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Flicker Demonstration
Video courtesy of the Department of Energy

What we're looking at here is this table top being lit by a lamp that exhibits 120 Hertz flicker. This is an LED product that has internal drivers that actually produce a lot of variation between its maximum output and minimum output per cycle. So there are different ways to examine this.

When you're looking at the still object, you don't really pick up the flicker so much. But as soon as you wave your fingers, you can quickly see multiple fingers. Although I have to say that not all flicker is as dramatic as that.

But you will see essentially stripes of bright and less bright when you fan your hand underneath the light source. Or you can wave a white bar. Or we can wave a shiny pen.

You can clearly see areas of the pen, and then a blank stripe next to the pen. And the bright stripe occurs when the light is on. The dark stripe occurs when the light is off, 120 times per second.

Or if you spin the top, if you see a checkerboard pattern on the top, that's indicating flicker. Because there are different frequencies of black and white squares on here, and depending on how fast the disk is spinning, you'll see a checkerboard from different bands.

Now this is an especially bad lamp. So it's showing off the worst of the flicker. So as it slows down, you'll see different areas that look like a checkerboard.

You are now looking at a light that is not flickering. As you can see, the shiny reflection of light produces a smooth, almost a figure eight of light as I swing it around. This one has internal electronics to make sure that the light output is continuous, rather than switching on and off at a high rate.

Now you can see that there are just gray circles. So none of the checkerboard pattern is appearing because of the stroboscopic effect. Since light is not turning on and off, you're not creating those dark and light bands.