Small phone, big universe.
This communication tool, once only used for making calls, has now become all-powerful, serving as the first window connecting people to the entire world.
The mobile screen, which bears the display function of the phone and serves as the entry point for touch operations, is even more important, being the “window of the window”.
In this article, we will discuss the secrets behind mobile screens. After reading this, you will be able to answer the following questions:
1. What forms do mobile screens take?
2. How do we measure the size of mobile screens?
3. What do concepts like resolution, PPI, and refresh rate refer to?
4. What types of mobile screens exist, and what are the underlying technical principles?
1. Forms of Mobile Screens
Since smartphones have dominated the market, phones, as centers for entertainment and information during fragmented time, require larger display areas, thus screens are getting bigger; meanwhile, traditional phones have a “forehead” for the receiver and front camera, and a “chin” for the Home button, resulting in larger phone sizes and difficulty in increasing the screen-to-body ratio.
Later, Apple came up with a solution to centralize a series of components on the forehead of the phone, allowing the screen to extend upwards on both sides, resembling a “notch”. Therefore, this type of screen is called a notch screen.
iPhone’s iconic “notch screen”
The “notch screen” fired the first shot in the battle for screen-to-body ratio. Subsequently, various Android phones rushed to imitate it, but soon found it ugly and boring, leading to various designs with even higher screen-to-body ratios.
Waterdrop Screen: The top of the screen avoids the front camera, and the black background of the camera connects with the upper frame, resembling a droplet hanging precariously, hence the name “waterdrop screen”.
Android phone’s “waterdrop screen”
Pearl Screen: This is essentially a waterdrop screen, but Huawei feels that the area occupied by the camera resembles a plump pearl, hence the name “pearl screen” to highlight its extraordinary quality.
Huawei phone’s “pearl screen”
Punch-Hole Screen: As can be seen from the previous forms, the handling of the front camera determines the shape of the screen. If we punch a hole in the screen and embed the camera, it becomes a “punch-hole screen”.
Android phone’s “punch-hole screen”
Waterfall Screen: The previous notch, waterdrop, and punch-hole screens all struggle with the camera at the top of the phone, but the sides still have bezels, making them less impressive. So someone thought, why not curve the sides down a bit, making the bezels invisible from the front? This type of screen has sides that resemble a waterfall, hence the name “waterfall screen”.
Android phone’s “waterfall screen”
Full-Screen: The front of the phone is a complete screen, with no notch, waterdrop, pearl, or punch-hole. Full-screen seems perfect but is achieved through a series of compromises: the front camera retracts into the body when not in use and slowly pops up from the top when in use, making this structure require precision and durability, thus adding unnecessary complexity for a perfect appearance. True full-screen can also overlap with waterfall screens.
Android phone’s true “full-screen”
2. Main Parameters of Mobile Screens
Mobile screens are measured in inches, with one inch equal to 2.54 cm. When we say a mobile screen is a certain size, we are actually referring to the length of the diagonal, not the edge length of the phone.
The size of the screen is measured by the length of the diagonal, in inches
As phones have evolved from feature phones to smartphones, mobile screens have also grown larger. Steve Jobs once claimed that the golden size for mobile screens was 3.5 inches, but now most flagship phones maintain screen sizes of over 6 inches, with many manufacturers even expanding screens to 7 inches.
Is it true that a larger screen size guarantees clearer image display? This introduces the concepts of pixels and resolution.
Pixel: The principle of screen display is to divide the effective area into many small grids, each grid displaying only one color, making it the smallest element of imaging, thus called “pixel”.
Pixels are small squares on the screen, each displaying one color
Resolution: The number of pixels in the length and width directions of the screen is called resolution, usually expressed as AxB. The higher the resolution, the smaller the area of each pixel, resulting in smoother and finer display effects.
Common display screen resolution definitions
For example, the iPhone X has a screen size of 5.8 inches and a resolution of 1125×2436, meaning this phone has 1125 pixels in width and 2436 pixels in length.
PPI: Different mobile screens have different sizes, and naturally, their resolutions vary. So how can we intuitively represent the pixel density, or clarity, of mobile screens?
The answer is simple: regardless of how large your screen area is, we convert it to the number of pixels per unit area, allowing for standardized and comparable metrics.
In fact, since screen size is expressed in inches, the industry standard converts the number of pixels per inch of the screen, called PPI (Pixels Per Inch), also known as pixel density.
The calculation of PPI is as follows: using the Pythagorean theorem, calculate the number of pixels in the diagonal direction based on the horizontal and vertical pixel counts in the resolution, then divide by the diagonal length (which is the inch size of the phone’s flat surface).
Below is an example using the iPhone 5, where the calculated PPI is 326. Of course, checking the product description for the PPI value is much simpler.
PPI calculation example (iPhone 5)
With a measurement method in place, what PPI should mobile screens have? Our expectation is, of course, that the images seen by the eyes are clear and smooth, with no visible pixels.
