Flat panel monitors on liquid crystals
Liquid crystals (LC)
These include fluids that have an anisotropy characteristic of solid crystals in a certain temperature range. Anisotropy or the dependence of physical properties on the direction, is due to the cigar-like shape of LC molecules and the ordering of their arrangement. The monitors use a change in the optical properties of the thermotropic (produced by melting) LCs under the action of an external electric field. LCD cells for flat screens of modern monitors are built on the basis of a twist effect in nematic LCs, which are characterized by an ordered
Fig. 8.3. The principle of the formation of a raster with the Δ-shaped arrangement of electron guns
volume orientation of molecules in the absence of external influences.
Let us explain the essence of the twist effect on the example of a twisted Nematic cell, which is a thin layer of nematic LC enclosed between the upper and lower glass substrates (Figure 8.4), which are grooved in mutually perpendicular directions. The upper substrate is called the polarizer, the lower - analyzer. The polarization vectors of the polarizer (E n ) and the analyzer ( E) are deployed by 90 °. Due to the rotation of the vectors E n and E n, in the absence of an external electric field, a molecule structure is created with a twist of 90 ° between the upper and lower layers of molecules (Figure 8.4, a) ). Such a structure is called the twisted structure of molecules.
With the absence of an external electric field, the incident light wave with arbitrary polarization passes through the upper substrate and acquires the polarization E,
Fig. 8.4. Explanation of the essence of the twist effect
coinciding with the direction (E a ) of the longitudinal axes of the molecules of the upper layer of the twisted structure. The twisted structure of molecules transmits a light wave, changing its polarization. When the lower layer of molecules is reached, the electric field vector E unfolds by 90 °. Since its direction coincides with the direction of polarization of the analyzer E a, the light wave freely passes through the lower substrate.
With the presence of an external electric field with a strength E b 0.3 V/μm, the molecules of the liquid crystal material change their orientation. Their longitudinal axis is parallel to the vector of the external electric field E b (see Figure 8.4, b). The twisted structure of the molecules disappears. When a light wave passes through a LC layer, its polarization is conserved. The vector of the electric field strength E of the light wave is rotated by 90 ° relative to the polarization vector of the analyzer E a. The luminous flux through the lower substrate does not penetrate.
The Twisted Nematic technology considered has several drawbacks:
• Low contrast, brightness and saturation of the image and their strong dependence on external light;
• a long time (up to 500 ms) changes in the structure of LC molecules, which does not allow displaying dynamic images on the screen;
• Limited angle of visibility of the luminous image, etc.
Improvement of LCD cells was along the way:
• increase in the angle of twisting of twisted structures up to 270 °. Such cells are called super-twisted nematic cells, and technology - Super Twisted Nematic (STN). According to the LC property, an increase in the angle of twist of the molecules increases the contrast;
• combining two cells with simultaneous twisting of molecules in opposite directions - Dual Super Twisted Nematic (DSTN) technology;
• Dual screen scanning, which allows to increase the performance of LCD cells. In this case, the screen is divided into even and odd lines, which are updated at the same time. Such scanning together with the use of more mobile molecules allowed to reduce the reaction time of the LCD cell to 150 ms and significantly increase the refresh rate of the screen;
• Use of Thin Film Transistor (TFT) technology for individual cell control of built-in thin-film transistors. Transistor wrench with a voltage of about 0.7 V allows you to switch the voltage to tens of volts. This technology was called technology of active cells. It allowed not only to significantly increase the brightness and contrast of LCD monitors, increase the angle of view, but also to create a color monitor based on the active LCD matrix. The element of such a matrix contains three LC cells, each of which is provided with a light filter of one of the three primary colors and is controlled by a thin-film transistor. By changing the level of the control signal applied to the transistor, it is possible to adjust the brightness of each triad cell.
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