I used a DSLR camera to take a short-exposure time photo of a CRT television screen displaying the menu screen of Super Mario Bros. on Nintendo NES.
The interesting observation here is that at any given moment the CRT is mostly dark. The faded area above the brightest scanlines is still visible for some time after it has been scanned due to phosphor decay.
NTSC time to scan a single scanline: 63.5us (microseconds)
Camera exposure time: 250us (1/4000s)
Visible scanline count: 250us / 63.5us = 3.9 scanlines
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