Ten years ago, during the iPhone 4 launch, Steve Jobs stated: “When you hold something 10-12 inches (about 25-30 cm) away, if its resolution reaches above 300 PPI, this ‘magic number’, your retina will not be able to discern the pixel dots.”
Steve Jobs holding the iPhone 4
This is Apple’s initial definition of the “Retina Screen”; the pixel density of the iPhone 4 also reached 326 PPI.
In reality, Jobs’ definition assumed that the viewer has a vision of 1.0, but many people’s vision is actually better than 1.0; and the distance from the screen needs to be 25 to 30 cm, while many people may get closer. Therefore, the 300 PPI value is not absolute.
The industry has not been confined to this value but continues to raise the bar; currently, mainstream phones have PPI ranging from 300 to 500, with Samsung’s flagships exceeding 500, and even Apple’s own iPhone X and 11 reaching 458 PPI.
Next, let’s talk about another parameter that has been hotly discussed recently: refresh rate.
Refresh Rate: This refers to the number of times the mobile screen refreshes the display in one second. For example, a common refresh rate of 60Hz means the content displayed on the screen refreshes 60 times per second.
Why do the displayed contents on the screen need to refresh quickly?
When an object moves quickly, the human eye can retain the image for about 0.1-0.4 seconds after the image disappears; this phenomenon is called visual persistence.
Due to the visual persistence effect, if a person sees multiple rapidly changing images, the content of the previous image is still retained in visual persistence, and the next image quickly appears, giving the impression of continuous animation; this is the principle of video.
The principle of video or animation
Therefore, there is a concept of “frame rate (FPS: Frame per second)” in video playback, which indicates how many continuous images are played per second.
When the frame rate is 16 FPS, the human eye perceives the images as coherent; a higher frame rate can yield smoother and more realistic animations. Generally, 25 to 30 FPS is acceptable, but increasing the frame rate to 60 FPS significantly enhances interactivity and realism.
Animated images at different frame rates
Thus, the refresh rate of the screen must exceed the frame rate of the video. Otherwise, if the video has already played to the next frame while the screen has not yet refreshed to display, remaining on the previous frame’s content, the user experience will naturally be poor.
Currently, the vast majority of video frame rates are below 60 FPS, so the refresh rate of mobile screens should not be lower than 60Hz. Theoretically, the higher the refresh rate, the more delicate and smooth the display and operation, which is why many flagship phones now use refresh rates of 90Hz or even 120Hz.
3. The Technology Behind Mobile Screens
If we look at mobile promotional posters, we can find that the terminology regarding screen materials and technologies is dizzying: TFT LCD, TFT, IPS, LTPS, OLED, AMOLED, etc.
What are the differences between screens using these technologies, and what are their advantages and disadvantages?
Currently, mainstream mobile screens can be broadly categorized into two types: LCD and OLED.
LCD: The full English name is Liquid Crystal Display, which refers to the well-known liquid crystal display.
OLED: The full English name is Organic Light-Emitting Diode, which translates to organic light-emitting diode, also known as organic electroluminescent display.
The three primary colors of light are red, green, and blue. By mixing these three colors in different proportions, we can obtain almost all colors in nature. Therefore, each pixel on a mobile screen is also composed of a mixture of these three colors.
The three primary colors of light and mixed colors
The diagram below shows the cross-section of each pixel under LCD and OLED technologies, illustrating the principle of how this pixel emits light.
Structure of LCD and OLED screens
In LCD technology, although the term “liquid crystal” (shown on the left side of the image) is prominent, liquid crystals cannot emit light; they require a back panel composed of LEDs (light-emitting diodes) to provide a white light source, also known as “backlight” (shown in the image).
Each pixel, based on the backlight, adds a layer of thin film containing the three colors of red, green, and blue; white light passing through these films transforms into colored light.
However, if the intensity of these three lights is equal, they will mix back into white or gray light, so it is essential to flexibly control the intensity of each light to mix various colors.
This is where liquid crystal material comes into play. Liquid crystals have a property that under the influence of an electric field, their molecular arrangement changes, affecting their light permeability; by adjusting the voltage, the amount of light passing through can be controlled.
For LCD screens, the liquid crystal layer sandwiched between the backlight and the film adjusts the brightness by altering the input voltage, and after passing through the colored film, different intensities of the three primary colors are obtained, resulting in a wide range of colors.
So, how do we adjust the input voltage for each pixel?
The so-called TFT (Thin Film Transistor) refers to a thin film transistor array on the glass substrate of the liquid crystal panel, allowing each pixel of the LCD to have its semiconductor switch, enabling “point-to-point” independent and precise control.
Thus, mainstream LCD screens are also called TFT-LCD. IPS and LTPS are different technical implementations under TFT-LCD, which will not be elaborated here.
Having discussed LCD, it’s time for OLED.
OLED screens are structurally much simpler than LCDs; they do not require backlighting, nor do they have liquid crystals or color filters. The internal organic material coating acts like colored small light bulbs, emitting light when electrified.
