I noticed my LED bulb appeared to pulsate with light radiating outward around it. This made me wonder about pulsation in light sources more broadly:
- Why do LEDs and artificial lights flicker or pulsate? Is this visible to the human eye?
- Does the Sun actually pulsate, and if so, does this affect its light output in any perceptible way?
- Do all light sources have some form of pulsation or oscillation, or are some truly constant?
- What causes the visual perception of pulsation or halos around bright lights - is it the light source itself or something about how our eyes/brain process light?
Of course! This is an excellent question that touches on physics, engineering, and human biology. The pulsating effect you observed is a combination of how the light is generated and how your eyes perceive it.
Here is a breakdown of your questions:
1. Why do LEDs and artificial lights flicker or pulsate?
Yes, many artificial lights flicker, but it's usually too fast for us to consciously perceive. This flicker is a byproduct of how they are powered.
- AC Power: Most homes use Alternating Current (AC) power, which reverses direction 50 or 60 times per second (50Hz or 60Hz). This means the power supplied to the bulb effectively turns on and off 100 or 120 times per second.
- LEDs and Drivers: LEDs are diodes, meaning they only allow current to flow in one direction. A simple LED circuit on AC power would only light up half the time. To solve this, LEDs use a component called a driver.
- Low-quality drivers may not fully smooth out the AC power, causing the LED to dim and brighten 100/120 times a second. This is the "flicker."
- High-quality drivers use capacitors to store and release energy, smoothing out the power delivery and creating a much more constant light with minimal flicker.
- Dimming (PWM): Many dimmable LEDs use a technique called Pulse Width Modulation (PWM). Instead of reducing voltage, the driver turns the LED on and off thousands of times per second. The shorter the "on" time, the dimmer the light appears.
Is it visible? For most people, flicker above ~80Hz is not consciously visible due to persistence of vision. However, our brains still process it, and it can sometimes lead to eye strain, headaches, or a general sense of unease. You can often see this flicker by:
- Viewing the light through your phone's camera (especially in slow-motion mode).
- Waving your hand or a pen quickly in front of the light, creating a "strobe" or "phantom array" effect.
2. Does the Sun actually pulsate?
Yes, the Sun does pulsate, but not in any way that is perceptible to the human eye.
The Sun is a dynamic ball of plasma with complex processes happening inside. The study of its oscillations is called helioseismology.
- Solar Oscillations: The Sun "rings" like a giant bell due to pressure waves (sound waves) reverberating through it. These cause the surface to move up and down by several hundred meters over periods of about five minutes.
- Light Output Variation: These oscillations cause minuscule variations in the Sun's brightness. Additionally, the Sun's total output varies by about 0.1% over its 11-year sunspot cycle.
Is it perceptible? Absolutely not. These variations are incredibly small and slow, requiring highly sensitive scientific instruments to detect. The "twinkling" we see from stars (and the Sun, if you could look safely) is caused by atmospheric scintillation, where turbulence in Earth's atmosphere distorts the light path, not by the star itself pulsating.
3. Do all light sources have some form of pulsation?
No, not all light sources pulsate in the same way. The constancy of a light source depends on its power source and mechanism.
| Light Source | Pulsation/Flicker? | Why? |
|---|---|---|
| Incandescent Bulb | Very little | The filament has thermal inertia. Like a hot stove element, it doesn't have time to cool down and dim significantly during the off-cycles of AC power. |
| Fluorescent Bulb | Yes | The gas inside is excited by the AC electricity. Older magnetic ballasts had significant 100/120Hz flicker. Modern electronic ballasts operate at much higher frequencies (20,000Hz+), making the flicker unnoticeable. |
| Candle Flame | Yes (Flicker) | The flicker is random and caused by the unstable process of combustion and air currents, not a regular electrical cycle. |
| Battery-Powered Light | No (Essentially constant) | Batteries provide Direct Current (DC), a steady, one-way flow of electricity. This results in a very stable, non-pulsating light output. |
So, a light source powered by a battery (DC) is about as close to a truly constant light source as you can get.
4. What causes the visual perception of pulsation or halos?
This is the most likely explanation for what you personally observed: the "pulsation with light radiating outward." This is almost certainly an effect of your eyes and brain, not the light source itself.
