I've built LED displays and the lowest frequency at which I wouldn't notice flickering was 500 Hz. This was at 1/8 duty cycle though, so you might get away with

I've built LED displays and the lowest frequency at which I wouldn't notice flickering was 500 Hz. This was at 1/8 duty cycle though, so you might get away with a lower frequency at a higher duty cycle. I find your claim that 50Hz was enough rather strange. Even incandescent light bulbs have noticeable flicker at below 50 Hz and those have considerable warm up / cool down times, as opposed to the LEDs.