Flicker in Red Light Therapy: What to Know

Flicker in Red Light Therapy: What to Know

With so much excitement around the science and benefits of red light therapy, it is natural to look for the downsides! One common item that comes up is the concept of flicker. 

*Warning, most of the embedded videos in this blog are showcasing flickering lights, please be mindful if you are sensitive to flicker or have photosensitive epilepsy*

Being something that is often difficult to detect and quantify, and even harder to assess the biological impact, this can easily cause unwanted fear. Even in the 1990's we had an interesting case of Pokemon Shock. [10]

GembaRed is the first PBM company and one of the few lighting companies to fully quantify and characterize our flicker waveform and transparently display it. This gives our customers important knowledge and peace of mind when using their lights.

The definition of Flicker, in the most basic terms of illumination, is a change in brightness. This can be quantified as a modulation or variation in luminous flux and intensity. [1]

You may see flicker in old light bulbs that need replacement, or flicker in candles and firelight. A video screen may flicker as scenes change or for visual effects. But that is not the kind of flicker we are talking about in this article. 

For this article we are talking about a regular repetition of flicker that is usually invisible to the eye, sometimes called electrical flicker. This occurs in practically all modern lighting sources to some degree. [1]

All lighting powered by the AC mains will have some flicker, by nature of the 60hz (USA) or 50hz alternating current. [1] This is the cycles of power from the outlets in the walls. The hz, or Hertz, is the cycles per second. 

If a light is flickering at 60hz, that means it changes in brightness 60 times per second.

Here we see a number of LED lights flickering at the same frame rate as our camera, 60hz. This flicker was not apparent to a casual observer.


Flickering fluorescent and LED bulbs have been associated with headaches at the workplace, particularly at lower flicker rates less than 120hz. When fluorescent lighting switched to higher frequency ballasts, it reduced the issue. The same thing goes for LEDs, the flicker depends on the design of the circuitry.

What to know:
  • All lights flicker as influenced by the 50/60Hz mains power supply. [1]
  • The human eye may perceive flicker at around 60Hz or below (depending on sensitivity), stroboscopic flicker could be detected up to several thousand Hz by eye or on camera. [2]
  • A camera or slow-motion video may be able to detect flickering lights ONLY if the frame-rate of the video syncs with the flicker rate of lighting. Just because you don't see flicker on a slow motion capture, does not mean it isn't there. [3]
  • Flicker can be more dramatic in LEDs than other sources of light not because of the flicker rate (Hz), but the amount of brightness change per cycle is higher than incandescent. [1]
  • The measurement of flicker is more than just in Hz, it can be characterized by its waveform and described also by Percent Flicker and Flicker Index. [1]
  • We can control or improve the flicker in LEDs with higher quality power drivers or designs. However even the best drivers may have "ripple" effect from the AC power. To eliminate it, a true DC power source would be required (like a battery). [1]
Why aren't we concerned when it comes to flicker and red light therapy?
  • Flicker is present in all AC driven lights [1], and hundreds of medical studies have used LED lights.[6] Since no medical studies we have found have failed for "too much flicker" and LED light therapy is deemed generally safe to use by the FDA, this lowers our concerns about flicker. As the LED light therapy market is already widely used in the consumer base and continues to grow [7], we generally only hear good feedback for this type of therapy.
  • If flicker was deemed an important variable for Photobiomodulation, then we would expect it would be characterized and reported for each study.
  • Pulsing is used commonly in Photobiomodulation as a method of thermal management and cellular activation. It is considered to be useful and beneficial in many situations. Pulsing is also characterized in Hz, or cycles per second. Pulsing parameters generally range from 10Hz to thousands of Hz.  However pulsing is different than flicker as pulsing is usually a square wave of on and off periods. In this way pulsing may be more impactful than flicker since flicker does not always reach zero brightness. The similarities between pulsing and flicker may lend us to assume something about the relative safety of flicker
  • Red light therapy is applied to the body for a short period of time on the order of 10's of minutes a day at the most. This is enough to stimulate a beneficial cellular activity from the energy applied. In contrast, a highly flickering light for many hours would not be appropriate for the workplace, task lighting, or driving as stroboscopic effects may interfere with the activity or cause neurological discomfort. [1]
  • GembaRed products focus on lower intensity light for stimulatory light therapy. Relatively speaking, flicker from a low intensity light will have less magnitude of brightness change than a high intensity light flicker
So why do we study it and measure our flicker if it is a non-issue?
  1. We are curious and love to quantify our devices! GembaRed sells the best-quantified devices on the market.
  2. To understand and improve on our devices and designs in the future.
  3. Some day when light therapy is sophisticated enough to measure subtle differences in efficacy based on waveform, remember that GembaRed was the first to characterize it.
  4. When we understand and visualize flicker, we can now manage it properly and know what to look for. "Knowing is half the battle" as the G.I. Joe would say.
  5. To encourage responsible use of red light therapy and reasonable discussion.

