Erica Bramley

• Introduction: Twisted Nematic Displays
• Why TN Displays Alone are Unsatisfactory
• How Compensation Films Improve TN Displays

The twisted nematic (TN) display is made up of a nematic liquid crystal which has an optic axis that undergoes a 90° twist, in between two substrates. Each substrate is coated with a conductive transparent electrode, indium tin oxide (ITO), which is then coated by a surfactant that is rubbed parallel to the substrate to create groves for the liquid crystal to align in. The optic axis of the liquid crystal undergoes this twist because the two substrates are placed so the rub directions are at 90° angles to one another. In the normally white (NW) mode of operation the optic axes of the polarizers along each side of the substrate are crossed. It is called normally white because in the unactivated state the display is transparent. In the normally black mode of operation the optic axes of the polarizers are parallel. In the unactivated state the display is black.

The performance of TN displays are measured by the contrast ratio, which is the luminance of the bright state divided by the luminance of the dark state. In the ideal cell the luminance of the bright state would be high (transparent) and the luminance of the dark state would be low (black) and this would be true for a large percentage of the viewing angles. The viewing angle depends on the distance of the observer from the display and the angle that the display is looked at. For a display approximately 3 m from the observer and the display width 2 m, the maximum viewing angle needed would be approximately 50° , if the observer is in the center of the display. For NW displays the luminance of the bright state is approximately constant for changing viewing angles but the dark state has a lot of light leakage off axis, therefore the contrast ratio varies greatly as a function of viewing angle.

As light propagates along the liquid crystal optic axis it is broken up into two normal modes which are generally elliptically polarized with the major axis of the polarization ellipse parallel to the nematic director. Only light that is linearly polarized propagates through the polarizers. Since the TN-cell has polarizers on each side a successful cell must have linearly polarized light coming out of it. To achieve this the retardation of the cell (the birefringence times the distance through the cell, D nd) must be much greater than the wavelength of the light divided by two. This condition is satisfied by the liquid crystal material used, which determines D n, and by the thickness of the cell. Once this condition is satisfied the bright state of the TN-cell in NW mode has a very high luminance, but the dark state does not have the low luminance expected. This is because the birefringence itself is dependent on the angle that the light propagates through the cell. Liquid crystals have two indices of refraction, an ordinary (n_o) and an extraordinary (n_e’) and the birefringence is, D n = n_e’ - n_o. The ordinary index of refraction is the same for all angles of the incident wavevector, k. The extraordinary index of refraction directly depends on the angle between the incident wavevector and the director of the liquid crystal. The bright state is virtually unaffected by the viewing angle.

Figure 1: The birefringence is dependent on the incident wavevectors angle.

The ideal TN-cell in NW mode would have an activated state with a uniform director that is normal to the substrates. The birefringence for the stated director would be zero, both n_o and n_e’ would have the value n_o at q = 0° . With this cell the polarization of the light would not change and the crossed polarizers would extinguish the output. But an actual TN-cell in NW mode has an activated director configuration that is uniform through only the center of the cell. The liquid crystal molecules near the substrates are at some angle to the normal of the cell due to the anchoring energy. The activated director configuration is illustrated below.

Figure 2: Twisted Nematic in the activated state.

To compensate for the change in birefringence due to the director pattern close to the substrate, a film is created that mirrors the director pattern of the activated TN-cell. This mirror image will change the birefringence in the opposite way the director does; therefore the phase difference of the light coming out of the cell-film combination will be the same as the phase difference of the light going into the cell-film combination. In principle, the ideal compensation film would have the exact director configuration of the TN-cell in the activated state. This can be achieved by two types of compensation layers a passive (no voltage) liquid crystal cell with a director that mirrors the activated state of the TN-cell or a polymer sheet. The disadvantage to the passive LC layer is the extra weight and thickness added to the display. Such a passive LC would also be more difficult to manufacture than the polymer compensation film. The disadvantage to the polymer films is that it is difficult to manufacture one that closely mirrors the TN director.

One film on the market that does come close to mirroring the TN director is the Fuji Wide View (WV) film. This film is made using a uniform splay of the director, which compensates for part of the TN director. The molecules used to create the film are discotic in shape and have a negative birefringence. A compensation film is placed on each side of the TN-cell, therefore each film compensates for half the cell (refer to figure 3). Some draw backs to the Fuji WV film is that the director pattern of the film does not have a twist to it like the director pattern of the TN and the splay of the director pattern is over a smaller range of angles. The first can’t be changed while the range of angles can be adjusted.

Figure 4: Splay compensation films on either side of a twisted nematic in the activated state.