How can we build a display with liquid crystals?

All the interesting characteristics explained in the previous sections make liquid crystals attractive for science to seek for useful applications. A well-known application of liquid crystals is the ubiquitous liquid crystal display, now comprising a world-wide billion dollar industry. All ingredients necessary for building Liquid Crystal Display (LCD) were treated and this section will briefly explain how LCD's work.

Construction of a liquid crystal display

On the picture below, a schematical drawing of an LCD is shown. The display consists of a thin liquid crystal layer (thickness ± 5 µm) sandwiched between two glass substrates. Principle of a twisted nematic LCD For control of the reorientation of the director, transparent electrodes are deposited on the glass substrates (white and red). In the picture, a so-called passive matrix is used. On the top and bottom glass substrates, row and column electrodes are respectively deposited. These are long parallel lines of electrodes, with a perpendicular direction on the top and bottom glass surface. The overlap between a row and a column electrode forms a single pixel (= picture element) of the display. In this simple example, 1 row electrode is used in combination with 2 column electrodes. This gives us a two-pixel display.

The left column electrode is at the same potential level as the row electrode. To the right column electrode (red), a different voltage is applied. In this way, an electric field is generated in the right pixel oriented perpendicular to the glass surfaces.

On the picture one can see that the rubbing direction of the alignment layers (green) on top and bottom substrate are chosen perpendicular to each other. Due to this choice, the director in the left pixel makes a homogeneous turn of 90° from bottom to top. Therefore, this type of LCD is called a 'Twisted Nematic LCD' (TN-LCD). If a voltage is applied to the electrode, the director reorients to become perpendicular to the surfaces (right pixel).

In order to control the intensity of the transmitted light, the whole stack is sandwiched between crossed polarizers (yellow). If unpolarized light enters the structure from below, the light becomes linearly polarized at the bottom polarizer. The light enters the liquid crystal layer with a polarization parallel to the bottom director. One can prove that if the thickness of the layer and the liquid crystal parameters are chosen appropriate, the linear polarization of the entering light will roughly follow the rotation of the director (= Mauguin regime). As a result the light is transmitted through the top polarizer and the pixel is in its bright state.

If we apply a voltage between the two electrode layers, the liquid crystal director is reoriented and the polarization of the light will no longer rotate through the liquid crystal layer. As a result, the light is absorbed at the top polarizer (= the analyzer) and the pixel becomes dark.

Adding the appropriate color filters makes a complete full color display as used in many liquid crystal displays nowadays. Each pixel is on its turn subdivided into three pixels for controlling respectively the red, green and blue components of the light.

Of course this is only one of the possibilities to build LCD's. Many other configurations exist using other types of liquid crystals, other molecule orientations or other manners to switch the molecules. The most common ones are the 'Super Twisted Nematic LCD' (with a director rotation in the layer > 90°) and the 'In-Plane Switching LCD' (molecules rotate parallel to the glass surface due to a horizontal electrical field).

Applications

Originally, LCD's were used in calculators or digital watches and had only a few black-and-white pixels. Nowadays, LCD's are widespread in all kinds of applications such as flat panel displays for desktop applications or notebooks, mobile phones, projectors, ... A few examples of devices using LCD-technology are illustrated in the pictures below.

Applications where liquid crystal displays are used


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Liquid crystal tutorial written by Chris Desimpel, based on the introduction of his Ph.D. thesis and literature.