Strobe: PWM Dimming, DC Dimming, and the Actual Impact of High-Frequency Dimming on Eyes

Strobe, or screen flicker, refers to the periodic fluctuation of screen brightness, which is closely related to the dimming technology of display devices. PWM dimming and DC dimming are the two most common dimming methods in current electronic products (such as smartphones, monitors, and laptops), and high-frequency dimming, as an optimized form of PWM dimming, has become a key focus of eye protection technology. This article will systematically explain the principles of the two dimming methods, focus on the actual impact of high-frequency dimming on eye health, and help you understand the "invisible flicker" around you.

1. What is Strobe (Screen Flicker)?

Strobe is the invisible or visible periodic bright-dark alternation of the display screen, which is essentially a side effect of the dimming process—display devices need to adjust brightness to adapt to different ambient light, and the way of adjustment directly determines whether strobe occurs and its intensity.

The human eye’s perception of strobe is related to the Critical Flicker Fusion Frequency (CFF): under normal visual conditions, most people can no longer perceive flicker when the frequency exceeds 80Hz, but in peripheral vision, dynamic gaze or high-contrast backgrounds, the sensitivity can extend to more than 200Hz superscript:3 (Data source: LED Light PWM Dimming Frequency Selection: Core Points Analysis). More importantly, even if the flicker is invisible to the naked eye, long-term exposure may still cause physiological reactions in the eyes, which is the core reason why strobe is closely related to eye health.

2. Two Core Dimming Methods: PWM Dimming vs DC Dimming

The fundamental difference between PWM dimming and DC dimming lies in the way of adjusting brightness—one achieves brightness control through "fast switching", and the other through "current adjustment", which also leads to differences in their strobe performance and impact on eyes.

2.1 PWM Dimming (Pulse Width Modulation)

Core Principle

PWM dimming works like "quickly turning a light on and off": the display screen does not actually reduce the brightness of the light source, but controls the brightness by adjusting the "duty cycle" (the ratio of on-time to off-time in a single cycle)superscript:3 (Data source: LED Light PWM Dimming Frequency Selection: Core Points Analysis). For example, if the screen is on for 5 milliseconds and off for 5 milliseconds in a 10-millisecond cycle, the perceived brightness is 50%; if it is on for 1 millisecond and off for 9 milliseconds, the brightness is 10%. The faster the switching frequency, the harder it is for the human eye to perceive the flicker, and it will mistakenly think that the screen is always on.

PWM dimming is widely used in OLED screens because each pixel of OLED emits light independently. PWM can accurately control the on-off time of each pixel, balancing brightness and color performance without causing uneven light emission superscript:3 (Data source: LED Light PWM Dimming Frequency Selection: Core Points Analysis). It is also used in some LCD screens, especially in low-brightness scenarios.

Frequency Classification & Characteristics

PWM dimming is usually divided into low-frequency and high-frequency according to the switching frequency, and their impact on the eyes varies significantly. IEEE Std 1789-2015 clearly divides the risk levels of PWM dimming frequency, and relevant experimental data also shows the different impacts of different frequencies superscript:3 (Data source: LED Light PWM Dimming Frequency Selection: Core Points Analysis):

• Low-frequency PWM dimming (＜1000Hz): The flicker is relatively obvious. According to IEEE Std 1789-2015, frequencies lower than 1250Hz are in the high-risk area. A study shows that the incidence of headaches among employees working under 100Hz PWM is 47% higher than that of the control group using DC drive superscript:3 (Data source: LED Light PWM Dimming Frequency Selection: Core Points Analysis). Long-term use may cause eye fatigue, dryness, headaches, and even induce discomfort in people with photosensitivity (such as epilepsy patients). Early mobile phones and low-end displays often used 480Hz or lower frequency PWM dimming, which was criticized for poor eye protection.