Here are the primary causes:
- Tear Film Instability: The most common cause. Your cornea is covered by a thin layer of tears. This film is not perfectly smooth and is constantly shifting, evaporating, and being replenished when you blink. As it moves, it slightly refracts the light entering your eye, causing bright points of light to appear to shimmer, pulse, or send out rays.
- Diffraction: Light waves bend (diffract) as they pass through a small opening—in this case, your pupil. This creates a faint pattern of rings and lines around a point source of light, known as an Airy disk. Small changes in your pupil size can alter this pattern, contributing to a sense of movement.
- Imperfections in the Eye: No one's eye is optically perfect. Tiny imperfections in the cornea or lens can scatter light, creating effects like halos (rings) or starbursts (rays). These can seem to shift or pulse as your eye makes tiny, involuntary movements (saccades).
- Floaters: The vitreous humor (the gel inside your eye) can have tiny clumps of protein called floaters. When they drift near your line of sight, they can cast shadows on your retina and interact with incoming light, causing a shimmering perception.
In summary, the technical flicker of an LED is real but usually too fast to see directly. The radiating, pulsating halo you described is a beautiful example of a visual artifact created by the interplay of a bright light source with the natural, imperfect optics of the human eye.
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1) Why LEDs and other artificial lights “pulsate” (flicker)
Most modern artificial lights are not powered by perfectly steady current, so their light output is often modulated in time:
- Mains-powered lamps (AC): In many countries the line frequency is 50 or 60 Hz. If the lamp’s light output follows the instantaneous power, you can get modulation at 100/120 Hz (because power goes like (V^2), giving two peaks per cycle).
- LEDs specifically: LEDs respond very fast to current changes, so any ripple in the driver current becomes light flicker. Common causes:
- Cheap/simple drivers that rectify AC with minimal smoothing → strong 100/120 Hz flicker.
- LED drivers using PWM (pulse-width modulation) for dimming → flicker at anywhere from tens of Hz to many kHz.
- Switching power supplies → can add higher-frequency ripple.
Is it visible?
- Many people won’t perceive 100/120 Hz flicker as obvious “blinking” when staring steadily, but it can still be noticeable:
- in peripheral vision
- as a stroboscopic effect on moving objects (fans, spinning wheels)
- on phone cameras (rolling shutter makes it very obvious)
- as eye strain/headache for some individuals (sensitivity varies)
- Lower-frequency PWM (e.g., <200 Hz) is more likely to be consciously seen.
2) Does the Sun pulsate, and does it change its brightness?
The Sun is not perfectly steady, but it does not pulsate like a lamp on the timescales and amplitudes you’re describing.
There are two relevant effects:
- Solar oscillations (helioseismology): The Sun has many acoustic modes with periods around ~5 minutes. These are real “pulsations” of the surface, but the associated brightness variations are tiny (parts per million to parts per thousand depending on what you measure and where). They are not perceptible by eye.
- Solar activity (sunspots, faculae, flares):
- Over the 11-year cycle, total solar irradiance varies by about ~0.1%.
- Solar flares can be dramatic in X-rays/UV, but the change in visible light at Earth is usually small and brief.
So: yes, the Sun varies, but not in a way that produces visible flicker or a “pulsing halo.”
3) Do all light sources have some oscillation, or can any be truly constant?
At a fundamental level, light emission is quantized (photons) and often produced by random microscopic events (spontaneous emission, thermal radiation). That implies:
- There is always some statistical “photon noise” (shot noise) if you look with sufficiently sensitive, fast detectors.
- Thermal sources also have microscopic fluctuations.
However, for everyday purposes many sources are effectively constant because the fluctuations are extremely small relative to the average when many photons are involved.
Whether you get macroscopic flicker depends mostly on the power supply / driving electronics (for LEDs, fluorescents) or on mechanical instabilities (some arc lamps, candles, etc.), not on an intrinsic “pulsation of light itself.”
Incandescent bulbs, for example, can have 100/120 Hz modulation from AC power, but the filament’s thermal inertia smooths it, so flicker is usually much less than in poorly driven LEDs.
4) Why you perceive pulsation/halos around bright lights
What you describe (“light radiating outward,” halo-like effects, sometimes seeming to pulse) is very often not the light source changing, but optics + physiology:
Optical causes (real light redistribution)
- Scattering in the eye: Light scatters in the cornea/lens/vitreous humor, producing glare and halos—worse with dry eyes, cataracts, or dirty/greasy surfaces.
