Many buyers receive samples that look bright and impressive, but after installing them in offices or shops and using them for a couple of days, they receive feedback about "eye strain" or a "feeling of head pressure." Upon investigation, the culprit turns out to be flicker. While "Flicker-Free" has become a standard marketing claim for LED bulbs, there is no globally unified definition for the term; what different manufacturers mean by "flicker-free" can vary significantly. This article breaks down the concept: what causes flicker, how it is measured, and how to evaluate it when making purchasing decisions.
What is LED flicker?
Flicker refers to rapid, periodic fluctuations in light output-alternating between bright and dim-over time. LEDs have an extremely fast response time; their brightness changes instantly in response to fluctuations in current. Consequently, they are more prone than traditional incandescent bulbs to reproducing power supply fluctuations directly in the light output.
Incandescent filaments possess thermal inertia; their temperature changes slowly during current fluctuations, resulting in a steady light output. LEDs lack this buffer, so the quality of the driver circuit directly determines the stability of the light.
Why do LEDs flicker?
There are three main causes:
Excessive power supply ripple: Mains electricity is alternating current (AC). After rectification, a ripple at 100Hz or 120Hz remains (the frequency doubles after rectification of 50Hz or 60Hz grids). If the filter capacitor is too small or omitted, ripple current flows directly into the LED, causing the light to flicker.
Low PWM dimming frequency: Many dimming systems adjust brightness by rapidly switching the light on and off. If the switching frequency is too low, flicker becomes noticeably worse at lower brightness levels.
Incompatible dimmers: Pairing a TRIAC dimmer with a low-quality driver often leads to flickering or sudden brightness jumps, particularly at low brightness settings.
Flicker invisible to the naked eye can still have an impact
Many people assume that "if I can't see the flicker, it's not a problem," but this is not entirely true.
While the human eye generally cannot perceive flicker at frequencies above 80–100Hz, the brain and visual system may still react to it. Prolonged exposure to such lighting conditions can cause discomfort-such as eye strain, headaches, and reduced concentration-though individual reactions vary significantly.
Under flickering light, fast-moving objects can exhibit the "stroboscopic effect"; for instance, rotating machinery might appear to be turning slowly or even standing still, posing a safety hazard in factory environments.
When recording video or live-streaming, flicker manifests as rolling horizontal bands across the image-an issue of particular concern for commercial shoots and e-commerce studios.
The true meaning of "Flicker-Free"
Ideally, "flicker-free" implies a light output stable enough to cause no perceptible or measurable impact on human vision or camera equipment.
In reality, absolute zero flicker is virtually non-existent; any light powered by alternating current (AC) exhibits some degree of fluctuation.
Therefore, "flicker-free" is more accurately understood as "flicker controlled to a very low level that meets specific standards." It is a marketing term, not an official certification.
Two light bulbs labeled "flicker-free" might differ vastly: one could have a fluctuation depth of only 1%, while the other has 30%. You cannot tell the difference from the packaging alone; you need to look at the data.
How to measure flicker: Key metrics
Percent Flicker: The ratio of the difference between maximum and minimum light output to the total output; lower values are better.
Flicker Index: Accounts for both waveform shape and the area of fluctuation, offering a more comprehensive assessment than Percent Flicker alone.
Pst LM: A metric for visible flicker; a value of 1.0 or lower indicates the flicker is imperceptible to the average person.
SVM: A metric for the visibility of the stroboscopic effect; lower values mean the "phantom image" phenomenon associated with moving objects is less pronounced.
Flicker Frequency: Generally, higher frequencies have less impact on people.
Regarding standards, IEEE 1789-2015 provides recommended limits for Percent Flicker at various frequencies; EU Ecodesign regulations set clear requirements for Pst LM and SVM; and certain energy efficiency and building codes in North America also address flicker. Which specific standard to follow depends on your target market.
How to achieve truly low flicker
From a product perspective, this relies on the following:
Constant current driver + adequate filtering: Use reliable electrolytic capacitors and an appropriate circuit topology to suppress ripple.
High-frequency PWM or analog dimming: Maintain a high dimming frequency or use constant-current amplitude dimming to avoid increased flicker at low brightness levels.
Driver and dimmer compatibility testing: Test with dimmers from various brands and different load quantities; do not limit testing to just one type in the laboratory.
Test the complete luminaire, not just the driver: The optical structure, thermal management, and driver form an integrated system; the final assessment should be based on the light output waveform of the assembled luminaire.
Note that reducing flicker often entails higher driver costs, which is one reason for price variations in the market.
How to verify "flicker-free" performance during procurement
Do not simply take the supplier's word for it; follow these steps:
Request test reports: Ask for flicker test data issued by a third party or laboratory, and carefully check which metrics were measured.
Check test conditions: Was the test conducted at full load or during dimming? Is data available for dimming levels such as 10% or 50%?
Confirm the applicable standard: Check whether the report is based on IEEE 1789, IEC, or another standard, and ensure it aligns with your target market's requirements.
Perform a simple preliminary check: Point your smartphone camera (in slow-motion mode) at the light; if distinct rolling bands appear on the screen, the flicker is likely excessive. This is only a rough screening method and cannot replace professional instrumentation.
Test samples in real-world scenarios: Conduct actual tests using the dimmers commonly used by your customers; this provides a more intuitive assessment than merely looking at specifications.
Summary
"Flicker-free" is not a single, unified certification; essentially, it means flicker is controlled at a very low level.
Flicker primarily stems from driver ripple, low-frequency PWM dimming, and dimmer incompatibility. Even if flicker is invisible to the naked eye, it can still have an impact-affecting visual comfort, video recording quality, and the perception of moving objects.
To accurately assess flicker performance, you should look at specific metrics such as flicker percentage, Pst LM, and SVM, rather than relying on the text printed on the packaging.
The most reliable approach is to request test reports, verify testing conditions, and test samples under actual dimming scenarios before making a purchase.
Our LED bulbs undergo flicker testing before leaving the factory; we can provide test reports aligned with your target market's standards and offer compatibility verification for various dimmers. If you are currently selecting products or wish to obtain samples for comparative testing, please share details regarding your application scenario, target market, and estimated order volume. We will respond within 24 hours with a suitable solution and a price quote.
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