AMOLED: Now that you know about OLED, the “AM” in front refers to the driving method of OLED, which stands for Active Matrix, usually using TFT as a switch to control the current through organic materials to achieve different color displays. Currently, all OLEDs used in mobile phones are AMOLED, so we can consider the two as the same thing.
Super AMOLED: This is Samsung’s improvement on AMOLED, removing the touch sensing panel in the middle and integrating the AMOLED sensing layer directly onto the screen, resulting in more sensitive control, thinner design, and better brightness performance under sunlight.
Dynamic AMOLED: This is another technology launched by Samsung, primarily used in high-end devices. This technology alters the organic materials in OLED, claiming to achieve a wider dynamic range, allowing for more dark detail display when there is a high contrast between light and dark in images.
Essentially, Super AMOLED and Dynamic AMOLED are largely marketing terms; they both belong to AMOLED, and AMOLED is the OLED technology used in mobile phones.
Various sub-screen technologies under LCD and OLED
Compared to OLED screens, LCD screens have several disadvantages.
1. Cannot display black: Since the liquid crystal layer cannot fully close, some backlight will always leak through, so LCDs cannot display pure black; they can only show dark gray. In contrast, OLED can achieve pure black display by controlling the switch of each pixel.
OLED screen (left) shows a significantly deeper night sky
2. Prone to light leakage: LCD screens’ backlight can easily leak from the screen and phone’s edges, creating light leakage, a phenomenon that was common in poorly made phones but is now rare.
LCD screen light leakage
3. Greater screen thickness: Due to the complexity of the technology, the LCD screen’s thickness is much greater than that of OLED screens because of the backlight and liquid crystal layers. While this thickness is negligible in TVs, in mobile phones, where thinness and limited internal space are crucial, a thinner screen allows for more components to be included, enhancing other performance aspects.
LCD screens are significantly thicker than OLED screens
4. Difficult to achieve curved screens: LCDs cannot be bent significantly, while OLEDs can. Therefore, for curved screen phones, only OLED screens can be used.
OLED is very suitable for achieving curved screens
5. Higher power consumption: Since LCD screens have backlighting, they must light up the entire screen during use, while OLEDs can individually control each pixel’s switch, resulting in significantly higher power consumption for LCD screens. In the image below, both the Nut R1 and Xiaomi Mix 2S use LCD screens, showing a noticeable disadvantage in battery life during long video playback.
LCD screens consume a lot of power, leading to poor battery life
6. Longer response time: Due to the longer response time of LCD screens, they can produce ghosting effects during fast-moving images. In contrast, OLEDs respond quickly, without ghosting.
Ghosting effect on LCD screens
Having discussed so many disadvantages of LCD, does that mean OLED is perfect? Certainly not; OLED has issues like burn-in and flicker.
1. Burn-in: Due to the faster aging of the organic materials used in OLED, if some pixels are under heavy load while others are relatively idle, there will be inconsistent aging across the screen, causing color display deviations in different areas. This phenomenon is called burn-in.
Burn-in phenomenon on OLED screens
However, under normal usage conditions, burn-in is a slow process, and by the time it becomes noticeable, three years may have passed, and most people would have already changed their phones.
Burn-in is a disadvantage of OLED, while for LCD, since the backlight lights up the entire screen and the aging time of liquid crystals is longer, burn-in issues are virtually nonexistent.
2. Flicker: For LCDs, to control screen brightness, you can directly adjust the brightness of the backlight. However, OLED is more complicated; it needs to achieve dimming by rapidly switching the screen on and off. To brighten, the screen is turned on more often, while to dim, it is turned off more often.
Dimming of OLED screens: the raised parts indicate when the screen is on
Since each on-off cycle is very short, although the human eye may not perceive each switching change, it can sense the average brightness over time, thus achieving the dimming effect.
For example, to achieve 50% brightness, the screen needs to be on for half the time and off for the other half. At lower brightness, the off time will be longer, causing the screen to flicker, sometimes to the point where it is visible to the naked eye, which can be uncomfortable for the eyes.
Flicker of OLED screens at low brightness
This phenomenon of OLED is called flicker, which is why it is often referred to as “eye-harming screen”. In contrast, LCD screens confidently call themselves “eye-care screens”.
Despite these drawbacks, the advantages outweigh the disadvantages, and currently, OLED screens have become mainstream, gradually squeezing the living space of LCD screens, particularly evident in high-end devices.
We can glimpse this from Apple’s configurations.
|
Release Year |
iPhone Model |
Screen |
|
2017 |
iPhone 8/8 Plus |
LCD |
|
iPhone X |
OLED |
|
|
2018 |
iPhone XR |
LCD |
|
iPhone Xs/Xs Max |
OLED |
|
|
2019 |
iPhone 11 |
LCD |
|
iPhone 11 Pro/Pro Max |
OLED |
From the above, we can see that starting from the notch screen of the X series, higher-end products use OLED screens, while lower-priced ones use LCD.
That’s it for this article. I believe everyone now has a better understanding of the main parameters and technologies behind mobile screens, which will help you in choosing a phone in the future.
Author: Yu Yao Cai Cai
Source: Wireless Deep Sea
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