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Antiferroelectrics |  |
As mentioned before, the switching time of SSFLCs is much less than that of other liquid crystal technologies. Shutters are capable of a 70 microseconds transition time. This is because the coupling of the spontaneous polarization vector to the applied electric field. The obvious advantage to this is the increased speed in switching from the white state to the black state and vice versa for video. Another advantage is the ability to use sequential coloring. This means that a color is created by rapid succession of the additive primary colors red, green, and blue. With the fast switching time, the eye fuses the sequence into a single color. If switching time was not fast enough, it would be necessary to sub-divide every pixel into red, green, and blue in order to create a color. This would necessitate making the display larger in order to get a less grainy color image.
None of the three resolution-limiting causes affect SSFLCs. There are no serial elements in directly driven matrices such as for SSFLCs. This means that there is no crosscapacitance effect, which is a crossmodulation induced by the capacitance of the serial element due to decrease in capacitance of a pixel decreasing. The problem with a matrix structure is that there exists a transverse electric field at the pixel edges. The transverse field can cause misalignment. However, SSFLC's thin cell gap reduces this effect to such an extent that it is not observed under realistic conditions. Thirdly, SSFLCs have a high aperture ratio, the ratio between the optically active area to the total area per pixel in a matrix. This is because it is a direct-driven panel so transistor, gate bus, and gate sources do not cover a lot of area as in the addressed matrix. The fact that these cases due not apply to SSFLCs make a pixel size of 5 micron by 5 micron feasible with the best resolution by standard graphic techniques corresponding to a size of 20 micron by 20 micron. The later, and highly realistic, pixel size would yield a resolution sufficient for high definition projection television when used in a 35mm slide. A comparison of images produced by an FLC/VLSI and a typical AMLCD device is shown in the introduction to the ferroelectric section.
Because in the relaxed state (white state) the effective cone angle is not near the optimum value of 45 degrees, contrast is limited. With the common values currently of 15 to 20 degrees, the contrast would be around 30:1. The black state does not suffer this limitation so its contrast could easily be 100:1. However, the black state is affected by partial switching in the refresh mode. This effect reduces the dynamic contrast even more, to a value of 7:1. The contrast can best be improved by increasing the effective cone angle. Also, appropriate addressing techniques can improve it. One good thing about the contrast of SSFLC devices is that it is nearly independent of viewing angle. This is because the optic axis is switched around an axis parallel to the light path. Ideally the optic axis would stay parallel to the cell plane. In other words, the optic axis never turns out into the direction of the observer where directional variations can cause large changes in the extinction. Thus, unlike common LCDs, SSFLC can be observed from small angles to the plane of the display without a significant loss of contrast. The tilting and chevron structure mentioned earlier weaken the SSFLCs contrast at sharp angles because they cause inhomogeneities in the director alignment.
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Antiferroelectrics | |