I recently had the pleasure of interviewing Ethan Biery, LED Engineering Leader, Lutron Electronics. The topic: flicker. I’m happy to share his responses with you here. The interview informed an article I wrote for the December 2016 issue of ELECTRICAL CONTRACTOR.
DiLouie: In your opinion, how big a problem is LED product flicker in the lighting industry as of 2016? What types of products (lamps versus luminaires, high- versus low-end products) are most likely to exhibit flicker? What types of applications are most likely to experience visual and/or stroboscopic flicker?
Biery: All light sources, including LED and incandescent, inherently provide stable light output when provided with a stable power source, but most real-world power is not perfectly stable, which is why flicker was a problem even before LEDs. However, LED light sources can be more prone to showing Temporal Light Artifacts (TLA) than other sources due to their fast response time.
The likelihood that an LED will flicker (visible or stroboscopic) is directly related to the quality of the driver, and indirectly related to the quality of the power source. While customers have little control over their building power quality, they do have control over the quality of the driver. Generally speaking, lower-cost (and lower-size) drivers are more likely to exhibit flicker, especially under noisy power conditions, because they may lack the necessary filtering to ensure stable power is delivered to the LEDs.
Highly detail-based tasks, especially those illuminated with a single light source (such as reading by the light of a bedside lamp), or applications with low light levels, are more likely to generate complaints around visible flicker. Tasks that involve motion or moving objects are more likely to experience the effects of stroboscopic flicker. Overall, however, the effects of flicker are highly dependent on the observer (and, like the sound of a dripping faucet, once flicker is detected, it becomes difficult to ignore).
DiLouie: How often is flicker an issue of either 1) LED lighting systems operating at full output or 2) LED lighting systems either dimming or reaching a certain dim level? Which is the greater concern?
Biery: The eye is naturally more sensitive to fluctuations in light when light levels are low, so any instability of the light is more likely to be seen as visible flicker when dimming to low levels (such as those below about 10%). Given the right environment, the effects of stroboscopic flicker can be seen at any light level.
DiLouie: How does visual and/or stroboscopic flicker manifest in traditional light sources and in LED light sources? What is it about the LED source that makes flicker a significant issue?
Biery: What determines whether or not TLA will occur is the quality of the power signal delivered to the light source, combined with how tolerant the light source is to any fluctuations in power. For example, high frequency electronic ballasts virtually eliminated the flicker problem that was once commonly experienced with low-frequency magnetic ballasts. In other words, merely changing the quality of the power source (the output of the ballast going from 60Hz to tens of kilohertz) greatly decreased the presence of flicker, although the lamps themselves remained unchanged.
Like all other sources, LEDs produce light output that is inherently very stable when fed with a stable power source. Because LEDs are very fast-acting, and any fluctuation in the power source to the LED will become an instantaneous fluctuation in the light output — this is what we see as flicker.
In contrast to LEDs, incandescent lamps are particularly tolerant of fluctuations in their power quality because their hot filament filters out many fluctuations (although some measureable amount of modulation is still present at a 60Hz line frequency). As a result, LEDs are more likely to show flicker then incandescent lamps are under the same conditions.
DiLouie: What detailed recommendations should electrical contractors follow to minimize flicker when selecting an LED driver?
Biery: When using fixtures, be sure to choose one with a high-quality driver. The LED driver plays a very significant role in delivering flicker-free, high-quality dimming performance. Less expensive, less complex drivers generally have fewer filtering components and use analog instead of digital circuitry, making them more susceptible to undesirable modulation and external electrical interference.
In their push to lower-priced designs, many screw-in retrofit lamps have reduced the amount of filtering done in their integral drivers, making them especially susceptible to flicker from power line fluctuations.
Also, be very wary of metrics. There is no established method for measuring flicker that completely captures all the aspects of TLA, and different applications can tolerate different levels of TLA. Simply citing an inadequate metric (such as Flicker Index or Percent Flicker) or seeing a spec sheet that says “flicker free!” won’t assure you a satisfying end result.
DiLouie: What are common causes of flicker that are external to the LED lighting system, such as the power supply and connected dimming controls? Why and how do these causes produce flicker?
Biery: Outside of the lamp or fixture, flicker can result from noise being introduced through power or control wires. For example, electrical noise generated from large motors or other power-hungry devices can reach a driver, pass through it, and cause flicker on the output. That same electrical noise may also introduce instability into a phase-cut dimmer waveform, causing flicker. Finally, even for drivers that use analog low-voltage control signals (such as 0-10V), noise coupled into the control wires can induce flickering from some drivers.
By nature, digital controls will consistently deliver higher performance than analog controls. Digital controls provide a more precise signal whose quality is less affected by noise and external interference, and therefore less likely to result in interference-induced TLA.
