Invisible Flicker Is Still Part of Lighting Quality
Moving from “the eye can’t see it” to measurable temporal light modulation
Author | Lawrence Lin
Chairman, GLGA | Board Member, GLG | IWBI WELL Light Concept Advisor | Founder & CEO, LRS
In a mock-up room, the naked eye cannot see the light flickering, yet a smartphone camera shows black bands across the image. The supplier says, “The phone isn’t professional, so that doesn’t mean there is a problem.” Meanwhile, the user worries that “invisible flicker must damage the brain.” Both statements are too absolute.
When light output varies over time, this is called temporal light modulation (TLM). It can produce visible flicker, the stroboscopic effect when objects are moving, or the phantom array effect during rapid eye movements. Whether it is perceived or causes discomfort depends on frequency, modulation depth, waveform, luminance, field of view, and the observer.
A Smartphone Can Provide a Clue, Not a Quantitative Measurement
A camera’s rolling shutter can amplify or create visible bands. It can therefore be useful for quickly identifying potential issues, but it cannot replace appropriate measurement equipment.
Rigorous measurement requires a sufficiently high sampling rate, an appropriate photodetector, clearly defined measurement conditions, and analysis of the light-output waveform over time.
“Flicker percentage” is not a universal metric either. The same modulation depth can produce very different visual effects at different frequencies and with different waveforms. Projects should use evaluation metrics and limits appropriate to the application.
Dimming Is Often Where Problems Appear
Many luminaires perform well at rated full power, but temporal light modulation can increase significantly when dimmed to 10% or paired with a different controller. Tunable-white systems can also develop new waveforms when two light sources are mixed.
Therefore, testing should not be limited to the “best factory condition.”
Residential nighttime settings, classroom demonstrations, office energy-saving modes, and low-light hotel scenes may all operate in low-dimming ranges—which are precisely the conditions that deserve to be tested.
Healthy Lighting Cannot Focus Only on Circadian Effects
If a system uses unstable low-level dimming to reduce nighttime mel-EDI, it may simply exchange one problem for another.
Integrated design requires the spectrum, luminance, controls, and LED driver to work together as a complete system.
What We Can Say Today / What We Cannot Yet Say
What we can say today: Temporal light modulation can be measured and may produce multiple forms of visual artifacts; the driver and dimming state are critical conditions.
What we cannot say: The absence of visible bands on a smartphone proves “zero flicker,” or that every form of imperceptible modulation causes the same type of health effect.
Three Things You Can Do Today
- Measure at 100% output, the typical operating level, and the lowest commonly used dimming level.
- Test the luminaire, driver, and controller as one complete system.
- Product documentation should state the measurement method, frequency, waveform, and operating conditions, rather than simply claiming “flicker-free.”
Evidence label: Established consensus / Open question.
Standardized methods exist for measuring visual artifacts and some of their effects; broader long-term health effects still require further research.
References
- CIE TN 012:2021：https://cie.co.at/publications/guidance-measurement-temporal-light-modulation-light-sources-and-lighting-systems
- CIE 249:2022：https://www.cie.co.at/publications/visual-aspects-time-modulated-lighting-systems
- IES TM-18-18：https://store.ies.org/product/tm-18-18-light-and-human-health-an-overview-of-the-impact-of-optical-radiation-on-visual-circadian-neuroendocrine-and-neurobehavioral-responses/
In the next article, we bring the discussion back to architecture: Daylight, electric light, and controls—which one is truly the protagonist of healthy lighting?
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