Light Pulsation: Physical Reality vs. Perception

Great question — this touches on electrical engineering, physics, and visual neuroscience. Let me address both parts.

Part 1: Do Light Sources Actually Pulsate?

The short answer is: some do, some don't, and the degree varies enormously.

Sources that DO physically pulsate

• LEDs (mains-powered): Many LED bulbs, especially cheaper ones, flicker at 100–120 Hz (double the AC mains frequency). This happens because the driver circuit does a poor job of converting AC to smooth DC. Higher-quality LEDs with good constant-current drivers flicker much less or imperceptibly.
• Fluorescent tubes: Older magnetic-ballast fluorescents flicker at 100–120 Hz and can be quite noticeable. Modern electronic-ballast fluorescents operate at 20,000–60,000 Hz, making flicker effectively invisible.
• PWM-dimmed LEDs: When dimmed using pulse-width modulation, LEDs rapidly switch fully on and off (often at 200–1000+ Hz). This is a real, measurable pulsation that some people can perceive, especially in peripheral vision or during eye/head movement (the "phantom array" or "stroboscopic" effect).

Sources that DON'T meaningfully pulsate

• The Sun and stars: These emit continuously via thermonuclear fusion. There is no periodic pulsation. (There are tiny stochastic fluctuations at the quantum level and acoustic oscillations studied in helioseismology, but nothing you'd ever perceive as flicker.)
• Incandescent bulbs: While powered by AC, the filament's thermal inertia is enormous — it stays above 2000°C throughout the AC cycle. The actual light output variation is roughly 5–15% at 100–120 Hz, which is almost always imperceptible because the filament never has time to cool significantly between half-cycles.
• Candles/fire: Continuous thermal emission. Visible flickering comes from air currents disturbing the flame geometry, not from any electrical pulsation.

Summary Table

| Source | Physical flicker? | Frequency | Perceptibility | |---|---|---|---| | Sun / stars | No (continuous) | N/A | None | | Incandescent | Tiny (~10% modulation) | 100–120 Hz | Almost never | | Cheap LED | Yes (significant) | 100–120 Hz | Often noticeable | | Quality LED | Minimal | 100–120 Hz | Rarely noticeable | | Old fluorescent | Yes | 100–120 Hz | Often noticeable | | Modern fluorescent | Technically yes | 20,000+ Hz | No | | Candle | Movement-based | Random | Yes, but not electrical |

Part 2: Why Do We See Shimmering Halos Around Lights?

This is mostly your visual system, not the light source, and several mechanisms contribute:

Optical factors (in your eye)

• Diffraction through the pupil: Light bends around the edge of your iris, creating subtle radial spikes and rings — this is essentially the same physics as a camera's starburst effect.
• Scattering in the cornea and lens: Microscopic imperfections, the natural fibrous structure of the lens, and the tear film scatter light outward, creating a glow or halo. This gets worse with age, dry eyes, or conditions like cataracts.
• Astigmatism and refractive errors: Even mild, uncorrected astigmatism stretches point sources into streaks or shimmer.
• Tear film instability: Between blinks, your tear film thins and becomes irregular, causing the light pattern to shift dynamically — this produces a real-time shimmering effect that resets with each blink.

Neural factors (in your brain)

• Adaptation and gain control: When you stare at a bright source, photoreceptors locally adapt and the surround region becomes relatively suppressed, creating dynamic contrast effects that look like pulsation.
• Microsaccades: Your eyes make tiny involuntary movements 1–2 times per second. Each microsaccade shifts the bright image slightly across your retina, causing the halo/glare pattern to "pulse" or shimmer.
• Temporal sensitivity of peripheral vision: Your peripheral retina is more sensitive to flicker than your fovea (central vision), which is why you may notice LED flicker more "out of the corner of your eye."

When to pay attention

Persistent or worsening halos around lights, especially at night, can be a sign of cataracts, corneal edema, glaucoma, or other eye conditions — worth mentioning to an eye doctor if it's new or changing.

The Bottom Line

The pulsation you see around an LED is likely a combination of real electrical flicker AND your eye's optical/neural processing. The sun and incandescent bulbs are essentially continuous emitters — any shimmer you see around those is almost entirely your visual system at work.