• High-frequency PWM dimming (≥1000Hz): It is an optimized form of PWM dimming. At present, high-end devices generally use 1920Hz, 2160Hz or even higher frequencies. The flicker is almost invisible to the naked eye, and the impact on the eyes is significantly reduced. According to IEEE Std 1789-2015, frequencies between 1250Hz and 3125Hz are the low-risk condition area (requiring modulation depth ＜8%), and frequencies higher than 3125Hz are the low-risk area, which is recommended for high-end scenarios such as long-term office and professional creation superscript:3 (Data source: LED Light PWM Dimming Frequency Selection: Core Points Analysis).

Advantages & Disadvantages

• Advantages: Wide dimming range, which can achieve precise brightness adjustment from 0% to 100%, especially suitable for low-brightness scenarios such as night use (the dimming depth can reach 0.1% superscript:3, Data source: LED Light PWM Dimming Frequency Selection: Core Points Analysis); good color stability, the color temperature and color gamut of the screen will not change with brightness adjustment, which is crucial for color-sensitive work such as design and photography.

• Disadvantages: Strobe is inherent (even high-frequency PWM still has subtle flicker); high-frequency PWM requires high-cost driving chips, which will increase the hardware cost of the device; when the frequency is in the range of 20Hz-20kHz (audible frequency band), it may produce slight "squeaking" noise due to the vibration of magnetic components. For example, a 100W street lamp driver with 1.5kHz PWM has a measured noise of 32 dB(A), which is obviously audible at night; after adjusting the frequency to 25kHz, the noise drops to 17 dB(A), which is completely integrated into the background superscript:3 (Data source: LED Light PWM Dimming Frequency Selection: Core Points Analysis).

2.2 DC Dimming (Direct Current Dimming)

Core Principle

DC dimming is like "twisting a brightness knob": it directly adjusts the current or voltage of the light source (such as LED backlight of LCD screen) to change the brightness. The brightness of LED lights is inherently proportional to the current—the larger the current, the brighter the light, and vice versa. During the dimming process, the light source keeps emitting light stably without on-off actions, so there is no strobe in theory. Compared with PWM dimming, DC dimming (also known as analog dimming CCDR) has poor color stability, and the light will turn yellow at low current superscript:3 (Data source: LED Light PWM Dimming Frequency Selection: Core Points Analysis).

DC dimming is more common in LCD screens, because the backlight of LCD emits light uniformly, and current adjustment is more stable; it is rarely used in OLED screens, because low current in OLED pixels will cause uneven light emission (such as partial pixels turning gray).

Advantages & Disadvantages

• Advantages: No strobe in theory, which is more friendly to people who are extremely sensitive to strobe (such as people who are prone to eye dryness and headaches); simple structure and low hardware cost, which is widely used in mid-to-low-end devices; according to human factors experiments, DC dimming has less interference on the eye’s optical system, and the fluctuation of visual imaging quality is weaker than PWM dimming.

• Disadvantages: Narrow dimming range, which cannot achieve extremely low brightness (it will be cut off when the current is too low, generally less than 10% of the maximum brightness superscript:3, Data source:LED Light PWM Dimming Frequency Selection: Core Points Analysis); color distortion is easy to occur at low brightness—LED lights have reduced luminous efficiency at low current, which may lead to screen yellowing or uneven color.

2.3 Direct Comparison of Two Dimming Methods (with Specific Data)

3. Actual Impact of High-Frequency Dimming on Eyes

High-frequency PWM dimming is widely promoted as an "eye-protecting technology" by major manufacturers, but it does not mean that it is completely harmless to the eyes. Its impact is mainly reflected in subtle physiological reactions, which are related to frequency, usage time, and individual differences. The specific impacts are as follows (with experimental data):

3.1 Positive Impact: Significantly Reducing Visible Strobe and Eye Fatigue

Compared with low-frequency PWM dimming, high-frequency dimming (≥1920Hz) has the most obvious advantage in reducing visible strobe. Since the flicker frequency exceeds the critical flicker fusion frequency of the human eye (＞80Hz), the eyes will not perceive the bright-dark alternation, thus avoiding eye fatigue caused by the continuous adjustment of the ciliary muscle to adapt to flicker. For people who use electronic devices for a long time (such as office workers and students), high-frequency dimming can effectively reduce symptoms such as eye dryness, soreness, and headaches, and improve the comfort of long-term use. At the same time, it retains the color stability of PWM dimming, which can meet the needs of color-sensitive work while protecting the eyes.

