PWM, or pulse-width modulation, is the go-to way to make LED-based lights appear brighter or dimmer. It’s the act of turning the LEDs off very quickly and the average “on time” is what determines the brightness.
While you might be able to notice this flickering at times, it’s generally not a problem in normal usage. Tindie seller future gadget labs, who describes himself as “a bit of a flashaholic”, came up with the PWM visualizer to translate light pulses into oscilloscope-readable voltage outputs.
If flickering light bothers you, this could make a great way to qualify your flashlights, or even illustrate how your home lighting is pulsing away at 60 Hz (in the US at least). It would seem though that an even more interesting use for this board could be to help synchronize video or still photographs so that you’re not capturing images during the short time when the lights are off.
The device accepts a wide range of low voltage inputs, outputting between zero and the input voltage. Per the product’s writeup, it’s been tested at 500kHz which should be acceptable for even those with incredible vision!