Observation of Flicker:

The most common way of talking about flicker is the cycles per second, flicker rate, or Hz. 

Observation of flicker is mostly subjective and typically can be detected by humans below 60Hz. Typically above 100Hz the light appears continuous in an optical phenomenon called Critical Flicker Fusion.

Stroboscopic flicker effects are when the light source may appear to be continuous the eye, but it can create a "strobe-light" or phantom array effect that can distort moving objects. This is how even invisible flicker can be irritating, or even cause issues with perception around intricate tasks or sporting events. [9]

Often, we can use a video camera or slow-motion capture to help us "see" flicker where the human eye cannot. This may be a quick way to assess flicker, however if we cannot see it on a camera merely shows that the flicker rate does not sync with the framerate of the camera. Some form of flicker may still exist but go undetected by the camera. [3]

The first part of this video was shot at 60 frames per second, with no observed flicker. The next part is at a slow motion rate of 120 frames per second, now observing flicker. So you can imagine, even if flicker is not perceived by the camera, does not mean it isn't there. [3]

Sometimes, we can even detect flicker from a still-image. In the picture below: see the light on the left is flickering as shown by the wavy horizontal lines. The light on the right side, our GembaRed Groove, is very photogenic with no distracting flicker lines.

We often see these wavy lines when people take a selfie in a full-body light therapy pod!

Here we see a happy user of the worlds most popular and well-studied full-body LED light therapy pod. A quick hashtag search on Instagram for this brand shows many customers in a flickering pod. The marketing material for this brand claims to be continuous, yet videos we find show obvious flicker.

Flicker Percentage and Index.

Did you know that all incandescent light bulbs flicker at 2x the AC power rate of 60/50Hz?[1] That seems fairly low rate and from our above discussion, would indicate that humans would be sensitive to such low flicker rate. So why do people say that incandescent lights are better for flicker?

That is where flicker percentage comes in as another important parameter to describe flicker. Flicker percentage is percent difference from the maximum brightness to the minimum brightness within a flicker cycle.

Incandescent bulbs use a filament that heats and cools with the AC cycles. Because the cooling effect is relatively slow, a typical incandescent may only drop in brightness by up to 30%. Compared to an LED which is sometimes called a Solid-State Lighting (SSL) source, an LED will instantaneously change its brightness based on the energy provided, and can drop in brightness by the full 100% in the cycle! 

Above we see an example of two sinusoidal light sources, both are cycling at the same flicker rate (Hz), but the orange one is only losing 30% of its brightness per cycle. So we start to see there is much more to the flicker story than just Hz.

Flicker Safety Flicker Frequency Hz and Flicker Percentage %


The above image shows the relative safety of flicker Frequency versus flicker Percentage. We can see that at very low flicker Percentage, nearly any Frequency will be safe and non-disruptive. Similarly, an extremely high frequency can also improve the safety of flicker. [14] So, we need to understand these two important parameters of Flicker Frequency and Flicker Percentage in order to correlate it to safety.

Controlling for Flicker

Most modern electronics and appliances must convert the AC from the outlet to DC to power the device. If you look at the description on your power adapter for your laptop or phone charger, it usually reads as having an "Input: 120V 60hz AC" and output is a lower voltage DC, direct current. This protects your devices from too much voltage or too much alternating current, which would damage the device or the battery in the device.

The same thing must now occur in LEDs. With incandescent bulbs, you typically power it by plugging it straight into the AC socket. Many LED lamps and bulbs have circuitry installed to do the conversion and drop the voltage and rectify the waveform. 

Nearly every LED brand may use different circuitry, sometimes a module called a Power Driver or Controller, to make this conversion. Depending on the size, space, components, quality, and cost of the power driver determines the characteristic of the waveform. No two LED devices are created equal.

Even with the best power driver converting AC to DC, there still may be a "ripple" effect from the AC that still causes some flicker. 

Here we see a demonstration with two nearly identical lights, one a generic white light with high flicker, the other a custom made red light with a higher quality driver and less flicker.

Flicker Index and Waveform

Since now we know flicker is a unique waveform that depends on the circuitry in the LED lamp, we can use professional quality equipment to measure the entire waveform. GembaRed is the first PBM company to measure the full waveform and publish the results.

With the waveform we can also calculate the Flicker Index, another important parameter that tells us more about the relative extent of flicker.

The Flicker Index is a more complex calculation over the areas of the waveform. It tells us if the flicker is a quick drop, or if it dominates more of the waveform. 

Our GembaRed panels all utilize the same power drivers, so this is the characteristic waveform that represents all of our light panels.

