Detecting flicker with cameras

You may have noticed instances on that flicker has been reportedly detected when referencing only a photo or video.  This is because in many cases flicker can be detected by cameras much better than the human eye.  Understanding how this works requires a brief explanation on how camera shutters work.

There are generally two types of camera shutters.  The first and more common is a rolling shutter.  The rolling shutter captures the image from top to bottom, either because the opening of a mechanical shutter assembly lets light through to the camera sensor through a gap that rolls from the top to the bottom, or because the sensor is only able to transmit the image to memory sequentially from top to bottom.  In any event, the end result is that the top of the image is slightly older than the bottom of the image.

The other type of shutter is the global shutter, which is far less common.  The global shutter is different in that it captures the whole image simultaneously.

Mechanism of detecting flicker using a rolling shutter

You may recall flicker as being a rapid variation in light output.  In some cases variation in light output could be so great that the light source actually turns off many times every second (known as 100% flicker depth).  If the light source turns off briefly a number of times over the duration the rolling shutter is in motion (capturing from top to bottom), then horizontal lines will appear.

If the flicker depth is something less than 100 (ie. the light source dims every cycle but doesn’t turn fully off), then instead of black horizontal lines you may instead see darker lines or some sort of horizontal disturbance.  If a camera is rotated to capture a tall object for instance, then the lines may appear vertical, but the lines will appear nonetheless.  In the case of video, you may see lines or perhaps just a shimmering effect.

The number of lines captured depends on the shutter speed of the camera and the frequency of the flicker.  For example, if the display cycles off twice during the time the rolling shutter is in motion, then the camera will capture two lines across the image.  Unfortunately, without knowing the shutter speed used for any particular photo or video it’s impossible to calculate the flicker frequency.

A fascinating video illustrating the action of a mechanical rolling shutter.

A subtle shimmer effect is evident in the following video at 3:35

A further video illustrating obvious flicker captured by a rolling shutter

Mechanism of detecting flicker using a Global Shutter

If you happen to be using a camera equipped with a global shutter it becomes more difficult to spot flicker.  This is because each frame may or may not be captured at the low or high point during each flicker cycle.  In fact, if your shutter speed is much slower than the flicker cycle then you may capture both the low and high points of each cycle in one frame, essentially capturing a blended average of brightness.  Only by using a very fast shutter speed and then comparing the overall brightness of each frame can flicker be spotted easily when using a global shutter.

Configuring your camera to detect flicker

To maximize your chance of detecting flicker with your camera your shutter speed must be relatively fast.  In many cases we recommend a shutter of of 1/2000th of a second or faster.  Not all cameras are capable of this speed.  High end smartphone cameras if paired with the right software allowing manual adjustments may be set to such a high speed.  Dedicated cameras such as any DSLR commonly achieve speeds of 1/2000th of a second or faster.

The next thing you’ll notice with such a short exposure time caused by the fast shutter speed is that the image appears dark, maybe even too dark to see.  This is because so little light actually reaches the camera sensor.  In order to fix this you’ll need to increase your ISO setting until the image becomes visible again.

If your camera is capable of recording slow-motion video, then that may also be effective at capturing obvious flicker.

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