Flicker at high enough frequencies isn't visible directly, but only as phantom array effect with very fast eye movement and high-contrast light. Flicker fusion

Flicker at high enough frequencies isn't visible directly, but only as phantom array effect with very fast eye movement and high-contrast light. Flicker fusion frequency isn't relevant. I couldn't see 10kHz flicker when I last tested it (with an LED and 555 timer), but I could see 5kHz flicker. The exact threshold will depend on the observer and the test conditions. I find high frequency flicker very annoying because the appearance and disappearance of the phantom array effect looks like unexpected motion to me, and if something is moving unexpectedly then it needs attention.