LCD (Liquid Crystal Display) technology has been one of the most important methods of displaying images in electronic devices for many years. Despite the rapid development of alternative technologies such as OLED and MicroLED, liquid crystal displays continue to dominate many applications, particularly in industrial, medical, automotive, and measurement systems. This is due to the maturity of the technology, high reliability, and the ability to tailor display parameters to the requirements of a specific application. Understanding the construction and operating principles of LCDs is fundamental to analyzing the differences between panel technologies such as TN, IPS, and VA.

What Are the Main Components of an LCD Display?

An LCD display (Liquid Crystal Display) consists of several interacting layers that work together to produce an image. The main components include two polarizers, two glass substrates, a liquid crystal layer, an RGB color filter, an LED backlight unit (BLU), and display driver ICs.

The front and rear polarizers polarize the light passing through the display. They work together with the liquid crystal layer to control the amount of light reaching each pixel. The liquid crystal layer is located between two thin glass substrates. When an electrical voltage is applied, the liquid crystals change their optical properties, controlling light transmission. The nature of this change depends on the panel technology used, such as TN, IPS, or VA.

In active-matrix TFT displays, color reproduction is provided by the RGB color filter, which contains red (R), green (G), and blue (B) subpixels. By controlling the brightness of individual subpixels, the display can reproduce a wide range of colors.

Because liquid crystals do not emit light, an LED backlight unit (BLU) is required. Its purpose is to illuminate the display area uniformly. Viewed from the rear of the module, the backlight structure includes a reflector sheet, a Light Guide Plate (LGP) with LEDs positioned along its edge, and a diffuser sheet that distributes the light uniformly across the panel.

The display is controlled by dedicated integrated circuits responsible for driving individual pixels and communicating with the display controller. These ICs receive image data and generate the control signals required by the LCD panel.

During operation, light generated by the backlight unit passes successively through the rear polarizer, liquid crystal layer, RGB color filter, and front polarizer. Controlling light transmission through the liquid crystal layer makes it possible to produce an image with the required brightness, contrast, and color reproduction.

What Are the Advantages of Using LCD Displays?

The popularity of LCD technology is primarily due to the favorable combination of image quality, production cost, and durability. Compared with many alternative display technologies, LCDs offer long service life, low power consumption, and high parameter stability over many years of operation.

Another important advantage is the ability to manufacture displays with a wide range of sizes, resolutions, and brightness levels. As a result, LCD technology is used in applications ranging from simple measurement instruments to advanced operator panels, medical systems, and industrial automation equipment.

Thanks to its relatively straightforward construction and low energy consumption, LCD technology has become one of the most widely used solutions for displaying images in electronic devices. LCDs are found in consumer electronics such as televisions, monitors, laptops, and smartphones, as well as in industrial, medical, and automotive equipment and automation systems. Their broad range of applications results from the ability to adapt parameters such as resolution, brightness, contrast, and viewing angles to the requirements of a specific application.

 

LCD Display Structure

Source: own elaboration.

How Does Crystal Orientation Affect Light Transmission?

Liquid crystals represent an intermediate phase between the liquid and solid states. They combine characteristics of both states: their molecules maintain an ordered orientation characteristic of solids while retaining the ability to move, as in liquids. Liquid-crystalline properties are exhibited only by molecules with anisotropic shapes, most commonly rod-like or disk-like structures. Depending on how the molecules are ordered, several liquid-crystal phases can be distinguished. In LCD technology, the nematic phase (Fig. 4) and its variants are of particular importance. In this phase, the molecules are oriented approximately in the same direction while retaining freedom of movement, allowing their optical properties to change under an applied electric field.

 

Change in Liquid Crystal Molecular Ordering as a Function of Temperature

Source: own elaboration.

Temperature has a significant effect on the phase of liquid crystals because increasing thermal energy changes the degree of molecular ordering. At low temperatures, the material exists in a smectic phase (Figs. 1, 2, and 3), characterized by a high degree of order. As the temperature rises, it transitions into the nematic phase (Fig. 4), in which the molecules retain a common orientation but can move more freely. Further temperature increase leads to the isotropic phase (Fig. 5), corresponding to the ordinary liquid state, in which the molecules lose their orientational order. This phase occurs above the liquid crystal clearing temperature; above this point, the LCD no longer operates correctly. LCD technology primarily uses the nematic phase because it allows the orientation of the molecules to be changed by applying an electrical voltage, thereby controlling the amount of light passing through the display.

The operation of LCDs is based on the ability of liquid crystals to change the polarization of light. The liquid crystal layer is positioned between two electrodes and two polarizers. The orientation of the molecules determines how they interact with polarized light and therefore how much light passes through the display. With no applied voltage, the molecules are arranged so that they rotate the plane of polarization, allowing light to pass through the second polarizer. When an electrical voltage is applied, their orientation changes, so the light is no longer rotated in the required manner and is partially or completely blocked by the second polarizer. Precise control of the voltage applied to individual pixels makes it possible to regulate the amount of transmitted light and, together with the RGB color filter, produce the required brightness, contrast, and full-color image.

 

LCD Operating Principle – Effect of Liquid Crystal Molecular Orientation on Light Transmission

Source: own elaboration.

 

This mechanism forms the basis of operation of modern LCDs. Differences between panel technologies such as TN, IPS, and VA result primarily from the way liquid crystal molecules are arranged and behave when an electrical voltage is applied.

What Role Does Voltage Control Play Here?

The operation of LCDs is based on controlling the orientation of liquid crystal molecules using an electric field. Each pixel in the panel is connected to electrodes between which an appropriate voltage is applied. The voltage level determines the orientation of the liquid crystal molecules and therefore the amount of light transmitted through the given pixel. Control is not limited to two discrete states. Different voltage levels allow different levels of light transmission, which in turn produce different brightness levels for each subpixel. Combined with the RGB color filter, this enables the reproduction of millions of colors and smooth tonal transitions.

In modern TFT-LCD displays, voltage control is implemented using thin-film transistors (TFTs), with each transistor assigned to an individual subpixel. The transistor acts as an electronic switch, charging the liquid crystal cell capacitance to a specified voltage during image refresh. After addressing is completed, the voltage is maintained until the next refresh cycle, ensuring stable image reproduction and consistent subpixel brightness.

The way in which a change in voltage affects liquid crystal molecular orientation depends on the panel technology. In TN panels, the molecules rotate toward a direction perpendicular to the display surface. In IPS technology, they rotate within a plane parallel to the glass surface while in VA panels, the molecules are nearly vertically aligned in the resting state and tilt when voltage is applied. These differences affect display parameters such as viewing angles, contrast ratio, response time, and color reproduction.

Summary

The operating principle of all LCDs is based on controlling the amount of light transmitted through the liquid crystal layer using an electric field. Changes in molecular orientation affect the polarization of light, enabling precise control of the brightness of individual pixels and the formation of an image. Although the fundamental operating principle is the same across LCDs, differences in the arrangement and movement of liquid crystal molecules result in different panel technologies, such as TN, IPS, and VA. Their construction determines key display performance characteristics, including contrast ratio, viewing angles, response time, and color reproduction.

 

For more information about LCDs and their potential applications in your devices, please contact Dawid Tutlewski: [email protected] / +48 886 223 908.