Flicker Guide
Light flicker — percent flicker, flicker index, visible thresholds, and measurement.
Reference
Metrics
- Percent flicker
- (L_max − L_min) / (L_max + L_min) × 100%
- Flicker index
- Area above mean / total area in one cycle (0 = steady)
- Flicker frequency
- 120 Hz on US mains (doubled), 100 Hz on EU, PWM f for dimmers
Perception thresholds
| Frequency | Effect |
|---|---|
| < 70 Hz | Directly visible — most people |
| 70 – 200 Hz | Invisible in central vision, visible peripherally / with eye movement |
| > 200 Hz | Safe for most people |
| > 3 kHz | IEEE 1789 "no observable effect" zone |
IEEE 1789 low-risk limits
- At 60 Hz
- Percent flicker ≤ 8%
- At 120 Hz
- Percent flicker ≤ 16%
- > 120 Hz
- Scales linearly with f
Sources
- Poor LED drivers — especially low-cost phase-cut dimmer compatible bulbs.
- Fluorescent (magnetic ballast) — 120 Hz flicker.
- Fluorescent (electronic ballast) — 25–100 kHz, imperceptible.
- Dimmable LEDs: some use low-frequency PWM (250 Hz – 2 kHz), problematic at low dim.
- Camera sensors can see flicker via rolling shutter — banding in video.
Notes
- Flicker can trigger migraines and epilepsy — low flicker lighting is an accessibility concern.
- Modern quality LED drivers aim for > 3 kHz PWM or true constant-current.
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