What is flicker and why do some LED lights blink invisibly?
Most electric lighting does not produce a perfectly constant light output. The phenomenon in which a light fixture’s output periodically fluctuates is called flicker. Historically, this issue has varied depending on technology:
- Incandescent bulbs: The filament was powered by alternating current from the mains (50 Hz), meaning the output fluctuated at 100 Hz. However, due to the huge thermal inertia of the heated tungsten, the light did not have time to dim significantly, and the real fluctuation (modulation depth) was minimal, only about 5–10%.
- Old fluorescent lamps: With magnetic ballasts, they visibly flickered at 100 Hz, which could be tiring for the eyes.
- Modern LED fixtures: The diodes themselves have no inertia—they respond instantly to changes in current. Therefore, the final quality of light depends entirely on the power supply (the driver).
Cheap LED fixtures without high-quality smoothing circuits can have a modulation depth of up to 100% at 100 Hz. In practice, this means the light turns completely off and back on 100 times per second. In contrast, premium drivers with active filtering reduce this fluctuation to nearly zero.
How invisible flicker affects health and reduces performance
The human eye consciously perceives flicker at frequencies up to roughly 60–80 Hz. The problem arises above this threshold. Even if we do not consciously notice rapid flicker, photoreceptors in the retina and associated neural pathways still respond, up to frequencies of around 3000 Hz.
Clinical studies confirm that long-term exposure to high-frequency flicker forces the brain to constantly process a large amount of unnecessary visual data. The mechanism likely involves continuous stimulation of cortical centers. In more sensitive individuals, this can cause:
- Headaches and migraines.
- Rapid eye fatigue and burning sensations.
- Reduced concentration on visually demanding tasks (reading, computer work).
For people with photosensitive epilepsy, the low-frequency range (3–70 Hz) is critical, as it can directly trigger a seizure.
What is the stroboscopic effect and how to identify a safe fixture?
A separate safety concern is the stroboscopic effect. If fast-rotating objects (fans, milling machines, saws in workshops) move under rhythmically pulsing light, the flicker frequency of the light can “catch up” with the rotation speed. The brain may then perceive the moving parts as slowing down or even appearing stationary. In industrial or school workshop settings, this is a serious safety risk.
Standards organizations and legislation have therefore introduced strict metrics. Older indicators like Percent Flicker are now replaced in Europe by two main standards (under the Ecodesign Directive):
- Pst LM (Short-term light modulation): Evaluates visible flicker up to 80 Hz (limit ≤ 1.0).
- SVM (Stroboscopic Visibility Measure): Evaluates invisible stroboscopic effect above 80 Hz (strict limit ≤ 0.4).
When choosing lighting for offices, schools, or healthcare, don’t focus only on power and color. It is essential to ask about the quality of the drivers. In Spectrasol fixtures, we exclusively use premium Flicker-Free certified drivers, which far exceed the strictest standards. This ensures perfectly smooth light flow without any strain on your nervous system.
