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For many, strolling around neighborhoods to look at the holiday light displays is an annual tradition that brings twinkling cheer.

But with the rise of LED lights in the last decade, some of the joy is lost. In its place, a slew of unintended consequences — from migraines and eye strain to nausea — have taken root. It can feel like the whole world was vibrating on and off.

Flicker is the catch-all term for changes in light output over time.

“One way to control the LEDs — and especially to dim LEDs — is to turn it on and off rapidly,” says 

, a lighting scientist and designer at the Pacific Northwest National Laboratory, who earned the nickname “The Flicker Queen” for her research on LEDs. “And it's the fact that it's going from full on to full off for a chunk of time, that makes you see it as flicker.”

In these lights, the electronics are everything. They determine how often the LED turns on and off.

For example, when dimming, the electronics might turn the LED on 50 percent of the time and off 50 percent of the time. “And your eye will see that is being dimmed down to 50 percent,” says Miller. “However, some people may see that on off operation as being distracting or disturbing.”

It's unclear just how many people are bothered by flicker, but it's potentially a big problem because LED lights are the future. The United States government has been working for more than a decade to 

, and it culminated in a regulation earlier this year that in effect banned the sale of most 

This transition to LEDs is a big win for the environment.

“They use about a fifth of the energy that incandescents used to use,” says Miller.

The Department of Energy predicts LED lights will cut carbon emissions by more than 200 million metric tons over the next 30 years. Not to mention help people save nearly $3 billion per year on their 

And to be clear, LED technology has come a long way, and many LED lights have solved the flicker problem.

“We know that this doesn't occur for most LED products because most LED products have better quality electronics built into them that maintain the light output over time, and those products people don't object to,” says Miller.

But for LED lights that do flicker, it can come in three forms.

Direct flicker (also known as low-frequency flicker) is when the viewer can observe the oscillation or jittering of the light source while looking directly at it. It happens when lighting modulates or changes between 3 and 80 times per second, or 3 and 80 Hertz.

“This one's an especially bad one,” says Miller, pointing to the middle of three sets of holiday lights in her lab. They flicker on and off 60 times per second. “It's producing this very jerky square waveform that is the kind of waveform that your brain was not designed to handle.”

Fortunately, she notes that it's uncommon to use these lights in buildings. It's fortunate because, she says, “there is a possibility that people who are affected by photosensitive epilepsy may actually have a poor reaction. So we really want to avoid this low frequency stuff.”

The second form of flicker is the stroboscopic effect. Miller explains that it happens “when you have a fixed gaze, but something is moving in the light that makes the uneven light output visible as jerky or unevenly-blurred motion.” It can happen with lights ranging from 80 to 2000 Hertz.

To demonstrate, Miller points to a set of LED holiday lights that claim on the box to be flicker free. They are 120 Hertz, so their flicker isn't easily visible by just looking directly at them. But when she waves her finger back and forth in front of the lights, instead of seeing a smooth motion, it looks like little tracers of her finger are scattered in an arc. 

The flicker of the LED lights make them work like super fast strobe lights, illuminating movement in such a way that it looks frozen in place. 

Another way to identify the effect is to record the light source using a phone's slow motion setting and then watch the video. By slowing it down, the phone reveals that the lights strobe on and off rapidly, which is what makes movement like Miller’s waving finger look jerky.

The third form of flicker is called the “phantom ray effect,” or “ghosting effect.” It happens when your eyes are moving relative to a light source. The appearance of a repeating pattern—what looks like a zipper or string of lights—is created as the lit object is displaced spatially on the retina as the eyes scan.

This effect is particularly noticeable while driving at night behind cars with LED tail lights.

“Suddenly, as your eye moves around the roadway normally, you will see a repeating pattern of lights,” says Miller. “So you don't see one tail light anymore. You see a whole string of tail lights spaced at some distance apart.”

Miller says it can be unnerving because it makes it hard to see where the actual light on a moving car is.

As for how to identify which LED lights — not to mention phone screens — are prone to flicker, Miller suggests searching the Internet for flicker-related websites, or using your finger or phone to test for the stroboscopic effect.

When purchasing, she recommends avoiding the cheapest bulbs, which she warns means being “prepared for the fact that you may have to change out your dimmer because not all old dimmers work politely with the new LED technologies.”

Finally, Miller suggests taking any lights you don't like back to the store. “The manufacturers need to know that people are unhappy with these products,” she says. “I still think it's going to be a subset of products. Even so, take them back.”

Curious about the technology you encounter in your everyday life? Email us at 

This episode was produced by Berly McCoy, edited by Rebecca Ramirez and fact checked by Brit Hanson. The audio engineer was Josh Newell.

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LED light bulbs are the future. They're better for the environment and the pocket book. But for some people, certain LEDs lights — particularly holiday lights — are a problem. They flicker in a way that causes headaches, nausea and other discomfort.

It's not just you: Christmas lights look different now and can give you headaches

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