If the amplitude of output current fluctuation is too large, it will cause flicker. If the current is high, the brightness will be high. If the current is low, the brightness will be low.

The change of brightness will give people the feeling of flickering. I wonder if you mean high-frequency stroboscopic or low-frequency stroboscopic? Generally, the human eye can see the low-frequency stroboscopic flashes within 120Hz, so some safety and energy efficiency and bio-photoelectric standards limit the frequency of flicker to be no less than 120HZ.

The high frequency flash can't be detected by the eyes. It can only be seen by high-speed camera (using mobile phone). So if it is low-frequency stroboscopic, as long as the power supply has any unstable operation, resulting in too large fluctuation of output current, there will be flicker.