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Nov 14, 2022

What is LCD Display

Here we go, the 100th time to ask what monitor to buy, this time really want to buy, 27 inch which is strong?

Nowadays, whether it is learning, work or entertainment, we can see the trace of the monitor. With the continuous development of the display panel manufacturers in recent years, the display market has also developed from the early years of the LCD alone to the current OLED, QLED flowering. For the masses of consumers, monitors and computers belong to the same long time frame of use of electronic products, until the time to replace the monitor, has been a variety of “new panels”, “new technology” around the dizzy.

As a consumer who understands most consumer electronics, I am often asked by friends how to buy a monitor. In particular, the epidemic has increased the demand for people to work at home, and more and more friends need to buy a monitor, but the average consumer really does not know how to start.

In fact, the composition of a product like a monitor is very well understood. If consumers understand the basic composition of the monitor before buying, then in the purchase, although not called comfortable, but also can be considered to have a good idea. This is also the original purpose of our production of this monitor report.

The basic components of a monitor

The composition of a monitor is not really complicated, and is broadly divided into 3 main components.

  • Panel
  • Driver board
  • Additional parts

The panel is the part that we see every day in our daily life, that is, the part of the monitor that mainly presents the content. Although different panels work differently, they all end up doing the same thing of “rendering” to the user.

The driver board, similar to the motherboard of a computer, is what makes the panel know what to display. The input interface of the display is often connected to the driver board, and the driver board will “translate” the display to the panel. The refresh rate, the backlight of the panel, or the color emitted by the panel pixels, and other panel-related values are also controlled by the driver board.

The extra parts, in a monitor are actually the generic parts such as the various cables, keypads, etc. For more advanced monitors there may be docking stations, light sensors, audio, etc.

It is because of the simple internal structure of the monitor that many geeky gamers will have DIY monitors to play with.

While the way a monitor is composed may seem simple at a cursory level, the more you learn about monitors, the more you realize that finished monitor manufacturers may be doing more than just “putting the pieces together” – why does A have a more vibrant color display than B when the monitor has the same LCD panel? Why does A cost more than B for a product that appears to have similar parameters? — we wanted to cover every aspect of the monitor.

But for the majority of this report, we will not be using screen shots to compare screens, because the human eye and camera CMOS do not perceive the same spectrum (nor is any CMOS spectrum the same as the eye), so the screen shot will definitely look different than what the human eye sees.

To ensure the accuracy of this consumer report, we also approached our old friends at TÜV Rheinland and with their help learned more about the details of display technology and some specific testing procedures that are in line with the current environment in which monitors are used. We hope to take this opportunity to pass on the correct and cutting-edge information in displays.

So in this report, we will first talk about how LCD, OLED and the paper-like displays that have become so popular in recent years, and find out what the manufacturers are talking about and what the advantages are when you are shopping for a display; in the next report, we will also bring you two very detailed guides to buying a display to help you in the next shopping season At the end of this report, we also prepared three pieces of monitor-related content, including monitors and eye health, understanding display monitor color calibration and how to do color calibration for monitors, etc. We hope to help you understand all aspects related to monitors in a scientific and correct way.

Next, let’s take a look at what details are hidden behind the most common LCD technology monitors in our daily lives.

It’s the age of LCD, but it’s not over yet

Before the iconic “big-ass” design of cathode ray tube monitors (also known as CRT monitors) were phased out, a variety of liquid crystal displays (LCD monitors) had been dominating our desktops for a long time. But to this day, LCD monitors are far from outdated and still dominate half of consumer displays.

The reason why LCD monitors are still going strong is actually very simple: it has low production cost and high production yield, so it is cheaper than other panels LCD can do.

To get to know the evergreen tree of the display, we have to start from its working principle.

How LCD works

The working principle of LCD can be simply divided into 3 parts to see in turn.

  • Backlight
  • Liquid Crystal
  • Color filter

Backlight layer

Regardless of the type of LCD, its light-emitting base is the backlight source, and we can simply understand the backlight as a flashlight. The backlight determines how bright a display can be and how many colors it can display, so the type of backlight has been constantly changing with the development of technology.

