Flicker Metrics: Percent Flicker, Flicker Index, PstLM, and SVM

The word flicker-free appears on countless LED product datasheets, but it rarely comes with numbers. Flicker is a measurable quantity, and different metrics capture different aspects of it. For specifiers, knowing what percent flicker, flicker index, PstLM, and SVM actually quantify is the difference between a vague marketing claim and a verifiable performance requirement.

Percent flicker, also called modulation depth or the flicker percentage, is the simplest metric. It is calculated as 100 times the difference between the maximum and minimum light output, divided by their sum. A steady output yields 0 percent; a light that alternates between fully on and fully off yields 100 percent. The metric is easy to measure and widely quoted, but it ignores the shape of the waveform and the frequency of modulation, so two very different waveforms can share the same percent flicker.

Flicker index, defined in the IES Lighting Handbook and IES RP-16, adds information about the waveform shape. It is the area above the average light level divided by the total area under the light output curve, giving a value between 0 and 1. A brief, sharp dip produces a lower flicker index than a prolonged dip of the same depth. Flicker index is more informative than percent flicker for comparing waveforms, but it still does not weight the effect by frequency.

The IEEE 1789-2015 recommended practice ties the allowable modulation to frequency. For modulation below 90 Hz, the recommended maximum percent flicker is 0.025 times the frequency. Between 90 Hz and 1250 Hz, the limit is 0.08 times the frequency. Above 1250 Hz, there is no restriction on modulation depth. This explains why high-frequency PWM dimming, typically 20 kHz or higher in quality constant-voltage drivers, is inherently flicker-safe regardless of duty cycle: at those frequencies the modulation is simply too fast for the human visual system to resolve.

PstLM is a short-term flicker severity value adapted for lighting, defined in IEC TR 61547-1. It applies the methodology of the IEC 61000-4-15 flickermeter, which was developed for mains voltage fluctuations, to the temporal light waveform. A PstLM of 1.0 represents the threshold where an average observer would begin to notice flicker, and values below 1.0 are generally considered acceptable. PstLM accounts for both frequency and depth in a way that simple metrics do not.

SVM, or stroboscopic visibility measure, is defined in IEC TR 63158. It evaluates flicker in the frequency domain, applying a weighting function to each spectral component of the light waveform based on how visible stroboscopic effects are at that frequency. The result is a single number where an SVM of 1.0 corresponds to the visibility threshold. SVM is particularly relevant for scenes with moving objects, where stroboscopic effects are most noticeable even when direct flicker is not perceived.

The different metrics answer different questions. Percent flicker and flicker index describe the raw waveform. IEEE 1789 provides frequency-aware design guidance. PstLM and SVM assess perceptual impact and are increasingly referenced in lighting specifications and procurement documents. A product can look good on one metric and poor on another, so a specification should name the metric and the limit rather than demanding flicker-free as an absolute.

When evaluating a DALI-2 constant-voltage driver for an office, school, healthcare, or video-production environment, ask the manufacturer for measured PstLM and SVM values across the dimming range, not just at full output. Drivers using PWM frequencies above 1 kHz, and preferably above 3 kHz, will normally satisfy IEEE 1789 and keep PstLM and SVM below 1.0. If the data is not available, treat the flicker-free claim as unverified.