Cameras · interactive
Why do LED lights flicker on camera?
Left: the phone's screen, built row by row as the scan line sweeps down in slow motion. Right: the LED's brightness at the same moments; the gold window is the light each row collects. Drag up or down on the stage to move the scan line.
1 See
Film a room lit by LEDs and dark bands can roll across your phone's screen, while the room looks perfectly steady to you. What is the camera catching that your eyes miss?
2 Change
3 Understand
Model notes and sources
Model. Many LED lamps run from a simple driver whose output ripples at twice the mains frequency, so the light pulses 100 times a second on 50 Hz power and 120 times a second on 60 Hz power. This model uses the simplest such driver: the LEDs light only while the rectified mains voltage is above their threshold (here 45% of the peak), so each pulse is a clipped half-sine and the light is fully off for about 30% of every pulse. The light is L(t) = 1 − m + m·max(0, |sin(πt/P)| − 0.45)/0.55, with pulse period P = 1/(2f) and flicker depth m (100% means the light goes fully off between pulses; 0% is a steady, flicker-free driver). A rolling shutter reads the sensor's rows from top to bottom over a readout time R. Row r (0 at the top, 1 at the bottom) is read at r·R and collects light for its exposure X just before that, so its brightness is ∫L dt over [t − X, t]. When X is a whole number of pulse periods (1/120 s or 1/60 s on 60 Hz power, 1/100 s or 1/50 s on 50 Hz), every row's window holds the same light and the bands vanish. Shorter or mismatched exposures leave bands. Band depth is 1 − (darkest row ÷ brightest row).
Fixed values. Readout R = 20 ms, about 1/50 s. Real readout times vary with the camera and mode: DxO Labs measured 30 ms for a Nokia Lumia 1020 using its electronic shutter and 12–23 ms for a Canon 650D's video modes. Video runs at 30 frames a second. On 60 Hz power that is exactly 4 pulses per frame, so every frame starts at the same point in the light's cycle and the bands stand still; on 50 Hz power each frame starts a third of a pulse later, so the bands roll. The film lets the bands drift slowly, as they do when a camera's frame rate isn't locked exactly to the grid. Auto exposure scales each frame so that the average row keeps the same brightness, which is why the brightest rows look blown out at very short exposures. Your eyes rarely notice flicker above about 90 Hz, so the room looks steady.
Not modeled. Pulse-width-modulation dimming (square-wave flicker, often at hundreds or thousands of hertz), drivers that smooth the ripple only partly, phosphor afterglow, sensor noise, and phones' automatic flicker detection. The photos are AI-generated illustrations; the phone's screen image is built from a pair of photos (LEDs on, LEDs off) mixed row by row, and the light curve and every number come from the model.
Sources: CREE via Mouser, “Flicker happens. But does it have to?” (driver ripple at twice the input frequency); Pacific Northwest National Laboratory for the U.S. DOE, Characterizing Photometric Flicker (2016); Sony support, How to reduce camera flickering (1/50 or 1/100 s on 50 Hz, 1/60 or 1/120 s on 60 Hz); L. Masson, F. Cao, C. Viard and F. Guichard (DxO Labs), “Device and algorithms for camera timing evaluation” (2014); Rolling shutter; Flicker (light).
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