The backlight has gradually changed from the very early cold cathode fluorescent tubes (CCFL) to the more commonly used light-emitting diodes (LED), with the LED reason is actually very simple, not only brighter, longer use, and less power consumption, lower heat.

LED backlighting is of course also color, but with the development of various technologies, white LEDs have become very common, white LEDs can be a good balance between the brightness of the display and the colors that can be displayed on the display, but also the vast majority of LCDs currently used by the program.

But want the display to show a wider color gamut, then the backlight will have to use blue LED. because blue light irradiated to the fluorescent material located in front, part of the fluorescent material is absorbed by the excitation of yellow light, another part of the penetration through the fluorescent material, and phosphor excitation of light mixed together to form white light. This mixture of white light is more pure, so it can display a larger area of color.

Of course, in addition to LEDs like this directly on the bottom of the backlight layer (also known as straight down backlight), there are many displays will use side entry backlight, this type of display backlight layer is essentially a row of LEDs mounted on the top or bottom of the screen, light through the light guide plate to illuminate the entire backlight. Compared to the straight down type, the side entry type display can be made thinner, lower power consumption, but the brightness compared to the straight down type will also be lower on a lot.

Liquid Crystal

The liquid crystal layer is also a key component in all LED panels. The liquid crystal molecules in the liquid crystal layer will rotate under the voltage applied by the TFT layer. All liquid crystals can be imagined as small sticks arranged in layers, which change the brightness of each pixel by constantly changing the angle of lying or standing up, completely lying down means that all light cannot pass, completely standing up means that all light can pass, so each liquid crystal molecule can be simply understood as a film that changes transparency at any time.

There are currently 3 different types of liquid crystals, which are

  • TN
  • VA
  • IPS

TN

TN is the first type of liquid crystal used in LCD screens, if you poke it with your hand there are “water ripples”, TN liquid crystal molecules will be spirally arranged when it needs to light up, and when it does not need to light up will return to vertical arrangement, the overall switching speed is very fast, so the advantage of TN screen is fast enough response, monitor manufacturers also use this to make the first high refresh rate monitors . But because the spiral arrangement will have the difference between clockwise and counterclockwise, so the brightness of different angles is not the same, so the disadvantage of the TN screen is that the panel viewing angle is relatively small, low contrast, color distortion is more.

VA

VA panel LCD molecules, on the other hand, stay down in all directions when they need to be lit up, and up when they don’t. The completely lying flat operating principle allows consistent brightness at all angles, so VA panels can maintain very good contrast. In addition, VA panel displays maintain consistent visibility at most angles without excessive color and contrast deviations. However, VA panels require the liquid crystal to lie down completely, so the response time is relatively much worse. Subsequent technology upgrades in MVA/PVA have also improved the response time and viewing angle, but at the expense of brightness.

IPS

As the most widely used LCD panel, it has the advantage of maintaining consistency and high color accuracy from all angles, and does not distort when touched, and can cover more colors. However, IPS has the longest response time of the 3 types of panels, and has the problem of light leakage. The latest IPS technology is IPS Black, which mainly improves the contrast of the IPS panel, that is, it will look a little darker.

Overall, there are pros and cons to each of these 3 panels, but mainstream monitors are still mostly IPS. LCD characteristics determine that it is impossible to completely block the backlight, which is the main reason why LCD still does not look completely black when displaying black, and the contrast ratio is relatively low.

Color filter

The last layer of the LCD is also the closest to our layer is the color filter, the color filter is usually composed of red, green and blue, the white backlight through the liquid crystal molecules, and then irradiate through the filter to get the color we want to present.

New backlight technology: zoned backlight, mini-LED and microLED

With Dolby Vison and HDR sources becoming popular on the consumer side, the ability of monitors to display HDR content has also become a part of the purchase criteria for enthusiasts. HDR content is characterized by brighter brightness, higher contrast and a wider color gamut. The color gamut and brightness are essentially to improve the backlight layer and color filter, but the contrast is not so good to deal with, the VA screen with the best contrast ratio can only reach 4000:1 contrast ratio in the best case. In contrast, the human eye can perceive the brightness range between 10^-2 to 10^6 (the unit here is cd/m^2, more commonly known as nit) and can feel the range up to 10^8. So the contrast ratio of the screen is far from adequate when displaying HDR content.

