for a long time, the material of mobile phone display screen has been divided into LCD and OLED. In the past two years, the proportion of OLED screen mobile phones has increased, especially in the high-end market. As mobile phone screens, OLED and LCD have their own advantages, and they adopt different technical means in many links, such as PWM and DC.

DC dimming is a way to adjust the brightness

In order for users to watch the content on the mobile phone normally under different lighting conditions, the brightness of the screen needs to be changed accordingly. One way to adjust brightness is called "DC dimming".

the principle of p>DC dimming is very simple, which is to change the brightness of the screen by increasing or decreasing the circuit power. Power = voltage x current, so changing voltage or current can change screen brightness.

PWM dimming is another way to adjust the brightness

On the PWM dimming screen, the brightness adjustment depends on the alternation of the screen's light and off instead of changing the power. When the PWM dimming screen is lit, it doesn't emit light continuously, but it is constantly lighting up and extinguishing the screen. When the turn-on and turn-off are fast enough, the naked eye will think that the mobile phone is always on.

in the process of turning on and off the screen, the longer the off-screen state lasts, the lower the brightness the screen gives the naked eye. The longer the lighting time, the shorter the screen off time and the brighter the screen.

the lower the speed of alternating light and off, the greater the possibility of adverse effects on human eyes. But this is not absolute, because everyone has different sensitivity to "flicker". For example, if you look at the same PWM screen, some people will feel tired if they are fine. If you belong to those people with very sensitive eyes, you may need to use high-frequency PWM to dim your mobile phone, or even DC to dim your mobile phone.

PWM is also divided into high frequency and low frequency

Many people think that the lower the brightness, the lower the frequency of PWM dimming, which is not good for glasses, which is wrong. Because for PWM screen, the only factor that affects brightness is "the proportion of time between bright screen and off screen in a flashing period". No matter how fast or slow the flashing frequency is, as long as the proportion of on-off time in a flashing period is kept unchanged, the screen brightness will remain unchanged.

for example, a PWM screen with a low flicker frequency can achieve a brightness of 51 nits (brightness measurement unit), and a PWM screen with a high flicker frequency can also achieve it. At this time, high-frequency PWM is healthier for human eyes, and only a few people will feel visual discomfort because of the flicker of high-frequency PWM. These people need DC dimming screen.

Whether it's harmful to eyes doesn't depend on the dimming method

China cuisine pays attention to color, flavor and taste, and no single item can determine the quality of dishes, and so can the screen. Dimming mode is only one aspect that determines the degree of eye protection of the screen.

If you look at the evaluation criteria of the authoritative screen evaluation agency DISPLAY Mate, you will find that they have many indicators to judge the quality of the mobile phone screen, such as screen reflection, brightness, contrast, color, color accuracy and viewing angle. This includes many indicators that can affect the comfort of the eyes.

LCD screens may also be PWM dimming

It should be noted that not all LCD screens use 111％ DC dimming. Some LCD screens will turn on PWM dimming mode when the brightness is lower than a certain value. However, basically all OLED mobile phones in recent years have adopted PWM dimming, because DC dimming will make the OLED screen appear abnormal in color, which is more unfavorable for human eyes to watch.

OLED and LCD have their own advantages

Aside from dimming mode, OLED screen has many advantages, and the favor of high-end models for OLED in recent years is not groundless. Generally, OLED screens have better color accuracy and image contrast accuracy. Due to the luminous principle, the OELD screen can light and control each pixel separately.

therefore, the color uniformity of OLED screen is better, and the most extreme counterexample is that the edge of LCD screen leaks light when displaying pure black. Controlling pixels separately can also make OLED turn off pixels that don't need to emit light, which saves more power when displaying most content. But when displaying pure white, LCD saves more electricity. Choosing LCD or OLED is a decision that needs all-round consideration, and can't be decided only by "PWM dimming or DC dimming".

how to simply judge whether the mobile phone is PWM or DC dimming

in order to judge what dimming mode the mobile phone is, we need another mobile phone. Turn on the camera on another mobile phone, enter the mode that can control the shutter speed (usually professional mode), point the camera at the screen of the mobile phone to be tested and light the screen. Adjust the shutter speed to a very fast level.

at this time, aim at a mobile phone, and the dark bar scrolling on the screen of the mobile phone indicates that the mobile phone is a PWM dimming screen, and all OLED have dark bars.

OLED screen: PWM dimming

For LCD screen with 111％ DC dimming, there is no scrolling dark bar at any brightness. On some LCD screens, when the brightness is below a certain level, dark bars will appear, because PWM dimming mode is turned on at this time.

LCD screen with p>DC dimming

This method can only distinguish whether the mobile phone screen is PWM dimming, but can't judge whether the screen is harmful to eyes. A screen with stripes is not necessarily unhealthy, and a screen without stripes is not necessarily healthy.

finally, although the PWM frequency will affect the thickness and number of stripes in the camera, you should never judge whether PWM is high-frequency or low-frequency. Because the focal length and shooting angle of the camera (shooting horizontally, vertically or obliquely) can affect the thickness and number of stripes.

eye protection needs to start from one's own usage habits

Most of the discomfort caused by watching mobile phones is not the pot of dimming, but often the screen color is too bright, the blue light level is too unhealthy, the screen brightness is too bright/low, the glasses are too close to the mobile phone or the mobile phone is watched for too long. If the habit of using the mobile phone is unhealthy, the eye-protecting screen will not save you.

after getting into good habits, you can choose the screen according to the sensitivity of your eyes and other aspects (color, contrast, etc.). Finally, keep in mind that a screen that is not suitable for you is not necessarily unhealthy, but it may be comfortable for others to use.