Every LED driver modulates its output at the line frequency. Done well, you never notice. Done badly, it strobes on camera, triggers migraines, fatigues the eye. IEEE 1789 and IEC TR 61547-1 finally gave the industry the metrics to spec against it.
Four ideas to carry into the next specification conversation you have.
Low frequency (< 100 Hz) + high depth (> 30%) = visible flicker. High frequency (> 1 kHz) is safer even at higher depth.
Percent = (max-min)/(max+min). Index = area under modulation/total area. Both are line-frequency dependent. Useful, not complete.
Stroboscopic Visibility Measure (CIE TN 006). A drill or a fan spinning under flickery light reads as static images, accident risk.
Short-term flicker (lighting) per IEC TR 61547-1. Office workers, hospital patients, the test that matters indoors.
Modern thresholds per IEEE Std 1789-2015 and IEC TR 61547-1.
| Metric | Threshold (premium) | Affected scenario |
|---|---|---|
| Percent Flicker (PF) | < 8% (low-risk zone) | General visibility |
| Flicker Index | < 0.1 | General visibility |
| SVM (Stroboscopic Visibility Measure) | ≤ 0.4 (no effect) | Workshops · sport · factories |
| PstLM (Short-term flicker) | ≤ 1.0 (imperceptible) | Offices · classrooms · hospitals |
| Modulation frequency | ≥ 1.25 kHz (low-risk) | All, IEEE 1789 risk plot |
| Modulation depth at 100 Hz | ≤ 0.3% (low-risk) | Mains-frequency ripple |
Five points the lighting industry only agreed on after 2015.
Magnetic-ballast T8 lamps modulated 100% at 100 Hz. Hours under them caused headaches and fatigue, but the industry had no name for it. LEDs reopened the conversation because some drivers were worse, and cameras finally exposed the difference.
The 2015 standard plots modulation depth (y) vs frequency (x) and divides the plane into "low risk," "no effect," and "NIOSH risk" zones. Below 100 Hz, almost any modulation is risky; above 1.25 kHz, even 100% modulation is safe. Read the plot, not a single number.
A 60 Hz LED video shot at 1/120 shutter shows bars rolling across the frame. Saudi commissioning teams now record a 5-second video of every fixture batch and flag anything that bars. Free, fast, and effective for screening.
A driver may pass flicker spec at 100% but PWM-gate aggressively below 30% dim, taking PF from 5% to 40%. Spec flicker requirements at full output AND at 20% dim, both must pass.
WELL Building Standard, BREEAM, and LEED v4.1 all now reference IEEE 1789 in their lighting credits. Saudi green-building procurement (Mostadam, Saudi Green Building Code) is following. "Flicker-free" is becoming a procurement gate, not a marketing claim.
Our premium driver range tests flicker-free in the IEEE 1789 low-risk zone at 100%, 50%, and 20% output, with the test report on every order.
Your eye averages the pulse away. A camera sensor samples it in milliseconds, so the strobe your eye misses lands straight on the frame.
The light pulses with the mains, hundreds of times a second. Here it is slowed right down so you can watch it dim and brighten.
The sensor exposes one row at a time, top to bottom: the rolling shutter, never the whole frame at once.
Rows caught on a dim pulse record dark, so the frame fills with the rolling dark bands you see on camera.
Almost every CMOS sensor, in phones and cinema cameras alike, exposes the image one row of pixels at a time, top to bottom, not all at once. While the sensor scans down, a flickering light keeps pulsing bright then dim. Rows captured on a bright pulse come out light; rows captured on a dim pulse come out dark. The result is a stack of horizontal dark bands rolling through the frame.
In a single photo the bands freeze in place and ruin a product shot or a portrait under the wrong panel. In video they drift up or down frame to frame, and the whole clip breathes between bright and dark. Slow motion makes it brutal: a 1000 fps shot under a 120 Hz light strobes hard between near-black and full exposure.
When the exposure time is not a whole multiple of the light's flicker period, every row and every frame catches a different slice of the pulse, so the brightness never evens out. Saudi mains runs at 60 Hz, so a cheap LED pulses at 120 Hz. Sync the shutter to that pulse, 1/60, 1/120, or 1/240 of a second, and the bands flatten. Off-multiple speeds like 1/100 or 1/250 bring them straight back.
Shutter tricks only patch one camera at one setting. A driver modulating above 1.25 kHz pushes the pulse faster than any normal shutter can resolve, so the sensor reads a steady level at every shutter speed, every frame rate, every slow-motion ramp. That is why studios, broadcast sets, and any space that gets filmed must spec flicker-free at the fixture, not chase it in the camera.
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