Now on mobile screens, we usually see two types of screens – LCD (Liquid Crystal Display) or OLED (Organic Light-Emitting Diode). Both have their own advantages and disadvantages, and the debate over which is better – LCD or OLED – has been ongoing for many years. Here, we will discuss some specific principles and pros and cons, so by the end, you should know which side you are on.
On the right, the XR uses LCD, while the XS series on the left uses OLED.
First, we need to understand what a pixel is. When looking at mobile specifications, the resolution usually refers to how many pixels there are horizontally × how many pixels there are vertically. In fact, if you look closely at your screen with a magnifying glass, you will find that each color on the screen is made up of red, green, and blue (commonly known as RGB).
A pixel is made up of three RGB sub-pixels (this is the case for the most common LCD, and OLED will be explained later), allowing it to display another color. The three colors RGB can basically cover all the colors that our eyes can see in daily life, and when combined in different ways, they can present different colors.
The above image shows the standard RGB arrangement used in most LCD screens – they are uniform in size, and each pixel has three sub-pixels, resulting in an average image quality.
Let’s first explain how LCD works. The image below can show that an LCD screen can basically be understood as being composed of five layers (this has been simplified a lot):
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The first layer is a regular light source (commonly LED), which only presents the result of making RGB as bright as possible. It radiates in all directions.
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This layer is a polarizing film; if a sub-pixel shines through it directly, there is no change. It acts like a filter, making the light emitted from the light source cleaner, allowing it to pass through in a straight line.
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This is the liquid crystal layer, which can adjust the direction of a beam of light. Its principle is to use voltage changes to adjust the direction. For example, it can adjust the light from a “-” shape to a “/” shape to pass through.
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This is another layer of polarizing film, but it is vertical. If the liquid crystal layer does not change the horizontal light and lets it collide directly with the vertical film, it will present the effect of not letting any light through. (However, many times it is not possible to prevent any light from leaking, leading to the problem of light leakage and the black not being pure enough in LCDs.) If different directional arrangements collide, it can achieve different color depth effects.
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This layer is an RGB filter, which, combined with different depths, can synthesize three different red, green, and blue colors.
In simple terms, it is a layer of light that has passed through the horizontal polarizing film, adjusted in direction by the liquid crystal layer’s voltage, then colliding with another vertical polarizing film to yield three different depths of white, and then passing through a filter to create different red, green, and blue colors.
If after all this explanation you still don’t understand, perhaps the next image can help you better understand. LCD relies on changing the direction of light and then passing through a film to adjust brightness. The filter’s effect is merely to add a layer of color to this colorless entity.
Having discussed LCD, let’s talk about what OLED is. The biggest advantage of OLED over LCD is its thinness, and also that OLED colors appear more vibrant and rich. Its structure is much simpler than that of LCD. Let’s first look at the most important emission layer, which is made of organic materials that can emit light on their own without relying on anything else! The cathode above and the conductive layer below act to provide electricity to the organic layer, which cannot work without power.
According to a detailed explanation from the China Display Network, the other layers work as follows:
Substrate (transparent plastic, glass, metal foil) – The substrate supports the entire OLED.
Anode (transparent) – The anode eliminates electrons when current flows through the device (increasing electron “holes”).
Conductive layer – This layer is made of organic plastic molecules that transmit the “holes” coming from the anode. Polyaniline can be used as the conductive polymer for OLED.
Emissive layer – This layer is made of different organic plastic molecules from the conductive layer that transmit electrons coming from the cathode; the light-emitting process occurs in this layer. Polyfluorene can be used as the emissive layer polymer.
Cathode (can be transparent or opaque, depending on the type of OLED) – When current flows through the device, the cathode injects electrons into the circuit.
The image above shows a detailed breakdown of the layers; in reality, OLED is very thin. As seen in the image below, due to the simple structure of OLED, it has much better flexibility compared to LCD screens. The actual structure of LCD is very complex, making it nearly impossible to produce a curved LCD (previously, Samsung applied for a patent for curved LCD, but it is unknown when it will be implemented).
