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When a snapshot of a monitor with refresh rate 60 Hz is taken with a camera with rolling shutter config operating at 30 FPS, what is the expected flicker frequency in the resultant image?

CONTEXT: I took images of a monitor screen with a camera with rolling shutter mechanism. I tried to figure out flicker frequency by plotting columns of the resultant images captured for a flat scene (same scene content throughout image). I find that flicker frequency is 240 Hz, when the refresh rate is 60 Hz. The device I used to capture the images runs at 30 frames/second. That's what I'm trying to figure out - why would it be 240 Hz?

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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).

See also: How a TV Works in Slow Motion - The Slow Mo Guys

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