The secret of LCD and OLED screens for mobile phone
A small mobile phone has a lot of meaning.
This communication tool, which was once used only to make phone calls, is now omnipotent and has become the first window for people to connect the whole world.
On the other hand, the mobile phone screen, which undertakes the display function of the mobile phone and the entrance of touch operation, is the “window of the window” and plays a very important role.
Here we will talk to you about the secrets behind the mobile phone screen.
After reading this article, you can get the answers to the following questions:
1. What are the shapes of the mobile phone screen?
2. How to measure the size of the mobile phone screen?
3. What do the concepts of resolution and PPI, refresh rate refer to?
4. What are the types of mobile phone screens, and what is the technical principle behind them?
I. The shape of the mobile phone screen
Since smartphones dominated the mainstream market, mobile phones, as entertainment and information centers for fragment time, need a larger display area, so the screen is getting bigger and bigger; at the same time, traditional mobile phones have a “forehead” for handsets and front cameras, and a “chin” for Home keys, making it too large for the phone to increase its screen share.
Later, Apple came up with a way to centralize a series of devices on the phone’s forehead and extend the screen on both sides, resembling “flush bangs”, so this kind of display is called “bangs”.
Pearl screen: it is actually a water drip screen, but Huawei thinks the part occupied by the camera is more like a full and round pearl, so it is called “pearl screen” in order to show its extraordinary temperament.
Digging screen( Infinity-O Display): as can be seen from the previous forms, the processing of the front camera determines the shape of the screen.
If you dig a hole in the screen and insert the camera on it, it will become a “hole digging screen”. Samsung calls it Infinity-O Display
Waterfall screens(curved screens): bangs screens, drip screens, and digging screens are trying to minimize camera areas on top of mobile phones but there are still borders on both sides of mobile phones, visual effects are not good enough.
Then someone thought that we could make a curved surface around both sides of the left and right sides and bend the screen down, then we will not see the border on your front side.
These screens are like waterfalls on both sides of the screen so were called “waterfall screens”. Also called curved screens.
Fullscreen: the front side of the mobile phone is a complete screen without bangs nor drops or pearls without digging holes.
Full screens seem perfect but they are achieved by a series of compromises: front cameras usually shrink inside mobile phones and pop up slowly from the top when used. This structure must be sophisticated enough to add unnecessary complexity to perfect appearance.
Fullscreen can also stack up with waterfall screens.
Surround screen: Surround screen refers specifically to the screen where the phone body is surrounded by a screen. The front, sides, and back are almost all screens. Xiaomi’s new concept mobile phone MIX Alpha uses a surround screen with a screen-to-body ratio of 180.6%.
Folding screen: It can be bent or even twisted 360 degrees. The display of the folding screen needs to ensure that it is not broken even after 200,000 times of folding. It is a flexible screen with high flexibility, and the structure of the screen needs to be designed separately.
Ⅱ. The main parameters of the mobile phone screen
Mobile phone screens are measured in inches, which is equivalent to 2.54 centimeters per inch. We usually say how many inches the screen of a mobile phone is, but it is not the length of the side of the phone, but the length of the diagonal of the screen.
The size of the screen is measured by the length of the diagonal, in inches.
As mobile phones evolve from feature phones to smartphones, the screens of mobile phones are getting bigger and bigger. Jobs once claimed that the golden size of mobile phone screens is 3.5 inches. As a result, the screen size of most flagship phones remains above 6 inches, and many manufacturers have even expanded their mobile phone screens to 7 inches.
As the screen size becomes larger, is the image display necessarily clearer? Let’s talk about the concept of pixels and resolution.
Pixel: The principle of the screen display is actually to divide the effective area into many small grids. Each grid displays only one color, which is the smallest element of imaging, so it is called “pixel“.
Pixels are small squares on the screen, each square displays a color
Resolution: How many pixels the screen has in the two directions of length and width is called the resolution, which is generally represented by AxB. The higher the resolution, the smaller the area of each pixel, and the smoother and finer the display effect.
Common display screen resolution definitions
For example, the iPhone X has a screen size of 5.8 inches and a resolution of 1125×2436, which means that the phone has 1125 pixels in the width direction and 2436 pixels in the length direction.
PPI: The size of different mobile phone screens is different, the natural resolution is also different, so how to intuitively express the pixel density of the mobile phone screen, that is, the definition?
The answer is simple, no matter how large the screen area is, it is converted into the number of pixels per unit area, and then the standard is unified and can be compared with each other.
In fact, because the screen size is expressed in inches, the industry-standard converts the number of pixels contained in each inch of the screen called PPI (Pixels Per Inch), or pixel density.
PPI is calculated as follows: using the Pythagorean theorem, the number of pixels in the diagonal direction is calculated by the number of pixels in the horizontal and vertical directions in the resolution, and then divided by the length of the diagonal (that is, the inch of the phone plane size Number).