By the way, OLED also has many different types; when talking about mobile phones, the main one is AMOLED. The AM in AMOLED stands for “Active Matrix”. The difference with AMOLED is that it can individually control the brightness and on/off state of each pixel. This is because manufacturers have applied another layer of expensive conductive organic material that can emit light on their screens. Currently, manufacturers making AMOLED include the well-known Samsung (breaking a new record in screen technology with every new phone), LG (known for issues like green screens), and the newcomer BOE.
After explaining so much, let’s look at the detailed advantages and disadvantages of LCD and OLED.
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Both OLED and LCD have the PWM issue
Many users have heard of the PWM flickering issue with OLED screens. PWM stands for Pulse-Width Modulation. Simply put, it is the flickering effect. OLED screens turn on and off every second to control their brightness (the lower the brightness, the faster the flicker), measured in Hz, indicating how many times it switches on and off per second. At low brightness, the lower the PWM frequency, the more it hurts the eyes (imagine a flickering car light). If it reaches a very high frequency that the brain cannot perceive, it would be perfect.
LCD can directly use DC modulation to control how bright the backlight is. Early on, OLED also used DC modulation, but it brought more problems. Not to mention the issues of screen burn-in and uneven brightness, especially concerning color display. When using DC modulation at lower brightness, the OLED screen can exhibit some color discrepancies. Some users have found a way to adjust the Samsung S8 to use DC dimming, which causes the screen to no longer flicker, but the displayed black color is not as pure.
Some LCD models also have flickering problems, but generally, LCD phones use very high frequencies and often combine with DC modulation to achieve a less eye-straining effect. However, some manufacturers are not very honest, using low-frequency PWM dimming on LCDs. Therefore, when choosing a phone, it’s important to do some research and not just choose based on the LCD label.
In some low-quality OLED screens, the flicker at low brightness can be as low as around 120Hz. If you are sensitive to flickering, you may want to avoid OLED screens. Even the top-tier Samsung AMOLED screens have flickering around 250Hz. Experts suggest that the healthy range for flickering should be above 1250Hz!
Of course, whether you are sensitive to flickering on OLED screens depends on your own eyes/brain. Some people always say, “LCD will never be a slave,” while others who are not affected by flickering have no faith in LCD screens. Personally, I have not noticed any flickering issues with my Mi 8 until I switched to another phone and changed the camera shutter speed to see the difference compared to my OnePlus 5T.
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OLED’s RGB arrangement issue
In ordinary LCD screens, the arrangement of sub-pixels is generally three rectangular RGB colors. This is not a problem. However, there are some differences in various OLED arrangements.
In OLED screens, the reason for not using standard RGB is mainly that the efficiency of blue materials is significantly lower than that of the other two colors, so different arrangements are adopted to reduce the use of blue. The “other arrangements” include Samsung’s famous PenTile RGBG arrangement and the RGB-Delta arrangement of the domestic Nut phone, as well as various customized arrangements from other manufacturers.
Since Samsung’s AMOLED is widely used in many devices, we will only introduce its PenTile RGBG (Red-Green-Blue-Green) arrangement here. Now, in Samsung’s diamond arrangement, each pixel only has two sub-pixels, with only green being complete in each pixel. Therefore, theoretically, the PenTile arrangement has 1/3 fewer pixels both horizontally and vertically. However, Samsung and other manufacturers have certainly developed some solutions for this issue, such as “sub-pixel rendering,” which borrows a sub-pixel from nearby pixels.
Manufacturers have indeed developed certain solutions to these problems, and we consumers don’t need to worry. If you search online for feedback about whether the Xiaomi MIX3’s 6.39-inch 1080P AMOLED screen has pixelation issues, many users respond that it is perfectly fine. Currently, flagship devices maintain pixel control within a range that does not produce significant pixelation. If you are not satisfied, you can always buy an iPhone/Samsung or similar foreign 2K OLED screen phone, or perhaps a domestic flagship like the Nut R1 LCD might suit you better.