- Diffraction: The pupil and eyelashes can create starbursts or rings.
- Tear film dynamics: A slightly uneven tear film can change from moment to moment (especially as you blink), making halos appear to shimmer or pulse.
- Glasses/contact lenses/windows: Smudges and micro-scratches strongly enhance halos.
Neural/perceptual causes (your visual system)
- Microsaccades (tiny involuntary eye movements) and adaptation can make a steady light seem to “shimmer.”
- Peripheral sensitivity to flicker: Even if you don’t see direct flicker, your peripheral vision may detect modulation.
- Afterimages/adaptation: Bright lights saturate photoreceptors; as they recover, you can perceive changing “aura” effects.
How to tell which it is
- Record it with a phone camera:
- If the camera shows banding/flicker, it’s likely real intensity modulation (driver/mains/PWM).
- If the camera looks steady but your eyes see halos/pulsation, it’s more likely eye optics/physiology.
- Try cleaning the bulb cover, any lampshade, and your glasses; blink deliberately; compare with another lamp.
Bottom line
- LEDs often flicker because of their drivers and AC/PWM modulation; it can be visible or subtly perceptible.
- The Sun varies, but not with visible flicker.
- No light source is perfectly constant at microscopic scales, but many are effectively constant macroscopically.
- The halo/pulsation you perceive is frequently dominated by scattering/diffraction in your eye and visual processing, sometimes combined with real LED flicker.
If you tell me the bulb type (dimmable/non-dimmable, any dimmer switch, country mains frequency) and whether your phone camera shows flicker bands, I can help narrow down the cause.
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Understanding Light Source Pulsation and Visual Perception
Great observation! Let's break down each aspect of light pulsation and perception:
1. LED and Artificial Light Flickering
Why LEDs flicker:
- Most LED bulbs are powered by AC (alternating current) and use drivers that convert AC to DC
- Cheap or poorly designed drivers can create flicker at 100-120 Hz (twice the AC frequency)
- PWM (Pulse Width Modulation) dimming rapidly switches LEDs on/off to control brightness
Visibility to humans:
- Most people don't consciously perceive flicker above 50-60 Hz
- However, peripheral vision can detect flicker up to 200 Hz
- Some individuals are more sensitive and experience headaches or eye strain
- You might notice it when moving your eyes quickly or seeing a light in peripheral vision
Other artificial lights:
- Incandescent bulbs: Minimal flicker (filament stays hot between AC cycles)
- Fluorescent lights: Noticeable flicker at 100-120 Hz, worse with magnetic ballasts
- Quality LEDs: Modern drivers with good capacitors produce minimal flicker
2. Does the Sun Pulsate?
Yes, but not in a way that affects our perception:
Solar oscillations:
- The Sun undergoes helioseismic oscillations with periods of ~5 minutes
- These are sound waves trapped inside the Sun, causing surface movements of a few km
- Brightness variations are incredibly tiny (~0.001% or less)
Solar activity cycles:
- The 11-year solar cycle causes ~0.1% brightness variation
- Completely imperceptible to human vision
- The Sun appears constant because these changes are far below our detection threshold
3. Are Any Light Sources Truly Constant?
Nearly constant sources:
- Incandescent bulbs (on DC power)
- Quality LED bulbs with proper drivers and capacitors
- Sunlight (for practical purposes)
- Laser pointers (battery-powered DC)
All sources have some variation:
- At quantum levels, photon emission is probabilistic and "noisy"
- Thermal sources fluctuate slightly with temperature variations
- However, these quantum and thermal fluctuations are far too small and fast for human perception
- For practical purposes, DC-powered incandescent lights are the closest to "perfectly constant"
4. Perception of Pulsation and Halos
This is where it gets interesting—most perceived pulsation is optical/neurological, not from the source:
Optical causes (your eyes):
- Diffraction: Light bending around iris edges creates radial patterns
- Lens imperfections: Astigmatism creates star-burst patterns around lights
- Pupil dynamics: Your pupil constantly adjusts, changing the amount of light entering
- Tear film: Uneven tear coating on your cornea refracts light differently
- Floaters: Cast shadows that move with eye movement
Neurological causes (your brain):
- Troxler fading: Staring at steady stimuli causes them to fade and reappear
- Microsaccades: Tiny involuntary eye movements make static images appear to shimmer
- Adaptation: Retinal cells adapt to brightness, creating perceived fluctuations
- Attention: When you focus on a light, increased neural activity can create perceived pulsation
Testing what you're seeing:
- Record with a phone camera - if pulsation appears in video, it's real flicker
- Look at reflection - if pulsation follows the reflection, it's the source
- Cover one eye - if it changes significantly, it's likely optical
- Look away then back - perception resets often indicate neurological processing
Most Likely Explanation for Your LED
Your observation is probably a combination of:
- Mild LED flicker (if it's a cheaper bulb without good driver circuitry)
- Optical diffraction creating a halo pattern around the bright source
- Your visual system's processing making you hyper-aware of the light when you focus on it
If the pulsation bothers you, try looking for LED bulbs labeled "flicker-free" or with specifications showing <5% flicker at all brightness levels.
