Diagnosing and Resolving LED Flicker Issues

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Diagnosing and Resolving LED Flicker Issues

Perhaps you see some vertical lines in your smartphone camera shots, observed some strobing in moving objects, or simply learned about the health risks of LED flicker.

In this article, we will explain how to evaluate the amount of flicker in your LED lighting and what you can do about it.

Causes of LED Flicker and Diagnosis

At its core, LED flicker is caused by a rapid on-and-off flashing of the LED. Although intended to flicker at a rate that is fast enough to go unnoticed, for some sensitive individuals and applications, the severity of flicker in some LED systems can be excessive and problematic.

Testing for flicker

The most scientifically accurate way to determine the extent and severity of flicker is to use specialized equipment such as an oscilloscope or light meter. At purchase prices starting at several hundred dollars per test unit, this is obviously not practical for most consumers concerned about flicker in their light bulbs.

Instead, we recommend a quick and effective way to determine flicker using just a standard iPhone and its built-in camera app.

Simply load the camera app, and swipe to the right twice to toggle the "SLO-MO" video setting. Then, point the camera at the light source in question, and hit record. Stop the recording after 2-3 seconds, and you're done!

Now, carefully watch the video you just recorded. If there is significant flicker in the light source, you will see it appear as a conspicuous flicker or repeated flashing on the slow motion video. If there is no flicker present, the light source should appear stable and continuously emit the same levels of light throughout the slow motion video.

See here for a video demonstrating the use of this method at a retail store.

Note: recent developments in smartphone software have helped eliminate the dark lines that used to frequently show up in still photos. While this new development is helpful for taking photographs, this makes it less effective in diagnosing flicker.


Where is the flicker coming from?

There are two sources of flicker in an LED - AC power or PWM - and it is important to determine the source of the flicker so that it can be directly tackled.

The first is caused by the fluctuation inherent in AC power. Alternating current, as its name suggests, alternates between positive and negative voltage/current in a sinusoidal waveform, 50-60 times per second. A 120 V AC input voltage actually is a reference to its average (RMS) voltage level - in reality, it can fluctuate anywhere between -170V and 170V.

As LED devices are inherently designed for DC (direct current) input, the AC waveform is inherently incompatible. As a result, electronics components are needed to "rectify" and smooth out the fluctuating AC signal.

With a simple rectified AC input, an LED system can function, but will momentarily cease or significantly decrease its light emission at each of the "valleys" that occurs 2x per cycle. The result is that an LED bulb may flicker between its on and off states at 100-120 times per second.

Figure 1: A graph showing the alternating current waveform, which repeats 50-60 times per second. Even a simple rectified AC signal will cause an LED to flicker at 100-120 times per second.

An LED bulb with more sophisticated electronics to convert the AC signal into a smoother, DC signal will produce less flicker. As such, the extent and severity of flicker caused by the AC signal is determined by the LED bulb electronics, which almost always is embedded in the bulb itself.

If you own any LED bulbs that screw in to a standard E26/E27 Edison base, or otherwise connect directly to mains electricity, the primary source of flicker is in the bulb itself, and will require replacement of the entire bulb.

The second source of flicker in an LED is pulse-width modulation, or PWM for short. PWM is an intentional manipulation of a DC power source, between on and off states. By varying the relative time between its on and off states, PWM can be used to create the perception of different brightness. In reality, however, the LEDs are either at 0% or 100% brightness - and to achieve the illusion of 50% brightness, the LEDs are at 0% brightness (off) half the time, and 100% brightness (on) the other half.

Figure 2: Chart comparing the on/off times for duty cycles of 50%, 75% and 25%. By shortening the time required to complete each duty cycle (PWM frequency) the effect of flicker can be reduced while maintaining the same apparent adjustment to brightness.

PWM is the most efficient and practical way to achieve the illusion of dimming, and the faster the PWM frequency, the less perceivable the flicker.  At high enough frequencies, PWM can safely be used for virtually all applications, including high speed cameras.

PWM induced flicker is caused by PWM LED dimmers and controllers. Virtually all LED strip dimmers and "color changing" controllers (CCT or RGB) that are installed on the low voltage DC side of the circuit utilize PWM to achieve their desired effects. Unfortunately, many PWM dimmers use a frequency that is too low. To eliminate the effects of PWM flicker, the frequency should be 25,000 Hz or higher, but most PWM dimmers will not include a PWM frequency specification, which is typically a sign that they use a frequency of several hundred hertz at best.


What can you do about the LED flicker?

The reason we stressed the importance of determining the source of the flicker (AC power or PWM?) is that we need to know which components of the lighting system need to be upgraded or replaced.

If the source of the flicker is in the LED bulb, you will need to replace the bulb with a new bulb that is specifically designed to smooth out the AC power and convert it to a truly stable DC current that in turn is used to drive the LEDs. When searching for an LED bulb, specifically look for "flicker free" designations as well as flicker metrics:

Flicker Percent - a percentage score that describes the relative difference between the maximum and minimum brightness levels (amplitude) within a flicker cycle. An incandescent bulb typically exhibits between 10%-20% flicker (because its filament retains some of its heat during the "valleys" in an AC signal).

Flicker Index - a measure that describes the magnitude and time interval during which an LED emits more light than its average light output during a flicker cycle. An incandescent bub has a flicker index of 0.04.

Flicker Frequency - the number of times that a flicker cycle repeats itself per second, measured in hertz (Hz). Most LED bulbs will run at 100-120 Hz due to the input AC signal's frequency. For bulbs that have a higher frequency, the effects of similar flicker % and flicker index values would be less pronounced because of their faster switching times.

Most LED bulbs flicker at 100-120 Hz. At this frequency, the IEEE 1789 recommended safe ("low risk") flicker percentage is 8%, and 3% to completely eliminate the effects of flicker.

If the source of the flicker is in the PWM dimmer or controller, likewise, you will need to replace the PWM dimmer unit. The good news is that the flicker is most likely not caused by the LED strips or other components, so your replacement costs will be limited to the PWM dimmer / controller.

The only meaningful PWM flicker metric is its frequency (because it is almost invariably a signal with 100% flicker), so when searching for a flicker free PWM solution, make sure that there is an explicit frequency rating. For a truly flicker-free PWM solution, we recommend a PWM frequency of 25 kHz (25,000 Hz) or higher.

See here for a full guide on flicker-free LED strip dimming.

In reality, standards such as IEEE 1789 indicate that PWM light sources with 3000 Hz is a sufficiently high frequency to completely mitigate the effects of flicker. However, one of the advantages of increasing the frequency to above 20 kHz is that it eliminates the possibility of audible buzzing or whining sounds emitted from power supply units. That's because the range of audible frequencies maxes out at 20,000 Hz for most people, and by specifying something at 25,000 Hz, for example, you can eliminate the possibility of annoying buzzing or whining sounds, which can be particularly problematic if you are particularly sensitive, or if your application is very sound-sensitive (e.g. recording studio).

See here for Waveform Lighting's line of flicker-free PWM dimmers and DMX dimmers.

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