Twisted nematic liquid crystal display 3(3)
	Direct driving of twisted nematic liquid crystal displays (TN-LCD's)   When direct-driving a liquid crystal display (LCD), each individual segment or icon in the display has an individual contact-pad at the edge of the device. Application of voltages to the individual contact-pads directly switches the appropriate segments or icons in the display ON & OFF. Maximum contrast and viewing angle can be attained via direct driving of a TN-LCD. However, as the total number of display segments increases, direct-drive may become impractical due to the large number of separate drive circuits and external interconnections required.   In the following example, a single back-plane (shown in red) is common to every display segment and the separate segments are all individually connected to a unique contact-pad on the edge of the display. An individual segment is activated by applying the voltage between the common back-plane and the individual segment contact-pad. In this example, fourteen contact-pads are required for the fourteen individual segments and one additional contact-pad is required for the back-plane.         The equivalent electrical circuit in the above example is shown below. Here, the contact denoted BP is for the common back-plane and application of voltage to the individual contacts A, B, C, etc will individually switch ON the display segments.      The use of the direct driving scheme means that by using R+C individual contact-pads, R+C individual pixels or segments can be switched ON & OFF (addressed).       Multiplex driving of Super Twisted Nematic Liquid Crystal Displays (STN-LCD’s)   When using a direct-driving scheme, each individual segment or pixel present in the display possesses an individual electrical contact-pad. However, when the number of individual pixels becomes large, it is not possible to use an individual contact-pad for each individual segment. In such case, a multiplexing driving scheme is therefore used instead.   In this case, the segments or pixels in the display are organised into rows & columns and each row and column is connected to a single row or column electrical contact-pad. By applying suitable voltage pulses to the individual contact-pads for each row & column, the individual segments of the display can be turned ON & OFF.   The most commonly used design for multiplexing is the passive-matrix display, which consists of a matrix of horizontal rows & vertical columns, as shown below.             By rapidly applying suitable voltage pulses to the row & column electrode contact-pads, an image can be scanned into the display row-by-row. In order to facilitate this process, the twist angle present in the helical structure of the liquid crystal material is often increased to above 240 degrees, forming the so-called Super-Twisted Nematic (STN) Liquid Crystal Display (LCD).   The use of the multiplexing driving scheme means that by using only R+C individual contact-pads, R*C individual pixels or segments can be switched ON & OFF (addressed).       Transmission, reflection & transflective mode for TN-LCD   A liquid crystal display (LCD) does not generate any light itself, but merely modulates the light that passes through the device. An external light source is therefore required. Furthermore, an LCD can be operated in either transmissive or reflective mode. In transmissive mode, a light source is placed behind the liquid crystal display. Examples of LCD’s working in transmission mode are displays frequently used in dark environments such as displays for car stereos, etc. Reflective mode uses a reflector placed behind the liquid crystal display (LCD) and it is the reflected light that is observed. Most LCD’s in watches and calculators operate in reflective mode.            Transmissive displays possess clear polarisers on the front and the back. The display therefore relies upon light coming through from the back of the display toward the observer in order to be seen. Most transmissive displays are negative-image, and coloured filters are sometimes applied to different areas of the display in order to highlight different icons.   Reflective display use rear-polarisers that include a diffuse reflector. This layer reflects polarized ambient light that has entered through the front of the display and passed through the liquid crystal material to the back-plate. Reflective displays require ambient light in order to be observed and exhibit high brightness, good contrast and wide viewing angles. They are particularly suitable for use in battery operated equipment where an adequate level of light is always available. Reflective LCD's cannot be backlit; however they can be illuminated from the front in some applications.   Transflective displays are displays that can be both reflective & transmissive. Here, a rear polarizer is used that includes a translucent material which reflects part of the ambient light and also transmits backlighting. As the name implies, it is a compromise between the transmissive and reflective viewing modes. Used in reflection, it is not as bright and has lower contrast than that of the reflective type LCD, but it can be backlit for use in low lighting environments.      Driving voltages for liquid crystal displays (LCD)   Impurity ions present in the LC material such as alkaline-earth metals cause a leakage current to flow across the cell gap when a voltage is applied to the cell. This ion migration may destroy the helical stacking structure and initiate irreversible degradation chemical reactions. If driven with a DC voltage, impurity ions present may migrate towards the alignment layers under the action of the electric field and become embedded at the cell surfaces. Upon removal of the applied voltage thereafter, an electrical field across the liquid crystal may persist due to the captured charges and hence hinder cell switching.   For this reason, LC cells are usually driven with AC square-wave voltages of between ±3.0 and ±10 volt whereby the polarity is rapidly switched at speeds of up to 200Hz in order to prevent impurity ion migration from occurring. It may be expected that activation of the LC cell with AC voltage might cause the molecules to rotate. However, the single molecular model considered so far in which each individual LC molecule interacts with the applied voltage is somewhat simplified and in practice interactions between the LC molecules themselves must also be considered.   When in the activated phase, the bulk molecules are oriented by the applied electric field and interact with each other so that adjacent molecular dipoles are aligned anti-parallel. An individual bulk molecule is therefore screened or shielded by the surrounding molecular dipoles and hence is not directly subjected to the externally applied voltage but only that due to the local environment. Polarity reversal of the driving electronics will therefore have no effect upon the local environment of the screened bulk molecules and the performance of the device is dependent upon the root-mean-squared (rms) voltage and not on the polarity of the external field.   However, during polarity reversal, the liquid crystal cell must first be discharged before being charged up again in the opposite sense, resulting in the removal of the voltage across the liquid crystal material for a short period of time. This enables partial molecular relaxation back to the helical stacking structure to occur prior to the cell becoming fully charged again. The molecules are therefore able to vibrate with a frequency of twice that of the driving voltage, resulting in the overall optical transmittance of the device to oscillate.   