3.2 Potential Negative Impact: Subtle Physiological Interference

Although high-frequency dimming cannot be perceived by the naked eye, human factors experiments have shown that it still has subtle impacts on the eye’s visual function, mainly reflected in the following aspects:

• Impact on visual imaging quality: High-frequency PWM dimming may affect the wavefront aberration distribution of the eyeball (such as increasing spherical aberration and coma), leading to a slight decrease in retinal imaging quality. Experiments show that the change of modulation transfer function (MTF) of the PWM dimming group is significantly higher than that of the DC dimming group, indicating that the fluctuation of visual resolution is more obvious.

• Cumulative fatigue effect: Although the difference in ciliary muscle adjustment load between high-frequency PWM dimming and DC dimming is not statistically significant, long-term exposure to high-frequency brightness oscillation may still cause cumulative fatigue of the retinal nerve. For people with sensitive eyes, this fatigue may be more obvious—especially for those who use electronic devices for more than 6 hours a day, the cumulative effect is more prominent.

• Risk for sensitive groups: For people with photosensitivity (such as epilepsy patients) or people with eye diseases (such as glaucoma), even high-frequency dimming may induce discomfort, because their eyes are more sensitive to light fluctuations than ordinary people. According to relevant statistics, about 3% of photosensitive epilepsy patients may have seizures induced by high-frequency PWM dimming (＜3125Hz) superscript:3 (Data source: LED Light PWM Dimming Frequency Selection: Core Points Analysis).

3.3 Key Misunderstanding: "Invisible = Harmless"

Many people think that as long as the flicker of high-frequency dimming is invisible to the naked eye, it is completely harmless to the eyes. This is a common misunderstanding superscript:3 (Data source: LED Light PWM Dimming Frequency Selection: Core Points Analysis). IEEE Std 1789-2015 clearly points out that even if the human eye cannot perceive it, low-frequency PWM (＜1250Hz) may cause nerve stimulation and visual fatigue, and high-frequency PWM also has potential risks if the modulation depth (brightness fluctuation amplitude) is too large (＞8%).

The harm of high-frequency dimming is cumulative and individual: for ordinary people, using high-frequency dimming devices for 2-3 hours a day will not cause obvious eye damage; but for people who use electronic devices for a long time (more than 6 hours a day), or people with sensitive eyes, the cumulative effect of long-term exposure may still cause eye discomfort.

4. How to Choose the Right Dimming Device?

The choice of dimming method should be combined with personal usage scenarios, eye sensitivity and device performance. The following suggestions can be referred to (with scenario-specific frequency recommendations superscript:3, Data source: LED Light PWM Dimming Frequency Selection: Core Points Analysis):

• People with sensitive eyes/long-term users: Priority is given to DC dimming devices (such as LCD monitors/laptops) or high-frequency PWM dimming devices with frequency ≥1920Hz (preferably ≥3125Hz, low-risk area), which can minimize the impact of strobe on the eyes.

• Professional creative work (design/photography/video editing): Choose high-frequency PWM dimming devices (≥1920Hz), which can ensure color stability while protecting the eyes, and avoid color distortion affecting work; for photography and video lighting, choose ≥20kHz PWM dimming to avoid rolling stripes caused by camera shutter synchronization.

