The term vertical frequency denotes the vertical refresh rate in monitors.
The European TV standard PAL for example was designed for a frame rate of 25 Hz (interlacing method). In order to display ergonomic and flicker-free pictures, refresh rates of at least 70 Hz are required, although individual viewers' susceptibility for this issue may differ. Nowadays, most computer monitors and some TVs exceed this reference value, whereas LCD monitors are already entirely free from flickering at a refresh rate of 60 Hz.
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