Many people habitually point their phones at lights and take photos. If black bars or flickering appear in the image, they immediately conclude: “This light has flicker—it’s unhealthy.”

But the truth is: the flickering captured by your phone does not mean the light fixture actually emits harmful stroboscopic effects.

This is merely a phenomenon caused by the camera and lighting being “out of sync.”

In other words, while your phone can “see” the flickering, it cannot “measure” it.

The human eye and a camera perceive light in entirely different ways:

• The human eye possesses “persistence of vision,” acting as a natural “filter.” As long as the flicker frequency of light exceeds approximately 80Hz, our brain automatically “smooths” it out, making it appear as steady illumination.
• Cameras, however, are far more literal. They function as sampling instruments, faithfully recording every instantaneous fluctuation in brightness from a light source.

So when the light flickers at frequencies of 100Hz, 120Hz, or even thousands of Hz:

• Human eyes → Ignore it.
• Smartphones → Capture it exactly as it is.

In reality, most lighting is not “completely stable”:

1.AC Power Supply

• Under a 50Hz grid, lighting exhibits 100Hz brightness fluctuations;
• under a 60Hz grid, these fluctuations reach 120Hz.

2.LED Driver Power Supply

• Some driver circuits lack stability, causing output current “ripple” that results in noticeable brightness fluctuations.

3.PWM Dimming

• Many dimmable LEDs use a “rapid on-off” method to control brightness.
• While imperceptible to the human eye, cameras can capture the “off” moment, resulting in visible stripes in the image.

↑Image: A smartphone screen using PWM dimming, which may exhibit banding when photographed.

So, the lights do indeed flicker, only we don’t usually see it.

The key here is the camera’s “sampling method”:

• Shutter Speed: If the shutter only covers a small portion of the light wave, it will cause uneven brightness in the image.
• Frame Rate (FPS): When the frame rate is out of sync with the frequency of light, rolling black bars appear.
• Rolling shutter: Most smartphone cameras use progressive scanning. Since light sources vary in brightness during exposure of different lines → stripes appear.

This also explains why strobing is particularly noticeable in slow-motion videos. High frame rates “amplify” each fluctuation in light.

Many people mistakenly believe that “stripes captured on a phone = faulty lighting.” This is actually a misconception for three reasons:

1. No Standard

• Scientific flicker detection employs a comprehensive set of metrics, such as flicker percentage, flicker index, SVM, and PstLM.
• The stripes visible in phone photos do not correspond to these metrics.

2. Results are inconsistent.

• The same light source produces completely different effects when photographed with different phones, shutter speeds, and frame rates.
• Without a unified standard, results cannot be compared.

3. Misinterpretation is common

• High-frequency PWM dimming: Safe for the human eye, but smartphones will inevitably capture flickering.
• Low-frequency fluctuations: May contribute to eye fatigue, but smartphones may not detect them.

Therefore, flickering captured by smartphones is merely a “phenomenon” and cannot serve as evidence to determine whether a light fixture is healthy.

The truly scientific approach involves utilising professional instruments such as stroboscopes or spectrometers. These devices can quantify light fluctuations and calculate metrics including Flicker Percent, Flicker Index, and Standard Voltage Deviation (SVM).

↑Image: Licht Ultra spectrophotometer for testing stroboscopic effects

These are the parameters recognised by international standards such as CIE, IEC, and IEEE, and can be used to compare the merits of different luminaires.

One might understand it thus:

• Mobile phone stroboscopic effect detection = a subjective observation, enabling you to ‘suspect’ a light source may be faulty;
• Professional stroboscopic testing = objective measurement, capable of truly ‘verifying’ whether a light source is functioning correctly.

• Lighting Industry: An increasing number of luminaires now bear the “flicker-free” designation, with high-quality driver power supplies becoming increasingly commonplace. This not only enhances suitability for photography but also alleviates visual fatigue.
• Camera Industry: Once global shutter technology becomes widespread, the banding issues caused by rolling shutter will be fundamentally eliminated.

• Many people like to use their mobile phones as “detectors”, but the reality is:

  Capturing stripes ≠ the light is necessarily harmful;

  Not capturing stripes ≠ the light is necessarily safe.

  Only professional instruments and standardised testing metrics can truly tell the story.

  So next time someone pulls out their phone to photograph a light and claims “this light has flicker”, you can confidently tell them:

  ‘Mobile phones cannot detect flicker.’

  Note: When testing flicker with a portable spectrophotometer, attention should be paid to the test direction. For comparing luminaire flicker, horizontal testing may be employed, whereas vertical testing is required for assessing flicker from the human eye’s perspective. Values obtained from these two approaches often differ significantly. This discrepancy arises because the vertical plane receives both direct and reflected light from multiple luminaires at wide angles. Furthermore, temporal light modulation (TLM) variations from different drivers are more readily superimposed in this direction, potentially generating beat effects. Consequently, flicker perception is typically more pronounced vertically than horizontally. Spatial lighting design should therefore prioritise addressing flicker perception from the vertical plane.