converters) require selecting the modulation cycle of interest.
Percent flicker is a necessary but insufficient metric on its own. A 100 Hz square wave with 30% modulation depth and a 100 Hz sinusoid with 30% modulation depth have the same percent flicker but very different perceptual impact — the square wave sweeps more energy through the eye's flicker-sensitive band. That's why flicker index (which integrates over the waveform shape) and PstLM (which applies a perceptual weighting filter) exist alongside it.
Where it appears
- FlickerSpec Used here
Reported on every FlickerSpec measurement (Quick and Certified, free and Pro). Drives the IEEE 1789 verdict together with dominant frequency.
Related terms
- Flicker index
Flicker index is a unitless number from 0 to 1 that captures both the amplitude and the waveform shape of light modulation. It is the area o…
- IEEE 1789-2015
IEEE 1789-2015 is an IEEE recommended practice that defines low-risk and no-effect frequency / modulation-depth thresholds for current modul…
- PstLM
PstLM is a single-scalar perceptual metric for how annoying flicker from a light source is to a static human observer. PstLM = 1.0 correspon…