What is an LCD screen Introduction to LCD technology

It stands for “Liquid Crystal Display”. LCD is a flat panel technology commonly used in televisions and computer monitors. It is also used in screens for mobile devices such as laptops, tablets and smartphones.

LCD screens not only look different from bulky CRT (cathode ray tube) monitors, the way they work is also significantly different. Instead of shooting electrons at a glass screen, an LCD screen has a backlight that provides a light source to individual pixels arranged in a rectangular grid. Each pixel has an RGB (red, green, and blue) sub-pixel that can be turned on or off. When all the sub-pixels of a pixel are off, it appears black.

When all sub-pixels are 100% on, it appears white. By adjusting the individual levels of red, green, and blue light, you get millions of color combinations.

How are LCD screens built?

An LCD screen includes a thin layer of liquid crystal material sandwiched between two electrodes on glass substrates, with two polarizers on each side. A polarizer is an optical filter that lets light waves of a specific polarization through while blocking light waves of other polarizations. The electrodes must be transparent, so the most popular material is ITO (Indium Tin Oxide).

Since the LCD screen cannot emit light by itself, a backlight is usually placed behind the LCD screen so that it can be viewed in a dark environment. Light sources for backlighting can be LED (light emitting diode) or CCFL (cold cathode fluorescent lamps). LED backlighting is the most popular. Of course, if you want to have a color screen, a color filter layer can be converted to an LCD cell. The color filter consists of RGB color. You can also add a touch panel in front of an LCD screen.

How do LCD screens work?

The first LCD panel technology in mass production is called TN (Twisted Nematic). The principle behind LCD displays is that when no electric field is applied to the liquid crystal molecules, the molecules rotate 90 degrees in the LCD cell. When light from ambient light or backlight passes through the first polarizer, the light is polarized and twisted with the liquid crystal molecular layer. When it reaches the second polarizer, it crashes. The viewer sees that the screen is black.

When an electric field is applied to the liquid crystal molecules, they untwist.  When polarized light hits the layer of liquid crystal molecules, the light passes straight through without being twisted. When it reaches the second polarizer, it will also pass, the viewer will see that the screen is bright.

Because LCD technology uses electric fields instead of electric current (passage of electrons), it has low power consumption.

A short Youtube video will explain how LCD screens work concisely and efficiently.

The basics of LCD screens

The most basic LCD introduced above is called a Passive Matrix LCD, which can be found mostly in low-end or simple applications like calculators, utility meters, vintage digital clocks, alarm clocks, etc.  Passive matrix LCDs have many limitations such as narrow viewing angle, slow response speed, darkness, but it is great for power consumption.

To improve the drawbacks, scientists and engineers developed active matrix LCD technology. The most widely used is TFT (Thin Film Transistor) LCD technology. Based on TFT LCD, even more modern LCD technologies are developed. The best known is the LCD IPS (In Plane Switching).   It has a super wide viewing angle, superior image quality, fast response, great contrast, less burn-in, etc.

IPS LCD screens are widely used in LCD monitors, LCD TVs, iPhones, pads, etc. Samsung even revolutionized the LED backlight to be QLED (quantum dot) to turn off the LEDs where light isn’t needed to produce deeper blacks.

Different types of LCDs

– Twisted Nematic Display: TN   LCD (twisted nematic) production can be done more frequently and use different types of displays in all industries. These screens are most often used by gamers as they are inexpensive and have a fast response time compared to other screens. The main disadvantage of these displays is that they have low quality, as well as biased contrast ratios, viewing angles, and color reproduction. But, these devices are enough for daily operations.

– In-Plane Switching Display:  IPS displays are considered the best LCDs because they provide good image quality, higher viewing angles, accuracy, and vibrant color difference. These displays are mainly used by graphic designers, and in some other applications, LCD displays need the highest potential standards for image and color reproduction.

– Panel Vertical Alignment: The vertical alignment (VA) panels fall anywhere in the middle between Twisted Nematic and in-plane switchgear technology. These panels have the best viewing angles as well as color reproduction with higher quality features compared to TN-type displays. These panels have a low response time. But, these are much more reasonable and appropriate for everyday use.

– The structure of this panel generates deeper blacks and better colors compared to the twisted nematic screen. And various glass alignments can allow for better viewing angles compared to TN-type displays. These screens come with a trade-off because they are expensive compared to other screens. And they also have slow response times and low refresh rates.

– Advanced Edge Field Switching (AFFS):   AFFS LCD displays offer the best performance and a wide range of color reproduction compared to IPS displays. AFFS applications are very advanced because they can reduce color distortion without compromising the wide viewing angle. Typically, this display is used in highly advanced and professional environments, such as in the cockpits of viable aircraft.

– Active and Passive Matrix Displays: Passive matrix type LCD displays work with a simple grid so that charge can be delivered to a specific pixel on the LCD. One layer of glass gives columns while the other gives rows that are designed using a transparent conductive material such as indium tin oxide. The passive matrix system has significant drawbacks, in particular the response time is slow and inaccurate voltage control. Screen response time primarily refers to the screen’s ability to update the displayed image.

– Active matrix LCDs mainly rely on TFTs (Thin Film Transistors). These transistors are small switching transistors as well as capacitors that are placed inside an array on a glass substrate. When the proper row is activated, a charge can be transmitted down the exact column so that a specific pixel can be addressed, because all additional rows that the column intersects are turned off, the capacitor next to the designated pixel simply receives a charge. burden. .

Benefits over other screens

LCD technologies have great advantages of low power consumption, thin and light, which made wall-mounted TVs, laptops, smartphones and pads possible. On its path to progress, it killed off the competition from many display technologies. We no longer see CRT monitors on our desks or plasma screen televisions in our homes. LCD technologies now dominate the laptop display market. But any technology has its limitations.

LCD technologies have slow response times, especially at low temperatures, limited viewing angles, backlight is needed. Focus on the drawbacks of LCD, OLED (Organic Light Emitting Diodes) technology was developed. Some high-end TVs and mobile phones are starting to use AMOLED (Active Matrix Organic Light Emitting Diodes) displays.

This cutting-edge technology provides even better color reproduction, clear image quality, better color gamut, and lower power consumption compared to LCD technology. Note that OLED screens include AMOLED and PMOLED (Passive Matrix Organic Light Emitting Diodes). What you should choose is AMOLED for your TV and mobile phones instead of PMOLED.