DERA LCD views wider via voltage controlled twist

DERA LCD views wider via voltage controlled twistLCD incorporates a diffraction grating for the first time. Steve Bush. Research company DERA in Malvern has invented a new LCD mode (EW, April 1) which offers a wider viewing angle than traditional twisted nematic types. Called voltage controlled twist (VCT) it requires the incorporation of a diffraction grating into an LCD for the first time. Although unusual, this should not cause too many problems for LCD manufacturers, says co-inventor Guy Bryan-Brown, because makers of holographic equipment already have the technology. Diffraction gratings are normally used for their optical effects, but this is not the case in a VCT display. Indeed, optical effects are minimised by making the grating of a material with the same refractive index as the liquid it is in contact with. Optical effects disappear in the same way that rubies in port are invisible. In the display the grating acts as an alignment surface for the liquid crystals. This is more usually done by brushing the inner surface of the display glass, but a low energy surface is required for VCT and surface treatments that achieve this also mask the effects of brushing. The X-Y viewing angles of VCTare said to be ?60? and ?30?, against ?35? and +20? to -40? found in unmodified TN displays. This kind of wide viewing angle is found in in-plane switched displays which are more complex and less optically efficient. Modified TN displays are also capable of meeting wide viewing angles, again by adding complexity. How it works The display relies on negative dielectric anisotropic liquid crystals, ones that align across the direction of any applied electric fields. normal TNdisplays use positive dielectric anisotropic crystals which align with the field. Apart from the diffraction grating, a VCS display is of normal construction, with polarisers outside the cell, orientated at right angles to each other and electrodes either side of the liquid crystal cell. With no field applied (V=0 in diag) the crystal orientation is controlled by the surface treatment inside the cell. The top boundary has a high surface energy which causes the crystals to lay along it. Surface brushing aligns them in in a single direction (across the page in the diagram). The lower boundary is coated for low surface energy which makes crystals at the surface sit at right angles to it. Interaction between crystals away from the boundaries causes them to assume intermediate orientations. No twist in imparted to light entering the cell in this state so the crossed polarisers block all light, leaving the display black. As increasing voltage is applied to the cell, crystals aligned along the field begin to rotate across it because of their negative dielectric anisotropic behaviour. There is still no twist imparted to light so the display still blocks it. The horizontal (across the page) orientation of the crystals is still being controlled by the brushing on the top boundary, but a point is reached where the crystals at the lower boundary are sufficiently horizontal to be affected by the orientation of the ridges of the diffraction grating. This occurs with about 2V applied. The crystals near the grating begin to twist about a vertical axis because they have a tendency to lay along grooves of the size used. The crystals start to twist the light and the light begins to emerge from the display. At around 5Vthe lower crystals are fully horizontal and fully aligned with the grating. The cell is fully ‘on’ and passes light. To operate the cell with minimum energy usage, it is run between 2V for ‘off’ and 5V for ‘on’.

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