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?