LED flicker

Many factories using “no radiation, no flicker, high brightness” to advertising the LED production.
Just for now, the LED work in pure DC and flicker-free, eliminating the visual fatigue by traditional strobe light. but there are alternatives to power supply for LED . the way is AC power supply. The LED is vulnerable to be influenced by interferences and noises caused by AC alternating ripple .
Now , given an example to showing the LED flicker in everyday life.
the ambulance also has a red light on top to warn people that it is coming. the signal frequency is about 8~10Hz. which people are not comfortable with. the frequency(cause alarm in people) . fluorescent lights flickered when digital cameras shoots the images. The same is true for noises caused by AC alternating ripple . the different point that they have different frequency .