Flicker Measurement: IEEE 1789, SVM, and What They Mean for Driver Selection

Flicker — the rapid, repeated variation of light output — has moved from a niche concern to a mainstream specification requirement in LED lighting. It is linked to headaches, eyestrain, and reduced visual performance, and in industrial settings, the stroboscopic effect can make rotating machinery appear stationary or slow, a genuine safety hazard. The industry has responded with measurement methods and recommended limits, the most cited being IEEE 1789 and the SVM (Stroboscopic Visibility Measure) metric. Understanding these numbers helps buyers interpret datasheets and distinguish a driver that is truly low-flicker from one that merely claims to be.

Two basic metrics describe the magnitude of flicker. Percent flicker (also called modulation depth) is the ratio of the peak-to-peak variation to the average light output: (Lmax − Lmin) / (Lmax + Lmin) × 100%. Flicker index is a more refined metric that accounts for the shape of the waveform, weighting the amount of time the light is below the average. Both are simple to measure with a photodetector and an oscilloscope — the light output waveform is captured, and the two numbers are computed from it. What they do not capture is the frequency content: a 5 percent ripple at 100Hz is perceptually different from a 5 percent ripple at 10kHz, and this frequency dependence is where the modern metrics come in.

IEEE 1789 (the IEEE Recommended Practice for Modulating Current in High-Brightness LEDs) was published in 2015 to address the biological effects of flicker. It defines two risk regions based on the modulation depth as a function of frequency. In the low-risk region, the recommended modulation limit is roughly 0.025 × frequency for frequencies below 90Hz, rising to 0.08 × frequency between 90Hz and 1,250Hz. For a 100Hz light output ripple, that means a modulation depth below about 8 percent; at 120Hz (the double-mains frequency in the US), the limit is around 10 percent. Above about 1,250Hz, the modulation is essentially not perceptible and no limit applies. The no-observable-effect level is stricter still, roughly one-third of the low-risk values. The practical takeaway for drivers: output ripple that would cause visible flicker at 100-120Hz must be kept below roughly 8 to 10 percent modulation, and any higher-frequency components above 1.25kHz are perceptually invisible.

The SVM metric (defined in IEC TR 61547-1 and CIE TN 006:2016) addresses the stroboscopic effect specifically — the appearance of moving objects under modulated light in the 80Hz to 2kHz range. SVM is computed from the frequency components of the light waveform, weighted by the human visual system's sensitivity to stroboscopic effects at each frequency. An SVM value below 1 means the stroboscopic effect is not visible to the average observer; above 1 it becomes visible. Many modern lighting specifications require SVM < 1, and some premium specifications require SVM < 0.4. The SVM calculation is more involved than percent flicker, but the interpretation is simple: below 1, the light is effectively stroboscope-free for practical purposes.

How does this relate to the driver? In a constant voltage system, the light output ripple is driven by the voltage ripple on the DC bus, which is a direct consequence of the driver's output filter design and its control loop. The output ripple of a quality 24V driver is typically below 240mV peak-to-peak, which on a 24V bus is 1 percent — far below the 8 to 10 percent modulation that would produce visible flicker. In contrast, a poorly designed or heavily loaded driver may have output ripple of 1V or more — 4 percent of 24V — and if the ripple is at 100Hz, the resulting light modulation may approach the perception threshold. Higher-frequency ripple, from the switching converter (65kHz and above), is well above 1.25kHz and is perceptually invisible, so it is the low-frequency ripple — related to the mains rectification and the control loop — that matters for flicker.

For a buyer, the practical checklist is short. If flicker matters for the application (offices, hospitals, video production, industrial areas with moving machinery), ask the supplier for the driver's output ripple specification and, for critical projects, the measured flicker metrics: percent flicker and SVM. Confirm the driver's output ripple is below roughly 1 percent of the output voltage, which is the common design target for low-flicker performance. And note that non-dimmable drivers have an inherent advantage here: without a dimming interface, there is no phase-cut waveform, no control-loop interaction with a dimmer, and no low-level dimming instability — the flicker performance is set purely by the driver's own filter design, which is a controllable, verifiable quantity.