Flicker / PWM Frequency
Definition
LED fixtures usually control brightness by pulse-width modulation (PWM), switching the LEDs on and off at high speed. The PWM frequency determines whether this flicker shows on camera. The lower the frequency, the more likely banding becomes. ENTTEC advises caution with any fixture whose published PWM rate is below 3 kHz, and Waveform Lighting recommends 25 kHz or more for high frame rates and fast shutter speeds. Manufacturers do not all state the PWM frequency of their fixtures.
In the field
A low PWM frequency shows on camera as horizontal bands scrolling through the image. The remedies are to change the fixture, to raise its PWM frequency where the menu allows it, or to adjust the camera shutter speed or angle so that each exposure covers a whole number of PWM cycles.
Related lessons
Sources
- RS DesignSpark, LED dimming with pulse width modulation : gradation des LED par PWM, rapport cyclique · rs-online.com
- Sony, Understanding flicker : bandes dues aux sources pulsées avec la lecture ligne par ligne du capteur · support.d-imaging.sony.co.jp
- ENTTEC, Why do LEDs flicker on camera? Prudence sous 3 kHz de fréquence PWM, exposition sur un nombre entier de cycles · support.enttec.com
- Waveform Lighting, An introduction to flicker free LED strip dimming : PWM de 25 kHz ou plus pour les cadences et vitesses d'obturation élevées · waveformlighting.com
- IEEE 1789-2015, Recommended practices for modulating current in high-brightness LEDs : risques du scintillement selon la fréquence · ieeexplore.ieee.org