1. Introduction: Definition and Taxonomy of Temporal Light Artefacts
Temporal light artefacts (TLAs) are unwanted changes in visual perception caused by a light source whose luminance or spectral distribution fluctuates over time. The CIE, in its International Lighting Vocabulary (CIE S 017/E:2020) and technical report CIE 249:2022, distinguishes three categories of TLA: flicker (direct perception of luminance modulation at frequencies up to approximately 80 Hz in foveal vision), stroboscopic effect (apparent change in motion perception of moving objects under modulated light, visible at 80 Hz to approximately 2 kHz), and phantom array (spatial pattern of alternating bright and dark regions perceived during saccadic eye movements under modulated light, also known as the "ghost" effect). These artefacts are a well-documented source of visual discomfort, headache, and reduced task performance, and have attracted increasing regulatory attention in the European Union's EcoDesign framework.
2. Physiological Basis of TLA Perception
2.1 Temporal Contrast Sensitivity Function
Human temporal vision is characterized by a band-pass temporal contrast sensitivity function (TCSF), with peak sensitivity at approximately 10–15 Hz under photopic conditions and a high-frequency cutoff (critical flicker fusion frequency, CFF) typically at 50–60 Hz for full-field stimulation (de Lange, 1958; Kelly, 1971). The CFF is dependent on luminance level, retinal eccentricity, and stimulus size. Under typical indoor lighting (100–500 cd/m²), the CFF for direct flicker detection ranges from 55 Hz (central fovea, 1° field) to 90+ Hz (peripheral vision, 10°+ field). This peripheral sensitivity means that flicker that is invisible in central vision can still be perceived subconsciously in the periphery, triggering eye strain and headaches.
2.2 Stroboscopic Effect Visibility Measure (SVM)
The stroboscopic effect becomes detectable when high-frequency modulation (80 Hz to 2 kHz) interacts with the motion of objects in the visual field. The CIE developed the stroboscopic effect visibility measure (SVM, defined in CIE TN 006:2016 and adopted in IEC TR 63158:2018) as a unified metric. SVM is computed as:
where Cm is the relative amplitude of the m-th Fourier component of the light waveform, Tm is the threshold amplitude for that frequency (based on the CIE TCSF), and a is the Minkowski summation exponent (a ≈ 3.7). An SVM value of 1.0 corresponds to the visibility threshold for 50% of observers. Values below 0.4 are generally considered imperceptible, 0.4–1.0 visible upon deliberate attention, and > 1.0 clearly visible. IEC TR 63158 specifies a standard measurement protocol using a photodiode-based detector capable of 20 kHz sampling or higher.
3. Regulatory Frameworks
3.1 IEEE 1789-2015: Recommended Practices for Modulating Current in High-Brightness LEDs
IEEE 1789-2015 provides risk-based categorization of LED light modulation:
| Risk Level | Frequency Range | Percent Flicker Limit (0–100 Hz) | Percent Flicker Limit (100 Hz – 3 kHz) |
|---|---|---|---|
| No observable effect level (NOEL) | All | f × 0.025% | f × 0.08% |
| Low risk level | All | f × 0.08% | f × 0.25% |
Note: f is the fundamental frequency in Hz. "f × 0.025%" means that at 120 Hz, the maximum allowed percent flicker is 120 × 0.025% = 3%.
3.2 EU Ecodesign Regulation (EU) 2019/2020
Effective from September 2021, the EU Ecodesiver (EC) Regulation (EU) 2019/2020 (and its amendment CRM-005-2022, under development as of mid-2026) sets mandatory limits for flicker and stroboscopic effect in light sources placed on the EU market:
| Parameter | Limit | Effective Date | Reference |
|---|---|---|---|
| SVM (stroboscopic effect) | ≤ 0.4 | September 2021 | Annex III, Part B, (g) |
| PstLM (flicker severity) | ≤ 1.0 | September 2021 | Annex III, Part B, (h) |
| Flicker frequency (≥ 50% modulation) | No requirement at present* | — | — |
* CRM-005-2022 proposes a new limit for modulation depth at frequencies 100–800 Hz based on weighted SVM equivalent; final adoption expected Q4 2026.
The PstLM metric is derived from the IEC 61000-4-15 flickermeter standard, adapted for lighting (LM: light modulation). It evaluates the short-term (10-minute) flicker severity on a scale where 1.0 corresponds to the threshold of perceptibility for 50% of observers. The measurement is based on demodulation of the 50 Hz or 60 Hz power-line frequency through a weighting filter that simulates the eye-brain visual system's sensitivity to voltage fluctuations → light output fluctuations. PstLM ≤ 1.0 is the EU Ecodesign mandatory limit.
4. Root Causes of TLA in LED Drivers
Unlike incandescent lamps (whose thermal inertia provides natural low-pass filtering of AC line modulation—typically 5–10% ripple at 100/120 Hz), LEDs respond to current modulation virtually instantaneously (nanosecond-scale rise time). The primary causes of TLA in LED lighting products include:
- AC-DC rectification with insufficient bulk capacitance: Half-bridge rectifiers without power factor correction (PFC) produce 100 Hz (50 Hz mains) or 120 Hz (60 Hz mains) ripple at the LED current. The modulation depth is inversely proportional to the storage capacitor value: C = ILED / (2 × fline × ΔV). For 100 mA LED current at 50 Hz with 5% ripple (ΔV ≈ 0.5 V on a 20 V string), C ≈ 1000 μF is required—a significant cost and board-space factor in compact drivers.
