Twisted nematic liquid crystal display 2(3)
	Electric field effects in liquid crystal cells   The ability of the director to align parallel with an external field is caused by the electric nature of the molecules. Permanent electric dipoles result when one end of a molecule has a net positive charge and the other end a net negative charge. When an external electric field is applied to the LC material, the dipole molecules tend to orient themselves along the direction of the field. Even if a molecule does not possess a permanent dipole, it can still be influenced by an electric field. In some cases, the field produces slight re-arrangement of electrons in molecules such that an induced electric dipole results. Whilst not being as strong as permanent dipoles, orientation with the external field still occurs. The effects of magnetic fields on LC molecules are analogous to electric fields. Since magnetic fields are generated by moving electric charges, permanent magnetic dipoles are produced by electrons moving within the atoms. When a magnetic field is applied, the molecules will therefore tend to align with or against the field.       Applying voltages to liquid crystal cells   A transparent conducting oxide film over the inner-surfaces of the substrates of the cell allows an electric field to be applied to the liquid crystal material and the cell can be switched between an optically active (OFF state) and an optically inert state (ON state). The layer used for the electrically conducting material is typically 2-20% tin-doped, indium oxide (ITO).       Linear polarisers   Light emitted from an incandescent lamp or from the sun is randomly polarized, i.e it includes waves that are oriented in all possible random directions. A polariser is an optical component that is used to convert randomly polarised (unpolarised) light into a polarized one. Here, only light that is polarised in one specific orientation is transmitted by the polarising component and light polarised in the opposite (perpendicular) sense is absorbed.   With reference to the diagram below, light polarised with the electric vector, E2 oriented along the x-axis is absorbed, whilst light polarised with the electric vector, E1 oriented along the y-axis is transmitted. The light that exits the second polariser is the component of the original wave that was polarised in the y-direction. In this example, the polarizing direction is defined as being along the y-axis.     Liquid crystal displays are used together with polarising sheets consisting of long, organic molecular chains uniaxially stretched in one direction so as to give uniform molecular alignment. Light oscillating parallel to the molecular axis suffers partial absorption from loosely bound electrons that are free to move along the chemical chains, whilst radiation vibrating perpendicular to this axis passes through unaffected. This results in initially unpolarised light becoming predominantly linearly polarised upon traversing through the polarising medium.   The two parameters that define the overall performance of a particular material are the polarising efficiency, defined as the percentage of light that becomes polarised along the major transmission axis of the device, and the total transmission throughput of the material for unpolarised incident radiation. An inescapable consequence of polariser operation is that a material possessing a high polarising efficiency invariably has a low overall optical transmittance and vice versa. Typical values for the polarising efficiency lie between 80% and 99.9%, whilst that for the total optical transmission lies between 50% and 30%, respectively.   Maximum light extinction is obtained from two polarisers positioned with their transmission axes lying perpendicular relative to each other and as this angle is reduced, the total transmittance of the system rises, reaching a maximum for when the polarisers are aligned parallel. In general, the overall optical throughput, I(q) of a pair of polarisers is given by Malus’ Law, where I(q) = I(0) cos2q. Here, I(0) is the maximum transmission of the device with parallel polarisers and q the angle between the transmission axes of the two sheets.   The liquid crystal material in an LCD controls the polarization state of the beam of light polarized by the front polarizer (entrance polariser). Controlling the orientation of the liquid crystal molecules will change the polarization of the light beam. As the polarized light from the front polarizer passes through an un-energized liquid crystal display, it will be rotated by 90o due to the liquid crystal material and be either absorbed or transmitted by the rear (exiting) polarizer. When the display segment is energized, the liquid crystal molecules will align with the electric field and no longer rotate the polarized beam of light.   Depending upon the direction of the exit polariser (analyzer), the light will either be absorbed or transmitted. In the case of a positive image reflective display, this generates a dark character on a light background. It is important to note that twisted nematic liquid crystal displays (TN-LCD’s) employ a polarizer applied to both the front and rear surfaces of the display in order to function as useful devices.   Positive image reflective liquid crystal displays possess a polarizer applied to the front of the display and a reflector applied to the rear of the display. Light entering the front of the liquid crystal display is first polarised by the entrance polariser, then passes through the liquid crystal material where it is reflected by the rear reflector back through the liquid crystal material again, thereafter reaching the front polariser once more. Whether or not the light will escape though the front polariser will depend upon the orientation of the polarisation of such light. This in turn is controlled by the state of the liquid crystal material inside of the liquid crystal display (LCD).     Operation of a twisted-nematic liquid crystal display (TN-LCD)   If a 90° twisted-nematic (TN) liquid crystal cell is placed between crossed polarisers, in the limit of large cell thickness the liquid crystal material is able to rotate the plane of polarisation and the device will be transparent. The cell is thus able to modulate light in a similar way to a half-wave-plate. Here, the cell is said to be operating in the normally white mode and possesses a high transmittance when in the inactivated state (OFF state). Positioning of the liquid crystal cell between parallel polarisers gives rise to the normally black mode where a low transmission state is obtained in the absence of a stimulating voltage (OFF state).   The illustration below shows light passing through a liquid crystal cell placed between crossed polarisers operating in the normally white mode both in the inactivated (light) and activated (dark) states.         Liquid crystal material in inactivated (OFF) state rotates the plane of polarisation of light passing through the liquid crystal cell and hence the exiting light is polarised in the same orientation as the exit polariser. The liquid crystal cell therefore transmits the light and appears to be bright.          Liquid crystal material in the activated (ON) state is aligned parallel to the electric field vector and is therefore unable to rotate to plane of polarisation of light passing through the device. The exiting light is therefore polarised in a direction that is perpendicular to the exit polariser. The liquid crystal cell is therefore unable to transmit the exiting light and the device therefore appears to be dark.     Twisted nematic liquid crystal displays (TN-LCD’s)   In order to obtain a display device, the transparent conducting layers (ITO) on both substrates of the liquid crystal cell are patterned into the required display pictures (segments) so that applied voltage only activates the liquid crystal material bounded by the overlap regions of the ITO on both substrates, hence forming the picture elements.   The illustration below shows how the conducting layers can be patterned on both substrates so as to show a numerical digit. In order to be able to display different numbers, voltage is directly applied to the contacts of each segment area independently.         In the diagram below, there are eight (8) individual contact-pads on the edge of the liquid crystal display device denoted (a) – (g) & COM. The COM contact-pad activates the back-plate (common plane) of the LCD via use of a common crossover which is a conducting connector inside of the LCD that electrically connects the two (2) substrates together.   Application of voltage to the individual contact-pads on the front-plate (segment plane) switches the individual segments of the liquid crystal display, denoted (a) – (g).            By careful patterning of the conducting layers (ITO) covering the two substrates of the liquid crystal cell, a variety of different picture elements can be obtained. An icon is a word or special symbol that is driven as a single segment. In the following diagram, the symbols DAYS, AM, PM, AUTO etc. are turned ON & OFF as complete individual segments so that only a single electrical contact-pad is required for the entire word or picture. Furthermore, an icon can be created from a simple text font, or created via use of special graphic images using any popular drawing package. A company logo or picture can be scanned in order to create custom icons.       Click here to continue to page three of this section.                    © Stephen Palmer 2008