Flicker in light sources is an old problem made new again in the LED age. Its effects range from annoying to debilitating, and solving it can be challenging. Even defining flicker is difficult. Whether a problem is likely to occur depends on the light source, lighting conditions, how sensitive occupants are to it and the tasks being performed.
The solution is to ensure proper installation to minimize chances of electrical noise, choose LED products with high-quality drivers, and pair these products with compatible dimming controls.
Last year, the Institute of Electrical and Electronics Engineers (IEEE) issued recommendations for minimizing flicker based on existing metrics, though this data is underreported in product information. New metrics are on the horizon that should help manufacturers test and describe their products. Meanwhile, specifiers should evaluate partners and their products carefully and test products for themselves.
Photometric flicker is the modulation of light source output or intensity over time. Its source may be external or internal to the lighting system. Disturbances to a light source’s electrical input, such as noise or transient events, can produce flicker. Alternatively, flicker may be an inherent characteristic of the lighting system itself.
Flicker may be visible or stroboscopic. Visible flicker is modulation of light output or intensity that is perceivable under static conditions (nonmoving observer and light source). Stroboscopic flicker, which may be visible or invisible, is perceptible only if the light source or user is in motion. Stroboscopic flicker is particularly concerning when it makes rotating industrial machinery appear to slow or stop. This happens when the object moves at the same (or multiple of the) frequency as the modulation. A related phenomenon is the phantom array effect, in which flicker is made perceptible by the motion of the observer’s eye.
Impact of flicker
In some cases, visible flicker can be desirable. Candlelight’s playful quality is a good example. In a majority of general lighting applications, it can be a problem.
Variations in supply voltage can cause flicker. Severity depends on factors such as the light source type, extent of the voltage change, how often the voltage changes, and the lamp’s gain factor—how much the light source’s intensity changes in response to a change in voltage.
A final factor is ambient light level. Lower light levels make a light source’s modulation easier to see. Dimming can cause flicker or make it more visible.
“Highly detail-based tasks, especially those illuminated with a single light source, or applications with low light levels, are more likely to generate complaints around visible flicker,” said Ethan Biery, LED engineering leader, Lutron Electronics Co., Coopersburg, Pa. “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.”
Flicker is generally noticeable up to about 80 hertz (Hz). Above that, it is not visible to most people. The on/off cycles fuse into a steady and continuous source of light. However, even if flicker is not visible, it can still produce a nervous system response at frequencies of 125 Hz or higher. As a result, invisible flicker affects some people, though they don’t know flicker is the cause. Stroboscopic effects can occur at frequencies from 80–2,000 Hz.
Studies suggest flicker is a contributor to eyestrain, blurred vision and impaired visual task performance. A small percentage of the population is particularly susceptible. Flicker-sensitive individuals can experience severe headaches and migraines.
An old problem returns
All alternating current (AC) light sources produce flicker. For decades, magnetic-ballasted fluorescent lighting systems imposed objectionable flicker on workspaces. The rapid adoption of electronic ballasts, most of which operate at a frequency of about 40 kilohertz, virtually eliminated the issue.
A 1989 study, “Fluorescent Lighting, Headaches and Eyestrain” (Wilkins, et al.), showed a significant reduction in self-reported eyestrain and headaches in a small office population after installation of high-frequency electronic ballasts.
With the rise of LED lighting, flicker has re-emerged as an important industry issue. LEDs have no persistence, so any change in forward current produces a nearly instant change in light output. Therefore, flicker can be more pronounced.
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.
—Ethan Biery, Lutron Electronics
The majority of LED luminaires feature dimmable drivers and are often paired with dimming controls. This can exacerbate flicker during deep dimming by significantly reducing ambient light levels, making flicker more noticeable.
LEDs can produce flicker through interactions with line-voltage dimmers, which “cut” the AC waveform during dimming, causing the LED to cycle rapidly.
“The LEDs themselves are not the cause of flicker,” said Yan Rodriguez, VP, product and technology, Acuity Brands Lighting. “It is the power supplies that cause the flicker in most products. If the driver is not designed well to deal with flicker, there are no controls that will make it better. Digital controls, whether wireless or wired, will not generally induce flicker in the system.”
Solving the problem
High-quality LED lamps and luminaires typically do not produce objectionable flicker. Nor should LED products paired with digital controls (as Rodriguez points out above) or line-voltage dimmers rated as compatible with LED lighting. However, these drivers, which feature components added to manage output, impose a higher cost while requiring a larger driver size.
“Some in the LED industry have recognized the impact flicker has on people and have designed products that mitigate that impact,” said Aaron Smith, director of technology, Finelite Inc., Union City, Calif. “However, other priorities, such as cost reduction, have stressed cost advantage over control performance.”
