Recently, the problem of green screen on Apple mobile phone has been widely concerned. Many people have encountered the problem of green screen on mobile phone. How did this happen? How does the screen work? When will the green screen appear? Is there any way to avoid this?

What’s going on with the green screen?

This green screen incident was initially discovered by some users: when the screen brightness of iPhone 12 is set to the highest, when some dark gray pictures are displayed, that is, when it is very close to black, the edge of the screen will appear green.

In fact, this phenomenon is a very common problem in OLED screens, and Huawei’s mate20pro has a similar situation before. When the user turns the brightness down and the background is gray, the screen will turn green.

Why do OLED screens have such problems? First of all, let’s give a brief introduction to our leading role today – OLED screen.

OLED screen

OLED (organic light emitting diode) – Electromechanical laser display, is a special design of self light emitting diode, power will be bright, different voltage, brightness is different.

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Each pixel of OLED is composed of three subpixels of red, green and blue, which can be understood as a small color light bulb that can be controlled independently. The red, green and blue colors are mixed in different proportions to form the colorful videos and pictures we see.

It is because there is no LCD layer and backlight layer, so OLED screen can be made very thin, and even can be made into a transparent display full of the sense of future technology!

But the current OLED technology is not perfect, there are still some problems such as mura (refers to the phenomenon of uneven gray scale display), screen burning and so on. Green screen is a typical problem.

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Why LED screen turns green

The screen turns green, which is called mura in the LCD industry, which means the display is uneven.

In the production process of OLED screen, the light-emitting raw materials are covered on the display panel by evaporation. However, the current evaporation process itself is difficult to evenly arrange each independent small bulb on the display panel, which leads to the difference in the number of small bulbs covered in different areas.

Moreover, each small bulb itself is composed of three sub-pixel, and the voltage control of these three sub-pixel is difficult to achieve complete consistency. This leads to the mura phenomenon in the low brightness gray screen.

Why is mura more likely to appear when the screen is gray? This needs to introduce a concept, namely RGB color.

RGB refers to the three colors of red, green and blue. We have learned that OLED is composed of these three colors. By controlling the brightness of the three colors through voltage, different colors can be formed.

We use a number in the range of 0-255 to represent the brightness of the three colors. 255 means the brightest, and 0 means no brightness.

We adjust the brightness of the screen on the mobile phone, which is actually to adjust the gray level. You can take out your mobile phone to try, observe a pure white picture, and then slowly adjust the brightness from the highest to the lowest. It is obvious that this is a process from white to gray.

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When we turn down the brightness, we actually turn down the voltage of three sub pixels at the same time. To call out different colors is to adjust the voltage of three sub pixels to the appropriate size in proportion.

If different colors are displayed in low brightness, the values of the three will be reduced accordingly, and the reduction range involves very complicated calculation. When the brightness is set to very low, the three values are very close, and finally they will appear gray.

As mentioned earlier, it is difficult to control the voltage (i.e. brightness) of the three sub pixels of OLED to be exactly the same. When one of the sub pixels is too high, for example, the green screen will appear. This is why the mura phenomenon is easy to appear on the screen when it is dark gray.

In fact, it’s not just green. Oppo’s findx has been pointed out that the screen turns red. The principle is similar to that of green screen, that is to control the voltage instability of red sub-pixel.

Solution

Almost all OLED display manufacturers, whether LG or Samsung, are unable to solve the mura problem from the production process of OLED. So engineers began to study the technology of going to mura.

For example, use a two-dimensional imager to take a picture of the OLED display, take a distribution map according to the actual display situation, and then feed back the data to the screen driver in the reverse direction, so as to dim the over bright places.

In this way, the uneven display of OLED screen with high brightness can be greatly alleviated.

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But the above method is generally suitable for high brightness, for low brightness when the green screen, smart engineers thought of another wonderful solution.

Suppose that our screen at 1nit (brightness unit, mobile phone screen peak brightness is generally 1000nit), there will be no mura phenomenon, but at 0.1nit, there will be mura phenomenon.

So we can make the screen work at 1nit, but only 10% of the time.

At this time, because the vision of the eye remains temporarily (when the object is in rapid motion, when the image seen by the human eye disappears, the human eye can still retain its image for about 0.1-0.4 seconds), the 10% 1nit in our eyes is 0.1nit.

This technology is called PWM (pulse width modulation), but it is not a very good technology, because it essentially achieves low brightness by flickering. If it is used for a long time, some people’s eyes will not be able to stand it.

The reason why the iPhone 12 has a green screen is that we set the brightness of the screen very high. At this time, PWM will stop working, but we still let it display dark gray, at this time, mura phenomenon will appear.

So, why is this rarely the case with OLEDs in the Android camp?

Since the last mate20pro rollover, Huawei has forced those gray areas that are most prone to mura to lower to darker gray when facing the mura problem in OLED with low brightness. At this time, it is difficult for the human eye to distinguish the tiny difference.

Before the improvement of OLED screen manufacturing process, mura phenomenon will always be accompanied by OLED screen. We can only avoid it by some technologies to remove mura, or let users perceive it as little as possible.

If there is a serious mura phenomenon when using your mobile phone, contact the after-sales service as soon as possible to solve the problem.

Editor in charge ajx

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