The human eye takes time to adapt to new illumination levels and if the intensity changes are faster than that of the adaptation period, the eye acts as an integrator perceiving the time-averaged luminance and preventing cell flickering from being observed. The critical value at which flickering becomes apparent depends upon the level of luminance, amplitude, size and wave form of the variation, but typical values lie between 35Hz and 45Hz for a square wave driving pulse. Furthermore, since many artificial lamps operate at 50Hz or 100Hz, it is also necessary to avoid these frequencies to prevent stroboscopic effects from occurring.   Whilst preventing cell degradation, operation of the cell with an AC voltage has the disadvantage in that the cell, which approximates to a parallel plate capacitor, must continually be charged and discharged upon polarity reversal. This produces a large power consumption when in the activated state (ON state). In general, the power consumption is a linear function of the cell capacitance which, to a first approximation, is inversely proportional to the cell thickness, d. Reduction of the cell thickness together with an increase in the driving frequency hence produces an increase in the current consumption of the device and reduces the operating life of the battery.   The ionic transport mechanisms occurring within an LCD are also important. The simplified model of a liquid crystal cell being represented by a parallel RC-circuit assumes that the ionic concentration of the liquid crystal material as well as the mobility of the ions are constant. A refinement of this model uses three RC-circuits connected in series, one for each of the two polyimide layers and one for the slab of liquid crystal material. A serial resistor is also required in order to simulate the ITO contacting layers.     Response times of liquid crystal cells (LCD)   There are two switching speeds associated with a liquid crystal cell. The first involves switching the cell from the inactivated to the activated state upon application of an applied voltage and typically takes less than 10ms for the liquid crystal to react (activation switching speed). The second is the reverse process whereby liquid crystal relaxation occurs upon removal of the electric field and takes around 10 times longer (relaxation switching speed). Optical shutters such as welding-filters that require very fast response times from the light to the dark state are therefore restricted to using liquid crystal cells that operate in the normally white mode.   An inescapable consequence of liquid crystal operation is the rapid decrease of the switching speed at reduced temperatures due to an increase in liquid crystal viscosity.   There are many parameters that influence the switching speeds of liquid crystal cells. In particular, intrinsic properties of the liquid crystal such as viscosity and dielectric anisotropy play a major role. However, other factors such as cell thickness and the magnitude of the driving voltage also have a significant effect upon the final value.     Optical angular properties of liquid crystal cells (LCD)   A loss of cell contrast is obtained when liquid crystal (LC) cells are viewed at oblique inclination angles away from the surface normal. This is a general problem of all LC cells. The origin of the angular dependence of transmittance arises due to the angular variation of optical birefringence in an LC layer. This can be understood from the following illustration which shows a schematic of the LC material where the bulk liquid crystal molecules are only partially oriented with the applied field, whilst those at the surface remain relatively unaffected.   Here, two viewing angles are considered, one being approximately parallel to the bulk molecules, the other making a significant angle with the director axis. Light passing through the cell in a direction lying predominately parallel to the bulk molecular director experiences only one refractive index no , irrespective of the incident radiation polarisation. No retardation is therefore induced by the cell and the device is optically inert, appearing dark when viewed between crossed polarisers.   However, light impinging on the macroscopic structure with components that are oscillating parallel to the molecular axis are exposed to both refractive indices depending upon the direction of polarisation. Retardation between the ordinary and extra-ordinary rays therefore occurs and the cell possesses a degree of optical activity, giving rise to a high level of transmittance when viewed in this direction.       This generates an asymmetric angular transmittance for the liquid crystal cell with one side appearing more transparent whilst the other side retains a large degree of darkness. The optical angular properties of the liquid crystal cell can be described by using positions of a clock face as a co-ordinate system in order to define & describe the viewing field.               A twelve o'clock viewing angle means that the optimum direction is above the normal to the display, whilst a liquid crystal display with a six o'clock viewing angle is best viewed from below the normal. The viewing angle is set during the manufacturing processes and cannot be changed thereafter by rotating the polarisers, etc.   When specifying viewing direction for a specific application, it is important to consider how the display will be used. For example, a calculator is usually positioned on a desktop or held in the palm and hence viewed from the six o'clock direction. Some instrumentation like a wall temperature thermostat, may be mounted below the viewer and hence be viewed from the twelve o'clock direction. Other viewing angles are also possible but are less common. A car clock display for example, which will always be positioned to the driver’s right, may have a nine o'clock viewing angle, or possibly a ten-thirty viewing angle if the clock is low on the dashboard.   A polar chart can be used as an alternative representation for the optical angular properties (viewing properties) of a liquid crystal display. Here, polar co-ordinates are used in order to define the viewing angle together with a colouring scheme to show the transmittance for that specific viewing angle.     The typical optical angular properties (viewing characteristics) of a 90o twisted nematic (TN) liquid crystal cell are shown in the following figure. Here, the twisted nematic liquid crystal cell is operating in the normally white mode (placed between crossed polarisers) and is driven with 5 volts.         In a direct-drive twisted nematic liquid crystal display (direct drive TN-LCD), viewing angle is less critical because the angular properties of the display are relatively good. However, viewing angle becomes critical when the display is multiplexed; the higher the multiplex rate, the greater is the viewing angle issue. In displays with extremely high multiplex rates, care must be taken when designing the drive circuitry. Special films can also be applied to the front of the liquid crystal display in order to enhance the overall angular properties of the display. However, such films are usually relatively expensive. © Stephen Palmer 2008