So we see here the optical waveform is very steady, with very little drop in brightness anywhere in the waveform. In fact, the flicker rate is hardly measurable and our 3rd party was not able assign a Hz rate!

The waveform of our GembaRed Rave, powered by an AC/DC converting Samsung phone charger, shows an even more steady waveform. This also had no assignable flicker rate.

Continuous Wave, Pulse, and Flicker

A continuous light source would be one that is DC operated, or from natural sunlight. In medical literature when comparing Continuous light and Pulsed light they refer to them as Continuous Wave (CW) and Pulsed Wave (PW).

Pulsed Wave is a popular option for PBM as it offers a method of thermal management where the time in-between pulses, however short it may be, allows the treatment tissue to stay constant temperature. Too much thermal application is counter-productive to the study of PBM. Often, this allows the researchers to use a higher peak intensity, and achieve the same average intensity as a CW light source.

For those simple reasons alone, PW tends to perform better than their CW counterparts. For example these two studies show how 10Hz and 100Hz pulsed laser performed better than Continuous Wave, in both brain healing in Traumatic Brain Injury (TBI), and Wound Healing treatments. [12]

Both studies above found that the 10Hz pulsed laser performed better than the 100Hz light. This shows that a light source at 10Hz, which would be easily detected and irritating to the eye, is preferable than the higher Hz light source. The 10Hz may be more advantageous because the "off" cycle is longer than 100Hz which allows for more tissue cooling between cycles.

However there is growing evidence that specific frequencies are uniquely biologically stimulating beyond thermal management, which may explain further the success and popularity of specific pulses such as 10Hz. This indicates that there are quantum biological processes in place that are being modulated by the pulsing frequency. [13] More studies would be required to help understand which pulsing parameters such as frequency (Hz), dwell time, and delay times (in milliseconds) can be uniquely taken advantage of for its biological resonance.

We can use the same metrics defined for flicker to compare to pulsed wave lighting. For example we know the frequency in Hz already as specified by the individual study, pulsing is fully off and on so it is 100 Percent Flicker. Many pulsing studies specify the Duty %, which correlates to Flicker Index. A common duty % is 50%, which correlates to 0.50 Flicker Index.  [11] [12]

Given that it is still difficult to compare parameters of Continuous Wave to Pulsing since most studies use Pulsing to increase the peak intensity, as well as not knowing the ideal pulse parameters that resonate with human biology, we at GembaRed prefer to keep our lights as Continuous as possible.

Summary Chart:

 Light Type Flicker Rate (Hz)

Percent Flicker

(0 to 100)

0 means no flicker

Flicker Index (0.00 to 1.00)

0 means no flicker

Generic Incandescent (1) 120 13.4% 0.04
GembaRed Rex LED Therapy Panel NA 1.2% 0.002
Pulse Parameters 10 to 10k 100% ~0.50
GembaRed Rave w/ Samsung phone charger NA 0.5% 0.001
GembaRed Rave w/ Apple phone charger NA 0.6% 0.001



The natural state of lighting may be minimal flickering environments such as the sun, campfire, and candle light. Unfortunately we are exposed every day to flickering lights based on AC power, as well as drastic changes in brightness when scrolling on our devices, watching videos, or playing video-games.

The art of biohacking has always been a balance of using technology to our advantage and activating our natural biological potential. We extract therapeutic wavelengths of red and near-infrared from the spectrum with electronics and aim it at our body for direct mitochondrial activation. This is certainly not a naturally occurring a process, yet highly effective for a therapeutic response. Similar to concentrated supplement extracts, high-intensity training, and other bio-stimulation such as brain training, responsible use is recommended to get the best benefit.

Flicker will be an ever-growing topic for daily lighting, especially as the conversion from incandescent to SSL takes over. Manufacturers will have to assume responsibility for the quality of the components to reduce flicker, and for measuring and verifying that their products are low in flicker. Fortunately, for GembaRed Customers, we are the first company to understand this and offer that piece of mind.

[1] SSL Technology Fact Sheet: Flicker. DOE. March 2013
[2] LEDs: Fighting Flicker. Architectural Lighting. April 23, 2014
[3] Flicker Free Lights And Why They Are Important To You. B&H Photo. 2015
[4] Flicker, Shimmer, and Ripple: Lessons in Light Quality. LED Journal. Andrew Smith. May 27, 2014
[6] Low Level Laser Therapy (LLLT) spreadsheet.  Vladimir Heiskanen et al. 


[7] Light Therapy Market to Perceive Substantial Growth During 2026. SBWire. 5/31/2018

[9] Stroboscopic Flicker
[10] Did Pokemon Actually Give Kids Seizures in the 90's.
[11] Flicker in Solid-State Lighting: Measurement Techniques, and Proposed Reporting and Application Criteria
"Designing to Mitigate Effects in LED Lighting: Reducing Risks to Health and Safety"