Why slow-motion video flickers under LED and fluorescent lights

If you've filmed slow motion indoors, you've probably seen it: footage that looked fine on screen while you were recording plays back pulsing like a strobe, or with dark bands drifting through it. Nothing is wrong with your iPhone. The camera is recording something your eyes can't see, which is that most artificial lights are flickering all the time.

Your lights are already flickering

Mains electricity is alternating current. The voltage swings back and forth 50 times a second in some countries and 60 times a second in others. That's the mains frequency, measured in hertz (Hz).

• 60 Hz: the United States, Canada, Mexico, parts of South America, South Korea and Taiwan, among others.
• 50 Hz: the UK and the rest of Europe, most of Africa and Asia, and Australia.
• Japan uses both: 50 Hz in the east, 60 Hz in the west.

A light running on that supply gets the most power twice in every cycle, once on the positive swing and once on the negative. So its output rises and falls at twice the mains frequency: 100 times a second on 50 Hz power, 120 times a second on 60 Hz. That's far too fast for your eyes to notice. A camera taking hundreds of short exposures per second notices it easily.

Why normal video hides it

At ordinary frame rates the camera has time to average the flicker out. A 30 fps video can expose each frame for 1/60 of a second, which covers exactly two pulses of 120 Hz light. Every frame collects the same total amount of light no matter where the pulses fall, so the brightness stays steady. Exposures of 1/120 s (one pulse) or 1/50 and 1/100 s on 50 Hz power do the same thing.

Many cameras deliberately choose these flicker-safe exposure times when they detect artificial light. That's why the same room can look fine in a regular video and terrible in slow motion. The catch: in very bright light a camera may need a shorter exposure, and then even normal video can flicker.

What changes at 120 and 240 fps

A frame can't be exposed for longer than the gap between frames. At 120 fps the longest possible exposure is 1/120 s; at 240 fps it's 1/240 s, and in practice the camera often goes shorter still. A light cycle on 50 Hz power lasts 1/100 s and on 60 Hz power 1/120 s. At 240 fps every exposure is shorter than a single light pulse, so each frame records only a slice of it.

Consecutive frames land on different parts of the pulse: one near the peak, the next on the way down, the next near the bottom. Each frame comes out a slightly different brightness. Then slow motion stretches time. The iPhone plays 240 fps slow-mo at an eighth of real speed and 120 fps at a quarter, so brightness changes that happened too fast to see are slowed into a rhythm your eyes pick up immediately.

Aliasing: why the flicker has a rhythm

The pattern you see depends on how the frame rate lines up with the light. Camera people call the result aliasing, the same effect that makes wagon wheels appear to turn slowly or backwards on film.

100 Hz light at 240 fps. The camera takes 2.4 frames per light cycle. Because that isn't a whole number, each frame catches the pulse at a different point, and the sequence of bright and dark frames only repeats every 12 frames (five light cycles). Played back at an eighth of real speed, the picture pulses about twelve times a second, with a slower rhythm running underneath.

100 Hz light at 120 fps. The frame rate is just above the light frequency, so each frame lands a little earlier in the cycle than the last. The camera sees a slow 20 Hz beat, the difference between 120 and 100, and at quarter-speed playback that becomes a throb about five times a second.

120 Hz light at 120 fps. One frame per light cycle. In theory every frame catches the same point of the pulse and the brightness holds steady. In practice the camera clock and the power grid never match perfectly, so the timing drifts and you get a slow wander in brightness or bands that creep through the frame.

120 Hz light at 240 fps. Exactly two frames per cycle. Depending on where they fall, you can get a hard bright-dark-bright-dark alternation, or almost nothing at all, and the pattern shifts as the timing drifts.

Rolling shutter turns flicker into bands

Phone cameras don't capture a whole frame at once. The sensor is read out line by line, top to bottom, over a few milliseconds. This is the rolling shutter, and it's also what makes fast-moving objects lean.

If the light changes during that readout, the lines read at the bright part of the pulse come out brighter than the lines read at the dark part. The frame ends up with stripes, called banding. How many bands you see depends on how the readout time compares with the length of a light pulse. Whether they sit still or roll depends on how closely the frame rate matches the light: a near match gives slow-crawling bands, a poor match makes them jump.

The sensor always reads along its own long edge, so in landscape clips the bands run horizontally. Hold the phone upright and they usually run from top to bottom instead.

Which lights flicker the most

• LEDs with cheap drivers. An LED responds to current almost instantly, so it flickers exactly as much as its driver lets it. Well-designed drivers smooth the power into nearly constant light. Budget bulbs, strips and fixtures often skip that smoothing, and the light can dip close to zero between pulses. That's the deepest flicker a camera will ever see, and two bulbs that look identical can behave completely differently.
• Dimmed LEDs. Many dimmers and LED controllers dim by switching the light on and off very quickly, a technique called pulse-width modulation (PWM). The lower the setting, the longer the off periods. To a high-speed camera, dimmed LEDs often produce hard-edged bands.
• Fluorescent tubes with magnetic ballasts. Older fittings flicker at 100 or 120 Hz. The tube's phosphor glows briefly after each pulse and softens the dips, but doesn't remove them, and the color can shift through the cycle. Modern electronic ballasts drive tubes at tens of thousands of hertz, which is much kinder to cameras.
• Sodium and metal-halide lamps in older sports halls, stadiums and car parks flicker strongly. They're a classic cause of flickering slow motion at sports events.
• Incandescent and halogen bulbs flicker only mildly, because the hot filament keeps glowing between pulses.
• Daylight doesn't flicker at all.

Why it's worse in slow motion

Putting it together, slow motion makes flicker worse in four ways at once:

1. Each frame's exposure is shorter than a light pulse, so it can't average the flicker out.
2. Consecutive frames sample different points of the pulse, so the brightness jumps around.
3. Slow playback stretches those jumps into the range where flicker is most noticeable.
4. The rolling shutter adds bands on top, especially with LEDs that switch hard on and off.

Lowering the frame rate, matching the light, or changing the light all help. The practical options are in how to prevent flicker when filming on iPhone. For clips you've already shot, see how to fix flickering slow-motion video on iPhone, or go straight to removing LED and fluorescent flicker.