Lamp flicker: Hidden in plain sight
Most people are probably not aware that when they switch on almost any lamp, they are witnessing an optical illusion. They think that the lamp light is of constant brightness, even though it is not. The lamp is actually continually cycling between bright and dim, so rapidly that they think that they can't see it, even though their eyes do see it.
When a light "flickers" it rapidly switches between bright and dim light. Sometimes flicker can be obviously visible, like that of a malfunctioning fluorescent light that switches on and off a couple times a second. However, the normal light flicker of incandescent, fluorescent, and most LED lights is hidden because it happens so rapidly that we don't know that we see it. Sometimes flickering light makes people sick, causing seizures, headaches, eye strain, nausea, spatial disorientation, and other neurological symptoms (see Background: Health Effects of LED Lights and Screens).
LED lights present both a risk to human health and an opportunity for better health. Some LED lights flicker more seriously or in a different way than incandescent or fluorescent lights, creating the potential to harm the health of sensitive individuals. However, LED lights can instead be engineered to be completely flicker-free, creating light that doesn't harm sensitive individuals and that is potentially better for human health than incandescent or fluorescent light.
While glare from LED headlights or streetlights is also a significant public safety concern due to loss of visibility on roads, a problem beginning to be recognized even by the lighting industry (see a Cree whitepaper), this website will focus on the potential health effects resulting from light flicker.
LED Light Bulbs
What makes some lights flicker?
LED lights can be engineered to either not flicker at all or to have varying degrees of flicker. The flicker is so fast that it's difficult to impossible for people to know that they see the individual flashes of light.
Unless specifically engineered not to do so, any light bulbs, including incandescent lights, flicker to some extent. This is due to the 60 Hz AC electricity supply in North America or 50 Hz in Europe and other parts of the world. Alternating current (AC) means that electrons alternately flow in one direction through a circuit and then switch to flow in the opposite direction. A bulb lights each time the electrons flow through the circuit, regardless of the direction. Since 60 Hz AC means that the whole forwards/backwards electron cycle repeats 60 times a second, the light bulb flickers cyclically from bright to dim 120 times a second (120 Hz; 100 Hz in Europe). In contrast, If is a bulb is powered by direct current, such as in a simple circuit powered by a battery (like in a battery-powered incandescent flash light), the bulb doesn't flicker at all - it will be constantly on at the same brightness as long as the battery is on.
The flicker of incandescent lights tends not to bother people because when the bulb dims, the recently-burning bulb filament dims slowly enough that it may only dim a little before it flashes back on again. An average incandescent light bulb has about 6.6% flicker (IEEE std 1789, 2015, Figure 2).
The flicker of fluorescent lamps varies, depending on their engineering. Older fluorescent lights with magnetic ballasts have about 40% flicker (Poplawski and Miller, 2011), largely at 120 Hz / 100 Hz, but may also produce 60 Hz / 50 Hz flicker, especially as they age (Brundrett, 1974). Decades ago, there were health complaints about the flicker of magnetic ballast fluorescent lights and newer electronic ballast fluorescent lights are engineered differently so that they produce less flicker. Electronic ballasts introduce >20,000 Hz flicker, which reduces, but does not eliminate, the magnitude of the 120 Hz flicker caused by the AC power. The residual 120 Hz flicker tends to be similar to that of incandescent lights (Poplawski and Miller, 2011).
In contrast to incandescent or fluorescent lights, if they are not deliberately engineered to do otherwise, LEDs will turn completely off during their dim phase, giving them 100% flicker (IEEE std 1789, 2015, Figure 3).
When the current supplied to an LED stops, the light immediately shuts off, creating 100% flicker, as the bulb is alternately on and completely off. Some of the LED bulbs sold at least in the US have 100% flicker. However, circuit engineering strategies exist for reducing LED flicker or for making LEDs essentially flicker-free (IEEE std 1789, 2015).
Given the known adverse health effects of flicker from magnetically ballasted fluorescent lights in the 1980s and early 1990s (reviewed in IEEE std 1789, 2015 and in Background: Health Effects of LED Lights and Screens), LED flicker might produce even higher risks. However, completely flicker-free LED lights might be even healthier than prior artificial lights.
The figure below shows graphs of light output vs. time which display the degree of 120 Hz flicker of several kinds of light bulbs. Each of the graphs has the same average light output, meaning that each of the lights would appear to be the same brightness. The nervous system tends to perceive the average light output when flicker is fast. The Talbot-Plateau Law is the concept that the apparent brightness of intermittent light is proportional to the fraction of time that the light is observed (proposed by William Henry Fox Talbot in 1834 as a result of experiments where a viewer's gaze is fixed on a single point on a rotating disk with white and black sectors or on the intermittent reflection of sunlight through a slit; refined further mathematically in quantitative experiments by Joseph Plateau in 1835 using an observer's fixed gaze on the edge of a rotating white- and black-sectored disk).
Incandescent lights always have a waveform shaped like a sine wave (a) due to the slow brightening and slow dimming of the burning bulb filament. Notice that the shape of the electronic ballast fluorescent waveform (b) has a different shape. Depending on the circuit engineering, an LED light bulb could have any of several different waveform shapes, including (d) sine wave and (e) square wave. There are many other LED waveform shapes as well, some of which are shown in Poplawski and Miller, 2011. Importantly, LED bulbs that have been engineered to be completely flicker-free (c) have constant light output, a completely flat line in the graph, corresponding to 0% flicker.
Flicker percent = 100% x (max-min)/(max + min) where max and min refer to the maximum and minimum light output, respectively. Notice that the flicker percent does not indicate the shape of the waveform. For example, (e) and (f) both have 100% flicker, but have different waveform shapes.
Flicker of incandescent, fluorescent (CFL) and LED light bulbs and constant light output of completely flicker-free LED light bulbs. The above figure was drawn by J. Hackett and may be reproduced for any purpose with appropriate citation of this website as the source.
Industrial LED Lighting
Drivers for LED strip lighting can produce completely flicker-free light (a) or can introduce flicker by always pulsing the direct current from the driver at any brightness (b and d) and/or by increasing the amount of time the light is off when dimming via pulsed width modification (c and d). The above figure was drawn by J. Hackett and may be reproduced for any purpose with appropriate citation of this website as the source.
How is color controlled in LED lights?
Are there limits on LED Light Flicker?
Metrics for quantifying the amount of flicker
Testing LED light flicker
Early recommendations for limiting LED flicker
The 2015 IEEE std 1789 recommendations (see IEEE figure 18) proposed strict limits for <90 Hz flicker that might cause epileptic seizures and proposed more modest limits for >90 Hz flicker. Studies of the effects of fluorescent lights had already indicated such >90 Hz flicker could cause headaches, eyestrain, possible behavioral changes in autistic children, and difficulties in concentration (reviewed in IEEE std 1789, 2015).