- Pulse-width modulation (PWM) dimming below 2 kHz: Many 0–10 V and trailing-edge dimmable LED drivers modulate PWM at 200–2000 Hz. At low dimming depths (< 20%), the PWM pulse width becomes extremely short, and the synchronous recharge of the output capacitor between pulses creates high-frequency oscillation (ringing) that increases the effective modulation depth.
- Power-line frequency harmonics from TRIAC dimmers: Phase-cut dimming using TRIACs (common in retrofit installations) clips the AC waveform, introducing odd-order harmonics that produce intermodulation products in the 50–500 Hz band—precisely where the human TCSF is most sensitive. Filtered versions of the IEEE 1789 waveform envelope show that a TRIAC dimmed to 50% conduction angle can produce 30–60% residual modulation depth at frequencies up to 800 Hz.
- Switching regulator instability: Boost or flyback converters operating in boundary conduction mode (BCM) or discontinuous conduction mode (DCM) without feed-forward compensation can exhibit subharmonic oscillation at half the switching frequency, creating low-frequency beat patterns with the line frequency.
5. Measurement Methods and Instrumentation
| Metric | Standard | Sensor Required | Bandwidth / Sample Rate | Key Requirement |
|---|---|---|---|---|
| SVM | IEC TR 63158:2018 / CIE TN 006:2016 | Photodiode + V(λ) filter | ≥ 20 kHz sampling | Linear response to 2 kHz minimum |
| PstLM | IEC 61000-4-15 + modification for lighting | Flickermeter or equivalent digital | 0.5 Hz – 40 Hz (flicker weighting) | Amplitude accuracy ±1% |
| Percent Flicker | IES RP-16-17 / NEMA 77-2017 | Any calibrated photoreceptor | ≥ 2 kHz (NEMA 77) | Max/Min/Avg over ≥ 1 sec |
| Flicker Index | IES RP-16-17 | Any calibrated photoreceptor | ≥ 2 kHz | Area-based metric (0–1) |
| ASSIST Flicker Perception | LRC ASSIST 2014 | Photodiode + DSO | ≥ 2 kHz | Frequency-weighted perception metric |
6. Design Strategies for TLA Mitigation
- Bulk capacitance optimization: Use electrolytic capacitors with ≥ 10,000 h rated life at 105°C for applications requiring < 5% ripple. Aluminum-polymer hybrids provide higher ripple current capacity in smaller footprints.
- Active PFC + LLC resonant converter topology: Produces near-sinusoidal output current at 2× line frequency with ripple typically < 2%, achieving SVM < 0.1 without additional filtering.
- High-frequency PWM dimming: Raise PWM frequency to ≥ 16 kHz (above human hearing and TLA visibility). This requires MOSFET gate drivers with ≤ 20 ns rise/fall times to minimize switching losses.
- Hybrid dimming: Combine current-mode analog dimming down to 20% of full output, transitioning to high-frequency PWM (> 4 kHz) for the 1–20% range, to avoid the low-frequency PWM artefacts that are most visible at low depth.
- DALI DT8 (Device Type 8) dimming: Uses logarithmic current control (0.1–100% linear output) with inherent low-flicker operation at all levels, per IEC 62386-209.
7. Standards and References
- CIE S 017/E:2020 ILV: International Lighting Vocabulary.
- CIE 249:2022: Visual Aspects of Time-Modulated Lighting Systems — Definitions and Measurement Methods.
- CIE TN 006:2016: Visual Aspects of Time-Modulated Lighting Systems — The Stroboscopic Effect Visibility Measure.
- IEC TR 63158:2018: Equipment for stroboscopic effect visibility measure (SVM) of lighting equipment.
- IEEE 1789-2015: IEEE Recommended Practices for Modulating Current in High-Brightness LEDs for Mitigating Health Risks to Viewers.
- EU Commission Regulation (EU) 2019/2020, Official Journal of the European Union, L 315/120.
- NEMA 77-2017: Standard for Temporal Light Artifacts (Flicker) in Lighting.
- IES RP-16-17: Nomenclature and Definitions for Illuminating Engineering (see sections on temporal light artefacts).
- Kelly, D.H. (1971). Theory of flicker and transient responses. Journal of the Optical Society of America, 61(4), 537–545.
- de Lange, H. (1958). Research into the dynamic nature of the human fovea → cortex systems with intermittent and modulated light. Journal of the Optical Society of America, 48(11), 777–784.
- Wilkins, A.J. et al. (2010). LED lighting flicker and potential health concerns: IEEE standard PAR1789 update. Proceedings of the IEEE Energy Conversion Congress and Exposition.
8. Related Articles
Sources: CIE S 017 · CIE 249:2022 · IEEE 1789-2015 · EU 2019/2020 · IEC TR 63158 · NEMA 77 · de Lange 1958 · Kelly 1971 · Wilkins 2010
Disclaimer: This article is for educational reference. Compliance verification should be performed by an accredited testing laboratory per applicable regulations.