Lighting
Strobe / PWM on Camera
Definition
Two sources of flicker affect cameras. A strobe produces deliberate rapid flashes that beat against the camera shutter and cause banding or partly lit frames. PWM is the on-off dimming of LEDs, invisible to the eye. Both interact with a camera's rolling shutter. When the flash or PWM period does not divide evenly into the exposure time of a frame, the sensor records part of the light as bands that roll through the image.
In the field
When an LED wall or fixture bands on camera, the first step is to set the camera shutter to a multiple of the mains or PWM period, for example 1/50 in a 50 Hz country. If that is not enough, the fixture PWM rate is raised above 3 kHz or the strobe is reduced.
Sources
- 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, réglage de l'obturation sur la période PWM · support.enttec.com
- RS DesignSpark, LED dimming with pulse width modulation : gradation des LED par PWM, rapport cyclique · rs-online.com