Many LED fixtures modulate their light output while they’re on. Most of the time you won’t consciously notice it. But some people do — as eyestrain, headaches, or a subtle sense that a room feels “off” — and a phone camera often reveals it immediately as rolling bands across the screen.
LED flicker is rarely about the LED itself. It’s almost always about the driver, the dimmer, and how the two were specified together.
What Flicker Is
IEEE Std 1789 describes flicker simply as variation in luminance over time. The International Commission on Illumination (CIE) defines it from the viewer’s side: the impression of unsteadiness caused by light that fluctuates. The broader term is temporal light artefacts (TLA) — unwanted visual effects caused by light that changes over time. In practice, “flicker” and “TLA” are often used interchangeably.
There are two main kinds:
- Visible flicker — light fluctuation at lower frequencies, roughly below 80 Hz, that you can see directly or on a lit surface. It’s irritating to most people.
- Stroboscopic effect — at higher frequencies, roughly 80 Hz to 2,000 Hz, the light itself looks steady, but moving objects appear to jump or leave a trail of images. Think of a spinning fan blade or a hand waved under the light.
Why It Matters
Research summarized in IEEE 1789 links flicker to headaches, eyestrain, fatigue, and reduced visual performance, with greater concern for longer exposure and for people who are sensitive to it. Low-frequency flicker is also a concern for photosensitive individuals. Beyond health, flicker shows up on video calls and phone recordings — increasingly important in homes and offices where cameras are always on.
What Causes LED Flicker
An LED driven by steady current produces steady light. Flicker comes from what feeds it:
- Driver design. Drivers with too little output filtering pass current ripple straight through to the light. Some drivers dim by switching the LEDs on and off (pulse-width or pulse-frequency modulation); at low frequencies or deep dimming levels, that switching can become visible.
- Dimmer incompatibility. In residential work, this is the most common cause. A phase-cut dimmer paired with a driver it wasn’t designed for can flicker, shimmer, or drop out at the low end.
- Power quality. Voltage fluctuations on the circuit, such as large loads switching on and off, can show up as visible flicker in sensitive drivers.
How Flicker Is Measured
Older metrics — percent flicker and flicker index — describe the waveform but don’t predict well what people will actually notice. The newer metrics are built around human perception:
- PstLM (IEC TR 61547-1) predicts visible flicker. A value of 1.0 is the level at which about half of observers would notice it; lower is better.
- SVM — Stroboscopic Visibility Measure (IEC TR 63158) predicts the stroboscopic effect. Again, 1.0 is the threshold of visibility for half of observers; lower is better.
NEMA 77 uses both metrics to set recommended limits for the North American market, and IEEE 1789 offers frequency-based guidance for low-risk operation. California’s Title 24 JA8 also includes a reduced-flicker requirement for qualifying residential light sources. None of these settles every question, but together they give specifiers something concrete to ask for.
How to Specify Around It
- Ask manufacturers for PstLM and SVM data — at full output and dimmed. Many fixtures that perform well at 100% behave very differently at 10%.
- Specify the driver and dimming protocol, not just the fixture, and confirm the dimmer appears on the manufacturer’s compatibility list.
- Treat a substitution as a new driver until proven otherwise. A cheaper “equivalent” often has a different driver.
- Test it. A quick field check is a phone camera in slow-motion mode pointed at the light: bands or pulsing suggest modulation worth investigating. It isn’t a measurement, but it’s a useful screen.
We confirm driver and dimmer compatibility before a fixture is specified, check low-end dimming on the control platforms in the Illumify Design Lab, and recheck at commissioning. It’s one of the reasons a fixture that fails at trim-out usually failed at specification first.
Illumify is a lighting design firm led by Jared Jin, P.E., LC, MBA, based in Norcross, Georgia. We carry lighting intent from design through commissioning for architects, interior designers, builders, and owners across metro Atlanta and the Southeast. Book a free 30-minute discovery meeting · info@illumifyhome.com · 770-341-0761