The figure below takes the iPhone 5 as an example, and the PPI is 326 by calculation. Of course, it is easier and more convenient to get the PPI value by looking at the product manual.
After figuring out the calculation method of PPI, which PPI should be used for a mobile phone screen? Our expectation is of course: the image that the eye sees is clear and smooth, and the existence of pixels is completely invisible.
10 years ago, at the iPhone 4 launch, Jobs said: “When you take something 10-12 inches (approximately 25-30 cm) away from you, it has a resolution of more than 300 PPI, and your retina can’t tell the pixels.” “
That’s Apple’s original definition of a Retina display, with a pixel density of 326ppi on the iPhone 4.
In fact, Jobs’s definition assumes that the vision of the person looking at the screen is 1.0, but in fact, many people have vision much better than 1.0, and the distance to see the screen needs to be 25 to 30 cm, and many people may get closer. Therefore, the value of 300 PPI is not absolute.
The industry is not limited to this value but keeps raising it. At present, the PPI of main mobile phones is between 300 and 500, and Samsung’s flagship phones even exceed 500. Even Apple’s own iPhone X and 11 have reached 458 PPI.
Let’s talk about another parameter that has been hot recently: Refresh Rate.
The refresh rate is the number of times the mobile phone screen is refreshed in one second. For example, the common 60Hz refresh rate means that the content displayed on the screen is refreshed 60 times per second.
Why should the content displayed on the screen be refreshed quickly?
When the object is moving fast and the image seen by the human eye disappears, the human eye can continue to retain the image of the image for about 0.1-0.4 seconds. This phenomenon is called persistence of vision.
Due to the visual persistence effect, when a person looks at several rapidly changing pictures, the content of the previous picture is still in the visual persistence, and the next picture comes into sight quickly, giving a sense of the continuous pictures. This is the principle of video.
The principle of video or animation
Therefore, video play has a concept called “frame rate” (FPS: Frame per second), which is how many continuous pictures are played per second.
When the frame rate is 16 FPS, the human eye will think that the image is coherent, and a higher frame rate can get smoother and more realistic animation. Generally, 25 to 30 FPS is acceptable, but increasing the frame rate to 60 FPS can significantly improve the sense of interaction and realism.
Therefore, the refresh rate of the screen must be greater than the frame rate of the video. Otherwise, the video is played to the next frame, and the screen has not been refreshed, and the user experience is naturally not good if it stays on the content of the previous frame.
At present, most video frame rates are less than 60FPS, so the refresh rate of the mobile phone screen should not be less than 60Hz. In theory, the higher the refresh rate, the more delicate and smooth the screen display and operation, so many flagship phones currently use 90Hz or even 120Hz refresh rates.
Ⅲ. The technology behind the screen of the mobile phone
If we look at the mobile phone posters, we can find that there are many different terms about screen materials and technologies: TFT LCD, TFT, IPS, LTPS, OLED, AMOLED, etc., which are dazzling.
What is the difference between the screens using these technologies and what are the advantages and disadvantages?
In fact, the current main mobile phone screens are classified in terms of major technologies, namely LCD and OLED.
LCD: Full name is Liquid Crystal Display
OLED: Full name is Organic Light-Emitting Diode
The three primary colors of light are red, green, and blue. By mixing these three colors in different proportions, almost all colors in nature can be obtained. Therefore, each pixel on the mobile phone screen is also composed of a mixture of these three colors.
The figure below is a longitudinal section view of each pixel of LCD and OLED, showing the principle of light emission of this pixel.
LCD and OLED screen structure
In LCD technology, although the word “liquid crystal” is very conspicuous, the liquid crystal does not emit light. A backplane composed of LEDs (light-emitting diodes) is needed to provide a white light source, also called “Backlight”.
On the basis of the backlight for each pixel, a layer of red, green, and blue films is added. White light passing through these films becomes colored light of red, green, and blue.
But if the intensities of these three types of light are the same, they will become white or grey when mixed. Therefore, the intensity of each light must be flexibly controlled to mix multiple colors.
At this time, it is the turn of the liquid crystal material to appear. Liquid crystal has a characteristic, that is, under the action of an electric field, its molecular arrangement will change, which will affect the permeability of light. Changing the voltage can adjust the amount of light transmitted.
For LCD screens, the liquid crystal layer sandwiched between the backlight and the film adjusts the passable light by adjusting the input voltage. After passing through the colored film, the three primary colors of light with different intensities can be obtained. After mixing, thousands of colors will be obtained.
So, how to adjust the input voltage of each pixel?
The so-called TFT (Thin-film transistor) refers to the thin film transistor array on the glass substrate of the liquid crystal panel, which allows each pixel of the LCD to be equipped with its own semiconductor switch, thereby achieving independent and precise control of “point-to-point”.
Therefore, the main LCD screen is also called a TFT-LCD. However, IPS and LTPS are actually implemented by different technologies under TFT-LCD, so we won’t repeat them here.
