We have probably all experienced flicker at some point, where we see a flickering light from a light source. But what is flicker exactly? And how can it be avoided?
Flicker describes the phenomenon where a light source turns on and off at very rapid intervals. Many of us are familiar with flicker from older fluorescent lights, where the problem arose due to the magnetic ballasts* (or coils) of the fluorescent tubes. Over time, the magnetic ballasts were replaced with more efficient high-frequency electronic ballasts, which solved the problem of flicker. However, the flicker phenomenon has reemerged with the advent of LED lights.
*A magnetic ballast or coil is a simple type of power supply that ensures the fluorescent tube is turned on and supplied with the appropriate amount of energy.
LED lighting has brought flicker back to life
When it comes to flicker, it is important to emphasize that it is never the LED light source itself that flickers, but rather the power supply of the lighting. This can be explained as follows: LED lights consist of light-emitting diodes that convert electrical energy into light. The diodes are controlled by a driver (electric converter/ballast) that ensures the diodes are supplied with the correct amount of current. When it comes to the flicker of LED lights, it is not the diodes themselves that cause the problem, but rather the driver that delivers the current to them. For many, it may seem like a more complex technical problem to solve, but there are many ways to eliminate it – it just requires us to understand what flicker is.
A Misleading Term
When we look at a lamp and see the light flickering, we perceive it as flicker. However, it's interesting to note that flicker is not an entirely accurate term for what the light is doing, but rather a term for what our eyes see. While our eyes perceive the light as flickering, what is happening in the lamp is a variation of light over time, which is scientifically known as temporal light modulation (TLM). When exposed to TLM, we can observe several phenomena beyond flicker. TLM also encompasses phenomena such as stroboscopic effects and phantom arrays. Flicker, stroboscopic effects, and phantom arrays are all phenomena referred to as temporal light artefacts (TLA). These artefacts are all caused by temporal light modulations.
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A Subjective or Objective Phenomenon?
Flicker, in technical terms, is defined as the "perception of visual instability caused by a light source with temporal fluctuations in brightness or spectral distribution for a static observer in a static environment." In simple terms, flicker refers to the flickering we experience when the brightness of a lamp fluctuates while we and our surroundings remain still. This means that if we or the lamp are in motion, our perception of flickering light can be attributed to other factors rather than flicker. Specifically, our eyes can only perceive flicker when the modulation frequency is below approximately 90 Hz. If the modulation frequency is above 90 Hz, our eyes are unable to detect the temporal variations in the light. Based on this definition, one can conclude that flicker is more of a subjective phenomenon rather than an objective one.
Visible and invisible flicker: A health risk
When discussing flicker, we are referring to both visible and invisible flicker. This is because there is flicker that we humans can perceive with our naked eyes, but there is also flicker that is not immediately visible to us. However, we often feel the consequences of both visible and invisible flicker in our bodies. This is because temporal light modulation (TLM) has not only visual effects, such as temporal light artefacts (TLA) but also non-visual effects. Visible flicker, first and foremost, is bothersome and irritating to most people because it disrupts our vision. Furthermore, both visible and invisible flicker also have biological effects on us. In general, flicker has negative consequences for our health. Prolonged exposure to flicker can cause eye strain, headaches, and migraines, among other things. Therefore, it is essential to be aware of flicker in the environment we occupy – and this can be done by measuring the phenomenon.
How do you measure flicker?
Presently, there are two standardized methods for measuring temporal light artefacts. The first method is known as the "perceptibility short-term light flickermeter" (PstLM), which is used to measure flicker. The second method is called the "stroboscopic visibility measure" (SVM) and measures stroboscopic effects. Both methods are designed so that a measurement result of 1 indicates a 50% probability of humans being able to perceive flicker or stroboscopic effects. A higher measurement result implies a higher likelihood of human perception, so it can be said that the lower the measurement result, the better. Currently, there is no measurement method available to quantify phantom arrays, and even more concerning is the absence of a suitable method to quantify the biological or cognitive impacts of temporal light artefacts.
WARNING! The following video contains flashing lights that may cause discomfort for individuals with photosensitive epilepsy.
New regulations and limits
In September 2021, new EU regulations came into effect to address the issue of flicker in light sources. These regulations aimed to set limits on flicker and refer to new standards for measuring it. As part of these regulations, new thresholds were established for flicker and stroboscopic effects. For example, the "perceptibility short-term light flickermeter" (PstLM) should not exceed a value of 1, while the "stroboscopic visibility measure" (SVM) should not exceed a value of 0.4. However, a criticism of these thresholds is that they are specified for non-dimmed light levels (light sources at 100% brightness), which means that flicker can still occur in approved fixtures when dimmed below 100%. These regulations do not account for the fact that light modulation becomes more challenging as the light is adjusted in intensity and colour temperature. Despite this criticism, it has become even more important with these new regulations to increase awareness of flicker and educate about how to measure it easily, quickly, and accurately. By doing so, the lighting industry can comply with the new rules, reduce health issues caused by flicker, and make LED technology safer and more sustainable for everyone.
Standards and Recommendations
In 2008, the Institute of Electrical and Electronics Engineers (IEEE) established a working group to delve into the topic of flicker and LED lighting. The formation of this group was driven by the desire to gather a diverse community of experts to discuss the effects of flicker. Comprised of members from the medical, lighting, photobiology, and electrical engineering fields, the working group aimed to explore the issue of flicker and provide LED lighting developers with a better understanding of potential health effects associated with their designs. The culmination of their efforts came in 2015 with the publication of a new standard, IEEE1789, titled "Recommended Practices for Modulating Current in High-Brightness LEDs for Mitigating Health Risks to Viewers." While this standard is not mandatory, it serves as a recommended guideline that delves into various aspects of flicker. For a comprehensive read of the IEEE 1789 standard, you can access it here.