About LCD

According to AbbreviationFinder, LCD stands for Liquid Crystal Display.


In low-cost equipment, such as calculators, “common plane” liquid crystals are used, as they always display the same type of information.

Active matrix and passive matrix LCD screens are used on computer screens or larger screens. In the first case, an electric current is passed through a mesh of conductors above and below the liquid crystal plate. In this way, at the point where the electric charges meet, the small liquid crystal “untwists”, allowing the light that comes from the bottom to pass through.

Passive matrix technology has, however, two serious disadvantages: signal response time and poor voltage control. The former implies that when rapid movements are made in the image, for example, when moving the mouse pointer from one place to another, “ghosts” are seen on the screen, following the pointer.

Also since the voltage control is imprecise, the pixels surrounding an activated point may be receiving some electrical charge, resulting in images with little contrast. For their part, active matrix LCD screens have transistors and capacitors for each point or pixel, which facilitates greater control of which liquid crystal is activated and which is not, as well as greater precision in the degree of polarization of each liquid crystal. reaching up to 256 degrees of brightness per pixel.

LCD screen

In high resolution color devices such as modern LCD monitors and televisions use an active matrix structure. A matrix of TFTs (thin-film transistors: ‘ thin-film transistors ‘) is added to the polarization and color filters. Each pixel has its own dedicated transistor, which will allow each line in the column to access one pixel. When a row line is on, all lines in the column are connected to a row of pixels and a correct supply voltage is driven to all lines in the column.

When the row line is deactivated, the next row line is activated. All lines in the row are activated sequentially during an update operation. The active matrix is ​​aimed at devices with a higher brightness and size than the passive matrix (aimed at devices of small size, and, in general, that have faster response times, producing much better images).

Colors of an LCD screen

For the LCD screen to display colors and variants thereof, each pixel is required to contain three sub-pixels, one for each basic color (red, green, and blue). If each liquid crystal sub-pixel in the active matrix can have 256 different levels, the number of possible colors is 256³, that is, 16.8 million. And such a range requires a significant number of electronic components in active matrix displays.

LCD projectors

There are two main types of projectors: reflective, where the image is reflected, enlarged and projected on a screen that is part of the same device (some large-size televisions use this system); and the frontal ones, where the light of a high intensity lamp passes through a liquid crystal, to be projected on a wall or white surface, in a very similar way to how equipment operates in movie theaters.

In both families of projectors a group of lenses magnifies the image and focuses it on the screen. In LCD projectors, a light source is used to illuminate a liquid crystal panel. The light passes through the panel through those pixels that are active, under the same operating principle as computer screens. The beam of light then passes through the set of lenses to focus on the projection screen. As can be seen in this case, the small LCD panel inside these projectors acts as a transparency.

Operation of the liquid crystal display

Most of the chemical compounds go from a solid state to a liquid state, melting at a defined temperature. However, there are some exceptions. In 1888, Friederich Reinitzer, an Austrian botanical, found that an extract Plant called cholesteryl bezoato passed to the melt, a cloudy phase within a temperature range over 30 ° C.

Scientists have since discovered that this phase called “liquid crystal” occurs in various compounds in different forms. In a solid crystal, the atoms or molecules are “ordered.” This means that they are located in a regular three-dimensional shape, forming the lattice of the crystal, with their atoms or molecules in a fixed position and with specific orientations.

When the crystal melts, this order disappears, and the substance goes directly into a liquid phase. By contrast, a liquid crystal exhibits an intermediate phase in which there is a partial order, within a temperature range, before moving to the liquid phase.

In liquid crystals (more commonly used in watch, calculator and computer screens), the disorder in the liquid-crystal phase consists of molecules out of position but with the same orientation as in the solid state. This is known as the “nematic” phase and is found in liquid crystals, such as alkyl, alkoxy, and cyanobiphenols, which contain elongated molecules oriented parallel to each other.

The propagation of polarized light when it passes through the liquid crystal (which in turn determines the orientation of the polarization planes), will depend on the polarization angle: the light is polarized parallel to the liquid crystal or at a right angle to the axis of the molecule. The ability of a compound to rotate the polarization planes, which can be manipulated with an electric field, is the basis for the operation of the liquid crystal used in different devices.

A liquid crystal display consists of a thin layer of liquid crystal, about 10 microns thick, placed between two glass plates. The area of ​​the plates determines the size of the apparatus: the largest, used by television screens, have a diagonal length of 360 millimeters.

Manufacturers evaporate transparent electrodes on the inner surface of glass plates, and thin layers of “polyamide” on the electrodes, to form the cell of the liquid crystal apparatus. The polyimide layers are specially treated to orient the liquid crystal molecules so that, in parallel planes, those on one side of the cell are located at 90 degrees to those on the opposite side. This is known as a “twisted nematic” cell, because the molecular orientation is “twisted” from one side of the cell to the other.

Finally, the polarizing filters flank the cell with their polarization axes in parallel. This means that the light is polarized as it enters the cell and escapes through the other side only if its plane of polarization is sufficiently rotated at a right angle.

When the electrodes do not apply an electric field, the twisted orientation of the molecules rotates the plane of polarization of the light and the light passes through the cell. On the contrary, when an electric field is applied the molecules are forced to be arranged parallel to it and parallel to the direction of the light. This means that the plane of polarization, which is at right angles to the polarization of light, is not affected by molecules: and light cannot pass through the cell.

In watches, the polarizing filter located behind the cell (known as the analyzer) is generally combined with a reflector, so that the apparatus can be seen from the same side of the incident light.

LCD Liquid Crystal Display