Your PWM-controlled LED looks steady to the eye but flickers badly on camera video. What's going on and how do you fix it?

The eye integrates flicker above roughly 100-200 Hz, but a camera doesn't integrate continuously, it samples the scene during a short exposure (shutter/integration) window per frame. If the PWM period is comparable to or longer than the exposure window, different frames (or different rows, with a rolling shutter) catch the LED in different parts of its on/off cycle, so brightness varies frame-to-frame or you get dark bands rolling through the image, even though the average over a full period is constant and the eye sees it as steady. This is a beat between the PWM frequency and the camera's frame/line rate. The fix is to raise the PWM frequency well above the camera's sampling rate, hundreds of hertz that satisfy the eye aren't enough; pushing to several kHz or tens of kHz means many full PWM periods fall inside each exposure window, so every frame integrates the same average and the flicker disappears. (If you can't raise the frequency, an alternative is switching to analog/current dimming so the LED never fully turns off.) The lesson is that "no visible flicker" is a weaker requirement than "no flicker to a camera", the sampler's integration time, not human perception, sets the real minimum frequency.