Large-scale installations of LEDs, especially in commercial spaces, often demonstrate the weakness of existing analog control technologies such as phase control and 0-10V. Not only are analog controls prone to compatibility problems and interference, but they don’t deliver the sophisticated features that building managers expect from their lighting control
DiLouie: What detailed recommendations should electrical contractors follow to minimize flicker when pairing lighting controls with LED lighting systems?
Biery: First and foremost, keep in mind that the LED driver plays a very significant role in delivering flicker-free, high-quality dimming performance, and simpler drivers are often more prone to flicker. Less expensive drivers generally have fewer filtering components and use analog instead of digital circuitry, making them more prone to external electrical noise sources, which can manifest as flicker. (The same often holds true for retrofit LED lamps, which contain an integral LED driver.)
Use trusted manufacturers for fixtures, drivers, and controls, and stick to what has worked for you and your customers in the past.
Work with manufacturers who guarantee compatibility and performance between system components, such as drivers and control systems.
Use digital control rather than analog whenever possible. Digital controls are less prone to external noise and therefore less likely to result in interference-induced TLA.
Consider doing project mockups for larger jobs. This allows experiencing and evaluating a proposed lighting and control solution in real time and in the actual environment. Mockups reduce the risk of finding performance problems on an installed job, when remediation becomes extremely difficult and expensive.
Work with a quality manufacturer who demonstrates a commitment to testing thousands of driver, fixture, bulb, and control combinations for compatibility and performance (including flicker), and who makes those results public and easy to access. One such example is the Lutron LED Control Center of Excellence (www.lutron.com/ledtool).
DiLouie: What detailed recommendations should electrical contractors follow to minimize flicker that may be produced by other external sources such as voltage fluctuations on the power line? What applications present the greatest risk?
Biery: For new installations, contractors should follow the recommended practice of separating wiring (including neutral wires) between lighting and non-lighting loads as much as possible. Likewise, control signals (especially analog-based control signals, such as 0-10V and phase control) should be run separately from the high-current power wires that supply electrically noisy sources. Common sources of electrical noise are motors, including those found in elevators, compressors, and HVAC equipment.
DiLouie: How can electrical contractors test for flicker in the field? What specific testing can they undertake to evaluate potential installations?
Biery: Unfortunately, there are no good field-measurement techniques for measuring flicker as it corresponds to human perception. The best tool is still the eye of an experienced lighting professional.
DiLouie: What can electrical contractors do to mitigate flicker after installation, if anything? What is the basic troubleshooting process and typical remedies?
Biery: Mitigating flicker is best managed prior to installation by choosing compatible system components, and high quality drivers and controls. However, if flicker is experienced in the field, answering some basic troubleshooting questions may help narrow down the cause.
• Is the flicker always present, or it is intermittent? If intermittent, does the flicker correspond to any other activity (such as the motion of a nearby elevator)? Are all fixtures flickering, or just those in one particular area? Does the light source still flicker when moved to a different area of the building? These may all indicate the flicker is related to an external electrical noise source, which must be identified and mitigated.
• Is flicker only experienced at a particular light level, or at all light levels? This may indicate incompatibility with the lighting control being used.
DiLouie: What metrics are available to electrical contractors seeking to evaluate flicker potential of LED products?
Biery: Many lighting professionals are looking for an industry standard that definitively identifies acceptable levels of TLA (both flicker and stroboscopic effects), and can be applied in a wide variety of applications. At this time, however, it’s best to proceed with considerable caution because existing industry metrics for measuring and mitigating TLA remain limited and somewhat controversial.
Until researchers agree on a way to measure and replicate results that are consistent with human perception, while also controlling for all contributing factors, many lighting manufacturers and industry organizations are hesitant to recommend a particular standard or guideline.
DiLouie: What are the shortcomings of these metrics? What is the industry doing to develop suitable new metrics? How do you anticipate they will be used?
Biery: Most existing metrics have little correlation with the human perception of flicker, and therefore offer little insight as to whether or not the flicker will be visible in the end application.
Flicker is a scientific measurement and needs to be treated as such in developing design standards and identifying appropriate metrics. Both NEMA and the Department of Energy (DoE) are actively working to characterize TLA and measurement devices in hopes of developing metrics that are reproducible and account for the wide number of variables in general-purpose lighting applications.
For example, the DoE has recently tested existing flicker meters against a benchmark to provide specifiers and their customers with better guidelines and recommendations regarding measuring and accounting for basic flicker parameters in their projects (Click here to learn more.)
DiLouie: If you could tell the entire electrical industry just one thing about the flicker and LED lighting, what would it be?
Biery: TLA should certainly be taken seriously, as it can be a source of occupant discomfort and dissatisfaction, but until the industry reaches a consensus on a standard that accounts for all the variables in a lighting scenario, merely citing existing flicker metrics is probably not sufficient. Instead, work with high-quality, experienced manufacturers that will recommend, install, and support the right LED solution for your project. In the meantime, look for further updates on IES, CIE, and NEMA developments in flicker research. As an industry, we all look forward to a robust, reproducible, and accurate standard for measuring TLA.