Low-cost circuitry options such as rectifier, reverse-parallel or AC direct power supplies are prone to flicker. Products that are constrained in size, such as LED MR16 lamps, feature fewer filtering components and use analog instead of digital circuitry, making them more susceptible to internal and external flicker.
Cost is not a sure determinant of whether the product will exhibit flicker. Ask the manufacturer what type of circuitry is used and what components the driver features to minimize flicker. Even a high-quality driver paired with an incompatible dimmer may produce flicker.
“The problem with flicker in luminaires and lamps really does not fall into applications but rather cost, size and dimming requirements, in that order,” Rodriguez said. “Generally speaking, low-end residential products, due to their cost restraints, will use a power supply technology that is more prone to flicker, compared to more expensive multistage switching power supplies found in commercial products. Lamps, due to their size constraints, will also employ topologies that are more prone to flicker. There are, of course, exceptions in a few high-end architectural lamps.”
In the field, testing and solving a flicker problem is challenging. Handheld flicker meters allow field-testing, but training is required to achieve proper calibration and interpret the results. In addition, current flicker metrics make it difficult to interpret results in a way that’s meaningful for an application. Still, these tools can be valuable for comparing lighting performance to best-practice metrics. Measurements should be taken without stray light entering the area from windows or other luminaires.
Another method is a flicker wheel, which is a flat wheel with a point at the bottom. When it spins, a pattern on the wheel will become either visible or “choppy.” A similar method is the pencil test—waving a pencil under the light source to see if it’s a blur or a discrete set of images—which would indicate flicker. These methods are simple but relatively inaccurate.
“Unfortunately, there are no good field-measurement techniques for measuring flicker as it corresponds to human perception,” Biery said. “The best tool is still the eye of an experienced lighting professional.”
Don’t forget about cameras, Rodriguez said.
“If the room is used for video conferencing, ask for it to be turned on,” he said. “There may be little noticeable flicker in the space, but it may be completely incompatible with the video system.”
A basic troubleshooting process can isolate whether the cause of the flicker is external (electrical) or internal (driver or driver interaction with a dimming control). Note if the flicker is constant or intermittent. If intermittent, does it occur in relation to another activity, such as a nearby elevator moving? Move the luminaire to another part of the building, and see if it is still flickering. The answers to these questions may indicate interference by an external source, which should be mitigated.
“In new installations, contractors should follow the recommended practice of separating wiring [including neutral wires] between lighting and nonlighting loads as much as possible,” Biery said. “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.”
If flicker occurs at dimming levels, evaluate the dimmer. If it’s a line-voltage dimmer, consider replacing it with a dimmer that has been tested and confirmed to be compatible with the specific LED products being installed. As a starting point, both the LED product and the line-voltage wall box dimmer should be tested in accordance with the National Electrical Manufacturers Association (NEMA) SSL-7A standard, which helps ensure desired performance. Also, low-voltage digital controls are generally far less prone to flicker from external sources.
Otherwise, the best way to avoid objectionable flicker is to specify LED products with high-quality drivers.
“If a lighting installation is demonstrating undesirable flicker, and the flicker is part of the fixture’s normal operation, there is really nothing that can be done other than replacing the installation with a new luminaire with acceptable flicker performance,” Smith said. “This has the potential to be a very costly issue for commercial applications. So it is vitally important that the luminaire be validated to have a low risk of producing flicker before installation. Ideally, this should be determined well upstream in the design phase of a project, long before any luminaires are delivered to a job site.”
One way to do that is get educated and choose one’s partners carefully, preferring trustworthy manufacturers that back their products. Ideally, specifiers will evaluate products based on standardized metrics.
Smith said IEEE PAR1789 2015 is a good starting point for product selection. Its recommendations are broken into three application needs. First, prevent seizures in light-sensitive individuals. Second, limit other biological effects of flicker. Third, prevent other biological events of flicker. For each, IEEE stipulates maximum percent flicker based on frequency. These recommendations are based on current metrics.
After its release, however, NEMA released a position paper stating the IEEE recommendation is too strict for many applications and could add unnecessary cost to the electronics. NEMA, the Department of Energy, the International Electrical Commission and the International Commission on Illumination are all working on flicker metrics or guidelines.
“Flicker continues to be a challenging issue for the lighting industry, mostly because lighting product flicker performance information is practically nonexistent for specifiers and contractors,” Smith said. “There is also a bit of controversy regarding the exact metrics that should be applied to determine flicker. The most informed specifiers will do their own investigating, implementing best-practice design for their application, contacting manufacturers, measuring flicker using a flicker meter, and evaluating mockups and physical samples.”