The Troubles Without OLED Screens: How LCD Stays Strong?

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Compared to LCD screens, OLED screens have higher color gamut and contrast, displaying more vibrant colors. The self-illuminating advantage allows it to show pure black without light leakage, resulting in extremely low power consumption on dark backgrounds, thus successfully conquering more and more mid-to-high-end smartphones.

However, OLED screens also have many inherent flaws, such as susceptibility to burn-in, higher power consumption when displaying pure white backgrounds, and prolonged exposure in dim environments can easily cause eye dryness and migraines.

During the National Eye Care Day (June 6), the well-known lens ranking website DxOMark released information regarding screen dimming technology and flickering issues. In professional laboratory measurements, the PWM frequency of the Xiaomi 10T Pro (Redmi K30S Supreme Memorial Edition) equipped with an LCD screen reached 2360Hz, while the Samsung S20U with an OLED screen only reached 241Hz, and the Huawei P40 Pro was just 362Hz.

The lower the PWM frequency, the greater the impact on the eyes. DxOMark stated that even those who are unconsciously aware of flickering may react to it, resulting in headaches or eye fatigue after excessive screen use.

PWM duty cycles at 25%, 50%, 75%, and 100% perceived brightness levels

In other words, users sensitive to screen flicker are advised to purchase smartphones with LCD screens rather than OLED screens. The high-frequency PWM dimming characteristics of LCD screens are also one of the reasons why LCDs are never subservient.

So, what is PWM dimming?

The Two Dimming Methods of Screens

The brightness of mobile phone screens is a variable that adjusts up and down based on the intensity of current environmental light collected by light sensors (automatic brightness mode). Of course, it also supports manual adjustment by dragging the brightness bar. Currently, the commonly used dimming methods in smartphones can be divided into two schemes: “DC dimming” and “PWM dimming”.

DC dimming is based on the principle of “power = voltage × current”, which changes the brightness of the mobile phone screen by increasing or decreasing the power (adjusted by current or voltage). Its advantage is that it does not flicker, while its disadvantage is uneven color at low brightness.

The PWM (Pulse Width Modulation) dimming scheme does not rely on changing power but instead alternates between “on and off” at a certain frequency in a very short time, adjusting the time ratio of “on” and “off” to achieve brightness adjustment from 0% to 100%. During this process, the screen does not actually darken or brighten. For example, by extending the duration of the off state (shortening the on time), the phenomenon of visual persistence in human eyes can create the illusion that the screen appears dimmer.

In the PWM dimming process, if the screen flashes 500 times per second, then the PWM flickering frequency is 500Hz.

The advantages of PWM dimming include simple structure, no color deviation issues, energy-saving, and low heat generation. However, its disadvantages are obvious: it exhibits flickering (screen flicker) phenomena, and the lower the dimming frequency, the more pronounced the flicker.

It is important to note that individual sensitivity to flickering varies. If the flickering frequency of a mobile phone screen remains below your body’s tolerance line, prolonged staring can easily lead to eye strain and headaches.

Low-frequency PWM is the most harmful to the eyes

In the mobile phone field, whether LCD or OLED, a mixed scheme of DC and PWM dimming is often adopted—DC dimming is used at high brightness, while PWM dimming is switched to at low brightness.

The problem is that, thanks to the characteristics of the screen, even when switching to PWM dimming, LCDs still belong to high-frequency PWM dimming, with flickering frequencies generally above 2000Hz, thus having limited impact on the eyes and cerebral cortex.

PWM dimming mode, flickering issues are very obvious

When OLED screens switch to PWM dimming, their flickering frequency is generally only between 100Hz and 250Hz. Due to the slow flickering cycle frequency, sensitive users may trigger health issues after prolonged exposure to the screen.

To solve this problem, many smartphones with OLED screens offer a “DC dimming at low brightness” setting option, forcing the screen to still use DC dimming at low brightness to eliminate flickering issues. However, this results in partial color deviation, sacrificing image quality for health.

Flickering phenomenon significantly reduced after enabling DC dimming

In summary, users of smartphones with OLED screens should avoid using their phones in dim environments to prevent triggering low-frequency PWM dimming due to low brightness. On the other hand, rather than relying on the global DC dimming feature for eye protection, it is better to turn on a bedside lamp.

For health, do not play with your phone in dim environments

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