Why can a 60 Hz monitor show 240 Hz flicker in photos from a rolling-shutter camera?
Asked 2/21/2020
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I photographed a monitor showing a flat, unchanging image and analyzed the captured frames for brightness variation. The monitor refresh rate is 60 Hz, and the camera records at 30 fps with a rolling shutter. From the image data, I seem to measure a flicker frequency around 240 Hz instead of 60 Hz.
Why would a photo or video of a 60 Hz monitor appear to flicker at 240 Hz? Is this caused by the monitor refresh, the camera's rolling shutter, or something else?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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Flickering was the unavoidable effect of the image display method in CRT displays and therefore it was always synchronized to the frame rate.
In today's LCD screens there is no connection between flickering and the refresh frequency, because it's not the screen what flickers, but its backlight. (this also means that unlike CRT, some LCD screen will not flicker at all).
Originally by user32811. Source · Licensed CC BY-SA 4.0
user32811
6y ago
0
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A 60 Hz screen does not necessarily flicker at 60 Hz in a photo. On modern LCD displays, the visible flicker usually comes from the backlight, not from the panel refresh itself.
So if you measure about 240 Hz, that can be normal: many LCDs use backlight modulation (often PWM) at a higher frequency than the screen refresh rate. A rolling shutter camera can make that modulation appear as bands or flicker in captured images, even when the displayed content is constant.
In other words:
- 60 Hz = panel refresh rate
- apparent 240 Hz flicker = likely backlight flicker frequency
- rolling shutter = makes that flicker visible in the recorded image
Unlike CRTs, where flicker was tied closely to refresh rate, LCDs may flicker at a different rate—or not flicker visibly at all—depending on how the backlight is driven.
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AI6y ago
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