• Night use/ low-brightness scenarios: Choose high-frequency PWM dimming devices with wide dimming range (dimming depth ≥0.1%), which can achieve precise low brightness without obvious flicker; avoid using low-frequency PWM dimming devices (＜1000Hz), which are more likely to cause eye fatigue at night.

• Budget-limited users: DC dimming devices are more cost-effective, which can meet daily use needs and have no obvious strobe, which is suitable for ordinary office and entertainment scenarios; if choosing PWM dimming, ensure the frequency ≥1250Hz (minimum safety line).

5. FAQ

Is high-frequency PWM dimming completely harmless to the eyes?

No, it is not completely harmless. High-frequency PWM dimming (≥1000Hz) is indeed much safer than low-frequency PWM dimming, and its flicker is almost invisible to the naked eye, which can significantly reduce eye fatigue. However, it still has inherent subtle strobe, which may cause cumulative retinal nerve fatigue after long-term exposure, and may also slightly affect visual imaging quality. For sensitive groups such as photosensitive epilepsy patients, even high-frequency PWM dimming (＜3125Hz) may induce discomfort. Therefore, high-frequency dimming is "less harmful" rather than "completely harmless" superscript:3 (Data source: LED Light PWM Dimming Frequency Selection: Core Points Analysis).

Why do some high-frequency PWM dimming devices still cause eye fatigue?

There are three main reasons: First, the frequency does not meet the low-risk standard—some devices claim to be "high-frequency", but the actual frequency is between 1000Hz and 1250Hz, which is still in the high-risk area specified by IEEE Std 1789-2015. Second, the modulation depth is too large (＞8%)—even if the frequency is higher than 1250Hz, excessive brightness fluctuation will still cause physiological discomfort. Third, individual differences—people with sensitive eyes are more sensitive to high-frequency light oscillation, and long-term use (more than 6 hours a day) will also amplify the cumulative fatigue effect. In addition, factors such as screen brightness, ambient light, and usage posture will also affect the degree of eye fatigue. For example, using a high-frequency dimming screen in a dark environment with excessive brightness will still cause eye strain, because the contrast between the screen and the environment is too large, which will increase the burden on the eyes to adjust.

Which is better, PWM dimming or DC dimming?

There is no absolute "better" or "worse", only "more suitable". If you pay more attention to color stability and low-brightness experience (such as night use), high-frequency PWM dimming (≥1920Hz) is more suitable; if you are sensitive to strobe and pursue eye comfort, DC dimming is a better choice. For most ordinary users, high-frequency PWM dimming devices (≥1920Hz) can balance eye protection, color performance and usage experience, which is a more comprehensive choice.

How to detect whether the device uses high-frequency PWM dimming?

There are two simple detection methods: First, use the "slow-motion shooting" function of the mobile phone—aim the camera at the screen, and if there are obvious black stripes scrolling, it means it is low-frequency PWM dimming; if there is no obvious stripe or only very faint stripes, it is high-frequency PWM dimming. Second, check the device parameters—regular manufacturers will clearly mark the PWM dimming frequency in the product specification sheet (such as 1920Hz PWM dimming). It should be noted that some low-end devices may falsely mark the frequency, so it is recommended to refer to professional testing data.

6. Conclusion

High-frequency PWM dimming is an optimized technology based on traditional PWM dimming, which effectively solves the problem of obvious flicker of low-frequency PWM and balances color stability and dimming range. However, it is not a "panacea" for eye protection—it still has subtle strobe and potential cumulative impact, especially for sensitive groups and long-term users. DC dimming, as a complementary technology, has the advantage of no strobe, but is limited by narrow dimming range and poor color stability.

When choosing electronic devices, we should not only pay attention to whether they use high-frequency dimming, but also combine our own usage scenarios and eye sensitivity to make a reasonable choice. At the same time, no matter what kind of dimming device we use, we should pay attention to controlling the usage time, keeping a proper distance from the screen, and ensuring adequate ambient light—these habits are the fundamental way to protect our eyes.