Making Sense of Flicker, TLA, and Stroboscopic Effects
Although it sometimes seems like flicker in lighting systems is a new phenomenon, flicker concerns have been around for a long time. If you go back 20 to 25 years ago, many of the fluorescent luminaires of that era used magnetic ballasts that controlled the lamp arc current with line-current frequency (60 Hz in the US). This line-current power for fluorescent lamps caused light output waveforms that were nearly sinusoidal with a frequency of twice the line frequency (120 Hz in the US). The majority of HID luminaires of that era also used magnetic ballasts which caused optical waveforms with a frequency of 120 Hz. Incandescent lamps connected to line voltage or controlled by dimmers also exhibited 120 Hz, nearly sinusoidal optical waveforms with modulation depths of varying amounts. Of course, prior to these technologies, candles, torches, and gas lamps exhibited flicker which is often considered to be desirable. Unlike most electrical light sources, the frequency and shape of the optical waveform for flame sources is usually random and difficult to predict.
Since flicker in light sources has been around for such a long time, you might wonder why there is so much discussion about flicker with LED sources. The answer lies in the fact that many of the magnetically ballasted and line-voltage or dimmed incandescent sources exhibit a nearly-sinusoidal optical waveform that has a frequency of 120 Hz. With these two given conditions, flicker could be reasonably characterized by three primary metrics (Percent Flicker, Flicker Index, and Operating Frequency). Percent Flicker and Flicker Index describe certain characteristics of the shape of the optical waveform, while Operating Frequency describes the frequency of the periodic optical waveform.
The concern with LED sources is that their optical waveform is primarily dependent on the electrical control circuitry used to power the LEDs. Therefore, the optical waveform developed by LED light sources can be of nearly any frequency, and the waveform can be of nearly any shape.
Current Practices for the Evaluation of Optical Waveforms
In recent years, several studies have been published describing human sensitivity to the shape and frequency of optical waveforms. Most modern techniques of optical waveform analysis take advantage of the Fourier Transform. This mathematical transformation allows for the analysis of the individual frequency components of the optical waveform. The components of any optical waveform can then be evaluated based on published studies of human sensitivity. Several studies defining human sensitivity to optical waveforms have been proposed based on various environmental conditions. NEMA 77-2017 summarizes the work done to date, and recommends metrics and limits based on three separate phenomena: Flicker, Stroboscopic Effects, and Phantom Array.
Flicker is caused by changes in the perception of light occurring at frequencies less than 80Hz. Studies have shown that humans are very sensitive to frequencies between 8 and 33 Hz, and can detect very small modulation in this range of frequencies (IEEE-1789-2015). The weighted human sensitivity to these frequencies have been combined into one metric (Pst) (IEC TR 61547-1).
Stroboscopic Effects and Phantom Array effects are perceived by humans when light sources produce optical waveforms with frequencies between 80Hz and 2kHz. The weighted human sensitivity to these frequencies have been combined into a metric called the Stroboscopic Visibility Measure (SVM) (CIE TN 006:2016).
These three phenomena are all components of a broader classification of human-perceptible fluctuations in the luminance or spectral distribution of a light source called Temporal Light Artifacts (TLA) (NEMA 77-2017).
Some Examples of LED Optical Waveforms
A very inexpensive controller could use the LEDs in the circuit as a half-wave rectifier. This configuration would cause a 100% modulation at 60 Hz which would definitely be above the human visibility threshold for flicker.
Another inexpensive controller could use the LEDs in the circuit as a full-wave rectifier which would cause a nearly 100% modulation at 120 Hz. Without further filtering, this configuration would certainly be above the human visibility threshold for stroboscopic effects (SVM) and would be a poor candidate for installation in indoor applications.
If the current supplied to the LEDs is switched on and off throughout the waveform, as is often done in Pulse-width-modulation (PWM) controls, the result is a sudden change in the light output waveform. Sudden changes in the light output waveform will result in higher harmonic content in the waveform which can cause the stroboscopic effects (SVM) to push above the threshold of human visibility.
Limits and Regulations
NEMA 77-2017 recommends limits to the TLA based on Pst and SVM measurements. The US EPA’s ENERGY STAR program has included a requirement for operating frequency, highest percent modulation, and flicker index for dimmable lamps in its Lamps V2.0 specification. The latest version of the Lamps specification (V2.1) requires the measurement and calculation of Percent Flicker, Flicker Index, Lamp Light Output Periodic Frequency, Pst, SVM, and the ASSIST Flicker Perception Metric (Mp). The latest version of the Luminaires specification (V2.1) requires the measurement and calculation of Pst and SVM (optional: the luminaire meets the NEMA 77 proposed requirements for TLA).
The California Energy Commission (CEC) includes a limit for flicker in luminaires based on percent modulation from frequency components below specified frequency limits.
DesignLights Consortium (DLC) is considering adding limits to TLA in its future criteria for listing products.
The Lighting Facts program has an optional informational field where the flicker index and fundamental frequency can be uploaded as supplemental information.
An IES committee is currently developing a standard for the measurement of optical waveforms, and several other IES committees are also working on developing recommended practices for TLA limits for various lighting applications. These recommended practices could be used by lighting designers as a guideline for recommended limits to TLA for applications such as office lighting, roadway lighting, industrial lighting, sports lighting, etc.
The industry appears to be moving in the direction of the limits proposed in NEMA 77-2017 which consolidates many other publications (including IEC, ASSIST, and IEEE). Specific guidelines for the measurement of optical waveforms will likely be deferred to the forthcoming IES document. Further limits for special applications will be set forth by industry-consensus recommended practices (currently being considered by various IES committees).
LightLab International Allentown, LLC has the ability to test lighting products for TLA according to many of the proposed metrics described in this article. Contact us to discuss your particular testing needs.