The essence of the problem of insufficient contrast lies in the fact that – the LCD can not completely block the backlight. The solution to this problem is actually very simple, we just need to let the backlight according to different areas of dynamic adjustment of their brightness on it, which is the principle of zoning backlight (Local Dimming).

Partitioned backlighting does not work by adjusting each LED individually, but by dividing all LED bulbs into groups and then dynamically adjusting the brightness of the bulbs in each group, each group being called a partition. In theory, it is also possible to adjust each LED individually, but the current display driver board does not have the power to do such detailed adjustment work, so it can only be adjusted in zones.

Each backlight area is responsible for the brightness of a certain display area of the screen, so that it can do the bright place enough volume, the dark place enough dark, so that the contrast ratio of the screen can be from the ordinary IPS 1000:1 into an exaggerated 1000000:1, enough to meet the requirements of HDR.

Of course, zoned backlighting has its own limits, and if you encounter an object that is smaller than the zoned area – such as a star in the night sky – there is no way for the zoned backlighting to continue to pull out the contrast. And if one area is lit, but the adjacent area is not, the halo will be seen because the LCD does not completely block the light.

There are many more mini-LED panel displays on the market today, such as Apple’s iPad Pro. mini-LED is still based on backlighting, but the size of the individual bulbs has been significantly reduced, which improves the fineness of zonal light control and reduces the impact of light leakage.

Currently, display manufacturers are already working on mini-LED panels that can individually control the light bulb, which are called active mini-LEDs and are expected to have good halo control as well as higher contrast.

On the basis of mini-LED, micro-LED further, not only the size of the bulb is reduced to the pixel level, and each color sub-pixel can be independently controlled, after a long time of use will not be like OLED organic decay, but the current micro-LED is very expensive, so it has not entered the consumer market.

Applications of Quantum Dot

In addition to these LCD implementations, there is also an LCD that uses quantum dot technology, which is sometimes referred to by Samsung as QLED screens.

Quantum dot (quantum dot) color principle is not complicated. The so-called quantum dots, on an extremely small inorganic nano-semiconductor crystals, different volume of quantum dots in the stimulation of electricity or light will emit a different color. Since the light emitted by quantum dots is highly pure monochromatic light, people use it in TV and monitor.

There are two ways to implement quantum dot LCD, one is to use quantum dots directly as backlight, and the other is to use quantum dots as color filters. Either way, one uses the blue color to stimulate the light source for the quantum dot light emission because the blue color is relatively high energy and can be used as the blue sub-pixel of the three RGB word pixels.

There is a special term to indicate the practice of using quantum dots as backlight: quantum dot film (QDEF), first of all, it needs the whole blue backlight, then the blue light shines to the quantum dot film layer located in front, part of it is converted into red light, green light, and part of the blue light is directly through this layer of quantum dot film, the three finally mixed together into the “purest”, “ideal” white light. These white light in through the liquid crystal molecules to illuminate the color filter located at the forefront, composed of color on the screen. This mixture of red, green and blue light will make the white light spectrum more pure, so the color gamut can be made wider, and the display brightness can be made higher.

Quantum dots as color filters are called quantum dot color filters (QDCF / QDCC), which are located in the panel, the whole blue backlight through the liquid crystal layer to illuminate the front of the quantum dot color filter, the color filter to add different sizes of quantum dots according to the specific situation of red pixels and green pixels, and as for the blue pixels are left directly empty, so that The blue color can be directly transmitted. So under the excitation of the blue backlight, red, green and blue can be excited to a high purity color, which can enhance the color gamut, brightness and viewing angle of the display to a very exaggerated level.

However, this kind of quantum dot color filter may encounter a problem, that is, quantum dots will be from the front of the monitor ambient light excitation, resulting in normal use of the environment of the black purple. And the cost of this monitor is very high, for consumers is not worth buying is also another issue.

Published by YooCare Editor & last updated on November 14, 2022 9:35 am

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