Comments
1. Why do LEDs and artificial lights flicker or pulsate? Is this visible to the human eye?
LEDs and most artificial lights flicker due to their power supply:
- AC mains frequency: Household electricity (50/60 Hz) causes LEDs to turn on/off rapidly unless perfectly filtered. Cheap drivers produce 100–120 Hz ripple.
- Pulse-width modulation (PWM): Many LEDs dim via PWM (rapid on/off cycles, often 100–2000 Hz) to control brightness.
- Other causes: Faulty drivers, overheating, or poor capacitors.
Visibility:
- Human eye flicker fusion threshold is ~50–90 Hz for most; above 100 Hz, it's imperceptible to steady gaze but shows as strobing on cameras or fast motion.
- Sensitive people (e.g., with migraines) or low frequencies see it as pulsation. Your observation might blend flicker with visual effects (see Q4).
High-quality LEDs (DC-driven) minimize this.
2. Does the Sun actually pulsate, and if so, does this affect its light output in any perceptible way?
The Sun does pulsate, but imperceptibly:
- Solar oscillations: Acoustic p-modes (pressure waves) cause surface vibrations every ~5 minutes, varying brightness by ~0.001% (10 parts per million).
- Longer cycles: 11-year solar cycle modulates output by ~0.1%, plus tiny granulation (cellular convection).
These are detected by satellites (e.g., SOHO), not naked eye. No perceptible pulsation—Sunlight appears constant due to averaging over vast scale.
3. Do all light sources have some form of pulsation or oscillation, or are some truly constant?
No, not all pulsate macroscopically:
- Inherently pulsating:
Source Pulsation Cause Frequency/Notes Fluorescent/CFL AC ballast 100–120 Hz Cheap LEDs AC ripple/PWM 100 Hz–kHz Stars (many) Pulsation instability Hours–days - Truly constant (steady output):
- Incandescent/halogen bulbs: Thermal inertia of filament smooths AC to near-DC.
- Continuous-wave (CW) lasers: Stable output via steady pumping.
- High-end LEDs/strobes: DC power or high-frequency (>20 kHz) PWM.
- Ideal blackbodies (e.g., cavity radiators): Steady if input is constant.
Quantum mechanically, all light has photon shot noise (Poisson fluctuations), but it's negligible for bright sources.
4. What causes the visual perception of pulsation or halos around bright lights—is it the light source itself or something about how our eyes/brain process light?
Your "pulsating with light radiating outward" is likely perceptual, not the source:
- Source-independent causes (eye/brain):
- Diffraction/aberrations: Pupil acts as aperture; point sources (e.g., LED) produce rings/rays (Airy disk). Common at night.
- Glare/halos: Scattering in cornea/lens (worse with cataracts, dry eyes, dirty glasses). "Radiating outward" matches starbursts from spherical aberration.
- Phosphenes/neural effects: Bright light overstimulates retina, causing perceived flicker/pulsation (entoptic phenomena).
- Foveal processing: Brain fills gaps in peripheral vision.
- Source-related: If flickering (Q1), it amplifies perception, especially in low light.
- Test: View through pinhole (reduces aberrations) or phone camera (reveals true flicker).
Bottom line: Halos/pulsation around LEDs is usually optical artifacts in your eye, not bulb instability. Consult optometrist if persistent.
Comments
Light Pulsation: Source Physics vs. Human Perception
Great set of questions! The answer involves both the physics of light sources and the neuroscience of vision. Let me break each one down.