After talking about LCD for so long, now it’s OLED‘s turn.
The structure of the OLED screen is much simpler than that of the LCD. It does not require a backlight, nor does it have a liquid crystal or color filter film. The internal organic material coating is like a small colored light bulb, which can emit light when it is powered on.
AMOLED: Everyone already knows about OLED. The previous AM refers to the driving method of OLED. Its full name is Active Matrix. TFT is usually used as a switch to control the current through organic materials to achieve different colors. All OLEDs currently used in mobile phones are AMOLED, so it can be considered that the two are the same thing.
Super AMOLED: Samsung’s improvement to AMOLED, canceled the middle touch sensor panel, and built the AMOLED sensor layer on the screen, so the control is more sensitive, thinner, and brighter, and the presentation effect in the sun is better.
Dynamic AMOLED: It is also the AMOLED improved technology launched by Samsung, currently mainly used in high-end devices. This technology changes the organic materials in OLEDs and is said to achieve a wider dynamic range. When the image contrast is high, it can display more dark details.
Essentially, both Super AMOLED and Dynamic AMOLED belong to AMOLED, and AMOLED is the OLED technology used in mobile phones.
Compared with OLED screens, LCD screens have many disadvantages.
1. Cannot display black
Because the liquid crystal layer cannot be completely closed, some backlight will always pass through, so the LCD cannot display pure black, only dark grey. And OLED can realize a pure black display by controlling the switch of each pixel.
The night sky under the OLED screen (on the left) is obviously deeper
2. Easy to leak light
The backlight of the LCD screen can easily leak from the screen and the frame of the mobile phone to form a light leakage phenomenon. This phenomenon is very common in the era of rough mobile phone workmanship, and it is rarely seen now.
3. Large screen thickness
LCD is limited by the backlight layer and liquid crystal layer due to its complex technology, and the screen thickness is much larger than that of OLED. Of course, this thickness is not worth mentioning on the TV, but in the mobile phone, where the pursuit of slimness and extremely limited internal space, the screen is thinner, and more other components can be plugged in to improve other performance.
4. It is difficult to realize a curved screen
LCD cannot be bent greatly, while OLED can. Therefore, for those mobile phones like curved screens, only OLED screens can be used.
5. Large power consumption
Since the LCD screen has a backlight, it must be fully lit when in use, and OLED can individually control the switch of each pixel, so the power consumption of the LCD screen is much greater than that of OLED. In the picture below, Smartisan R1 and Xiaomi mix2s are both LCD screens, and battery life is obviously at a disadvantage under long-term video play.
6. Long response time
Due to the long response time of the LCD screen, smear will occur when the screen slides quickly. The OLED responds quickly, clean, and without smear.
The smear phenomenon of the LCD screen
After talking about the shortcomings of so many LCDs, is OLED perfect? Of course not, OLED mainly has the problem of burning screen and strobe.
1. Burning screen
Because the organic material used in the OLED screen will be aging faster if there is a large pixel workload, some are relatively idle, there will be an inconsistent degree of aging of the entire screen, resulting in different areas of color display deviation. This phenomenon is called burning the screen.
However, under normal use, screen burning is a slow process. Three years have passed when it is obvious that most people should change their phones.
Burn-in is the shortcoming of the OLED screen. For LCD, the backlight is fully lit, and the aging time of LCD is longer, so there is basically no problem of burn-in.
For LCD, to control the screen brightness, you can directly adjust the brightness of the backlight. But OLED is more troublesome. It needs to switch the screen at high frequency to achieve dimming. If you want to brighten, you turn on the screen more times, and if you want to dim, you turn off the screen more times.
Since the time for switching the screen each time is very short, although the human eye cannot perceive the change of the screen each time, it can feel the average brightness and darkness over a period of time, thus achieving the effect of dimming.
For example, to achieve a brightness of 50%, it takes half the time to turn on the screen and half the time to turn off the screen. At a lower brightness, it takes longer to turn off the screen. The screen flashes once and again, and it can even be visible to the naked eye，then the eyes will be very uncomfortable.
Strobe at low brightness of OLED screen
This phenomenon of OLED is called a strobe, hence the name “eye-damaging screen”. In contrast, LCD screens generously call themselves “eye protection screens.”
Although there are shortcomings, the shortcomings do not conceal the advantages. At present, the OLED display has entered the mainstream and gradually occupies the market space of LCD display. This is especially obvious in high-end mobile phones.
Regarding this issue, we can get a glimpse of Apple’s configuration.
iPhone 8/8 plus
iPhone Xs/Xs Max
iPhone 11 Pro/Pro Max
As can be seen from the above table, starting from the iPhone X series, high-end products all use OLED screens, while low-priced products use LCD.
Okay, so much for this article. We believe that everyone has an understanding of the main parameters and technology of the mobile phone screen, and we hope that it will be helpful when purchasing a mobile phone.