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Do you know that vague feeling of tiredness in your eyes after a long day in front of the screen? Do you get migraines more easily under certain light sources? Or do you know that liberating feeling when you step out of a neon-lit supermarket back into the clear daylight?
All these phenomena can be linked to a property of artificial light that is rarely noticed in everyday life. This refers to light modulation, or, to put it more simply, flickering.
Light modulation describes the change in brightness of a light source over time. Daylight is subject to a natural kind of modulation: it gradually gets brighter in the morning and darker again in the evening. These changes occur slowly and continuously.
Light flickering must be clearly distinguished from this. It is a technically induced, rhythmic light modulation that often occurs so quickly that it is not consciously perceived.
While the flickering of a malfunctioning fluorescent tube is clearly visible, the flicker frequencies of most modern artificial light sources are so high that we cannot see them directly. However, this does not mean that these rapid fluctuations in brightness have no effect.
Light flicker does not happen by chance, but is a result of the way artificial light is produced and controlled. While daylight is available from sunrise to sunset, artificial light must be generated and regulated electrically. Depending on the power supply, electronics, and dimming method, this can lead to rapid fluctuations in brightness.
Many modern light sources, especially LEDs and screens, regulate their brightness via timed light output. The light appears uniform, even though it is not physically constant. The degree of flicker in a light source depends less on the light source itself than on the quality of the control system.
In his book “Die Kraft des Lichts” (The Power of Light), physician and light researcher Dr. Alexander Wunsch devotes an entire chapter to the topic of light flicker. In it, he describes in detail how light modulation occurs, what role frequency and modulation depth play, and why these properties are relevant for the evaluation of artificial light sources.
Light flickering occurs when a light source is operated with alternating current or pulsed direct current. The luminous flux is then not emitted continuously. Instead, rhythmic fluctuations in luminance occur, meaning that the light alternately becomes brighter and darker. Two aspects are relevant for this value, namely the frequency at which the brightness fluctuates and the magnitude of the amplitude or modulation depth at which the light becomes brighter and darker – only gradually or from 100 percent to 0 percent and back again, as with a stroboscopic flash. An incandescent lamp powered by mains electricity flickers at twice the mains frequency, i.e., at 100 Hz.
Unlike flash lights, incandescent lamps have a low modulation depth because the filament reacts very slowly to changes in the amplitude of the alternating current. The plasma inside fluorescent lamps reacts much more quickly, resulting in stronger light flickering with a higher modulation depth. LEDs react even more quickly to the slightest fluctuations in the power supply. Whether or not an LED light source flickers therefore depends largely on the electronics. If it is operated with pure direct current, the LED is flicker-free, but if it is connected to a low-quality electronic driver, it mutates into a stroboscopic flash.
When dimming an LED, lighting technicians often use the aforementioned pulse width modulation. This involves switching the LED on and off in rapid succession and changing the ratio between the light pulse and the dark pause. Although the light from an LED dimmed in this way appears darker to the eye, in reality the lamp alternates between full brightness and being inactive.
This digital form of dimming is a kind of optical illusion, because it exploits the same property that makes us believe we are seeing fluid motion when we see twenty frames per second, for example in the cinema.”
(Dr. Alexander Wunsch, The Power of Light, p. 167 ff.)
Although most people are not consciously aware of light flickering at higher frequencies ...
Whether and how light flickering affects humans cannot be answered scientifically in general terms. Research shows a differentiated picture in which the technical properties of light, duration of exposure, and individual sensitivity interact.
One thing is certain: our visual system is not designed to reliably perceive rapid fluctuations in brightness. Even when light appears steady, temporal modulations can be detected in the brain. Studies and anecdotal reports link such lighting conditions to visual fatigue, headaches, and concentration problems, especially during prolonged exposure or in sensitive individuals. They also play a role in safety-related work environments because flickering light can distort movements and create stroboscopic effects. However, it has not yet been possible to define a clear threshold above which light flickering is considered harmful.
Natural daylight is a helpful point of reference. The sun does not flicker. Its brightness changes slowly and continuously. Artificial light sources that come close to this principle are therefore often perceived as calmer and more pleasant. In this context, light flicker is not a clearly defined risk factor. The effect always depends on the use of light, the strength of the modulation, and individual sensitivity.
When light flicker is not visible and its potential effects depend heavily on the context, uncertainty often remains in everyday life. You may feel tired or restless, but find it difficult to attribute these feelings to any specific characteristic of the light. The eye alone is not enough to reliably assess light quality.
An obvious consequence is not only to observe light, but also to measure its properties. This approach becomes particularly illustrative when the temporal modulation of light is not represented in abstract numbers, but translated into another sensory perception. When light flickering is made audible, differences can be perceived immediately—even without prior technical knowledge.
This approach is the basis for a simple device developed by physician and light researcher Dr. Alexander Wunsch. By transmitting light modulation into the audible range, direct, intuitive feedback is created: calm, flicker-free light produces hardly any signal, while strongly modulated light sources produce clearly audible patterns.
This was the basis for the creation of the LiMoTest solar. It makes it possible to experience a property of artificial light sources that would otherwise remain hidden, thus enabling a more objective classification of light – beyond subjective impressions or technical promises.
When we talk about light, we usually think first of brightness or light color. But light also has effects through properties that are less obvious—such as light modulation. Light flicker is a prime example of this. It is often invisible, not always easy to classify, but relevant enough to take a closer look.
In everyday life, this means one thing above all else: light quality cannot always be assessed at first glance. Measurability can help to reveal differences that are invisible to the naked eye. At the same time, a look at current technology shows that there are definitely alternatives available. Today, flicker-free, dimmable LED lamps with good color rendering are available that provide calm light and can be used flexibly.
A conscious lighting environment is not created by doing away with artificial light, but rather through careful selection. Those who know what matters can use light in a targeted manner to suit the room, its use, and their own preferences—whether at work, at home, or on the go.
Image credits:
Cover image: -Black Track Lighting by Alex Makhalov via Canva.com
In the blog: -Futuristic Tunnel by Liuuu_61 from Pexels via Canva.com
-Own graphic for light modulation
-Person working… by Stefanut Sava’s Images via Canva.com
-Own product photo
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