(Reference and this image from Ifanr Technology)
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OLED burn-in issue
Let me clarify that “burn-in” and “image retention” are two different concepts. Both occur due to displaying the same content for a long time, but the former leaves a permanent imprint, while the latter will disappear after displaying different content for a while.
You might wonder: “Why does only OLED suffer from burn-in?”
The essence of OLED is that each sub-pixel emits light on its own, and they will all age after prolonged use. Among them, blue ages the fastest, while green is the most resilient. This is also the main reason why Samsung adopts the PenTile arrangement. In contrast, LCD uses a whole backlight panel to emit light, and the colors are displayed through various filters. Moreover, the LED backlight used in LCDs is generally very durable and does not have significant issues.
In the image above, you can see that the burn-in phenomenon of the Pixel 2 XL (which uses LG’s screen) is quite severe; even in other displays without a navigation bar, this bar can still be faintly seen. This is what happens in phones that have not undergone significant optimization for burn-in. Here, I will provide some tips for those using OLED phones to prevent burn-in:
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Change your wallpaper regularly. If you remain on the home screen or frequently switch apps, remember to change your wallpaper to refresh your mood!
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Do not use high brightness for extended periods. Due to the PWM issue, many users prefer using their phones at maximum brightness, but I would advise against that. The best recommendation is to set it to automatic brightness, so the pixels won’t be continuously working at fixed levels.
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Do not display the same content for long periods. This solution does not require much explanation.
If you observe closely, Samsung’s navigation bar changes gradually.
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LCD’s Weakness – Full-Screen / Curved Screen
Currently, domestic manufacturers are pursuing extreme full-screen designs, and more people are beginning to understand the three different bottom frame solutions: COG, COF, and COP.
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COG stands for Chip On Glass. This is the most common technology, directly placing the IC chip and wiring on the surface. Phones using this technology include the iPhone 8, Xiaomi MIX first generation, and other phones with large bezels.
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COF stands for Chip On Film. This technology places the IC chip at the back, with a soft line connecting it to the screen. The process is not difficult, and the cost is completely reasonable.
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COP stands for Chip On Pi. This unique process is exclusive to OLED, folding a bit of excess screen down to the back, so the wiring and chip are behind the screen.
The COP process is quite difficult to master, and I believe it will appear in more OLED phones in the future. Currently, phones that use COP include only the iPhone X, XS, XS Max, and OPPO Find X. Therefore, if LCD continues to be used, it will be very difficult to achieve a bezel-less and curved screen. Moreover, OLED screens can also be made into curved screens, making the left and right bezels narrower, which is practically impossible with LCD.
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LCD’s Disadvantages – Display Issues
Since LCD cannot emit light on its own, it can only turn the entire screen on/off simultaneously, leading to several display issues.
First, there is the problem of displaying black. If a black image is displayed while the screen is turned on, the backlight will always emit a little light (mainly because the liquid crystal layer cannot completely close), leading to light leakage. Therefore, displaying black on an LCD is like this:
Since OLED can control each sub-pixel’s on/off state, when displaying black, it simply turns off the unnecessary pixels. This gives OLED an advantage over LCD – that is, the always-on display.
In sleep mode, some phones can still use a few pixels to show the current time/date/new notifications. Since all other pixels are turned off, only a few rows of white pixels are active, so the power consumption is very low. If LCD were to implement an always-on display, it would likely become a battery hog.
Moreover, under normal circumstances, the quality of OLED screens is generally much better than that of LCDs. Especially in terms of color accuracy, choosing an OLED TV for watching is the most enjoyable, but OLED large screens tend to be much more expensive than LCDs.
In summary, both LCD and OLED have their own advantages and disadvantages, and the PWM flicker issue does not only exist on LCD screens. As we gradually move into the era of full-screen mobile phones, LCD will eventually fade out of the mobile market due to its bezel and curved screen issues. Personally, I believe that in the future, the flicker frequency of OLED screens will increase, making them less eye-straining, while the lifespan and burn-in issues still need to be resolved.
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