1. Why Do LEDs and Artificial Lights Flicker?
Most artificial lights flicker because of how they're powered:
- AC mains power oscillates at 50 or 60 Hz (depending on country). Incandescent bulbs dim slightly each time the current passes through zero, but their filament's thermal inertia smooths this out significantly.
- LEDs respond almost instantaneously to current changes, so they are more susceptible to flicker. Cheap LED drivers that don't properly smooth the rectified AC signal can produce flicker at 100 or 120 Hz (double the mains frequency, since brightness peaks on both the positive and negative half-cycles).
- Fluorescent lights also flicker at 100/120 Hz, though electronic ballasts in modern tubes push this to tens of kHz, well above perception.
Is it visible? Most people can't consciously perceive flicker above ~60–80 Hz when staring directly at a light. However:
- Flicker can become visible in peripheral vision (which has higher temporal sensitivity)
- It becomes obvious during saccades (rapid eye movements) — the light source appears to strobe or leave discrete trails
- Some people are more sensitive than others, and LED flicker has been linked to headaches and eye strain even when not consciously perceived
- A flicker index and percent flicker are used to quantify this; high-quality LED drivers aim for <5% flicker
2. Does the Sun Pulsate?
Yes, but not in any way you'd ever see:
- The Sun exhibits solar oscillations (studied through helioseismology). The dominant mode is the "5-minute oscillation" — pressure waves (p-modes) cause the solar surface to rise and fall with periods around 5 minutes.
- These oscillations cause velocity changes of ~1 m/s and brightness variations on the order of a few parts per million — completely imperceptible to the human eye.
- The Sun also varies in brightness over the 11-year solar cycle by about 0.1%, and solar flares cause localized brightening — again, not something you'd perceive by looking at overall brightness.
Bottom line: The Sun pulsates, but the effect on light output is astronomically (pun intended) small.
3. Are Any Light Sources Truly Constant?
In principle, no light source is perfectly constant, but some come extremely close:
| Source | Pulsation/Variation | Notes |
|---|---|---|
| Cheap LEDs | High (up to 100% flicker) | Poor driver design |
| Quality LEDs (DC-driven) | Extremely low | Essentially constant for practical purposes |
| Incandescent bulbs | ~10–15% modulation at 100/120 Hz | Thermal inertia helps |
| Lasers (stabilized) | Near-zero variation | Used as laboratory standards |
| Stars | Varies wildly | Some (Cepheid variables) pulsate dramatically; others are remarkably stable |
| Blackbody radiation | Thermal fluctuations at quantum level | Negligible macroscopically |
A well-regulated DC-powered LED or a stabilized laser is as close to "truly constant" as you'll find. At the quantum level, all light emission involves discrete photon events that are inherently stochastic (shot noise), but this is not meaningfully "pulsation" — it averages out over any macroscopic observation.
4. Why Do You See Pulsation and Halos? (The Most Interesting Part)
What you described — light radiating outward around the bulb — is almost certainly not the light source itself but your visual system. Several phenomena contribute:
Optical (in your eye)
- Lenticular halos: Light scattering in the crystalline lens and cornea creates radial streaks and glows around bright point sources. This increases with age as the lens becomes less transparent.
- Diffraction: Light diffracting around the edges of your pupil and through the fibrous structure of the lens creates star-like patterns.
- Tear film: A thin, uneven tear layer on your cornea acts as a variable diffraction grating, and blinking causes this to shift — potentially creating a perceived "pulsation" as the optical halo changes shape.
Neural (in your brain)
- Neural adaptation: When staring at a bright source, retinal cells adapt and recover in cycles, which can create a perceived oscillation in brightness.
- Microsaccades: Tiny involuntary eye movements shift the image across different photoreceptors, modulating the perceived halo.
The likely explanation for what you saw
Your LED probably does flicker at 100/120 Hz due to its driver, and the halo around it (from corneal/lenticular scattering) may make that flicker more visually salient — the "pulsating outward" appearance is the optical halo dynamically interacting with real flicker and neural adaptation.
TL;DR: Most artificial lights do flicker due to AC power, the Sun pulsates but imperceptibly, no source is perfectly constant at the quantum level, and the "radiating pulsation" you see is primarily your eye's optics scattering light into a halo that your brain interprets as pulsating outward.
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