Nematic color displays

Fig. 10.16. A nematic LC display device. The pink colored plates axe two polarizers whose optic axes are perpendicular to each other. The two blue colored plates are glass plates with a transparent plastic and indium tin oxide coating, the latter serve as electrodes. A nematic LC is put in between and the arrangement of its molecules is shown. The first one is the arrangement of a twisted stage of molecules under No field condition. The second is the arrangement when there is field. The yellow arrow shows the condition of the polarized light during its transmission when there is no field and its obstruction when the field is on...

Many cellulose derivatives form Hquid crystalline phases, both in solution (lyotropic mesophases) and in the melt (thermotropic mesophases). The first report (96) showed that aqueous solutions of 30% hydroxypropylceUulose [9004-64-2] (HPC) form lyotropic mesophases that display iridescent colors characteristic of the chiral nematic (cholesteric) state. The field has grown rapidly and has been reviewed from different perspectives (97—101). 

Thermotropic cholesterics have several practical applications, some of which are very widespread. Most of the liquid crystal displays produced use either the twisted nematic (see Figure 7.3) or the supertwisted nematic electrooptical effects.6 The liquid crystal materials used in these cells contain a chiral component (effectively a cholesteric phase) which determines the twisting direction. Cholesteric LCs can also be used for storage displays utilizing the dynamic scattering mode.7 Short-pitch cholesterics with temperature-dependent selective reflection in the visible region show different colors at different temperatures and are used for popular digital thermometers.8... 

The first application described was as temperature sensors by using a chiral nematic liquid crystal, which displays different colors at different temperatures. It is also worth noting that many common fluids are in fact liquid crystals. Soap, for instance, is a liquid crystal, and forms a variety of liquid crystal phases depending on its concentration in water. 

Ho, J.-C. et al., Pentacene organic thin-fihn hansistor integrated with color twisted nematic hquid crystals display (CTNLCD), SID Int Symp. Dig. Tech. Papers, 35, 1298,... 

Figure 9.12. Schematic representation of (a) a standard twisted-nematic liquid crystal display and (b) a related display equipped with a drawn nanocomposite. 1, incoming light (unpolarized) 2, polarizer 3, glass plate coated with an electrode layer and an orientation layer for the liquid-crystalline molecules 4, liquid-crystalline molecules forming a 90° helical twist in absence of a voltage or a linear array parallel to an electric field in presence of an electric field 5, nanocomposite with oriented arrays of metal particles. See color insert.

P. S. Drzaic, Nematic droplet/polymer films for high-contrast colored reflective displays. Display, 2-13 (1991). 

Fig. 9. Schematic representation of (a) a conventional color twisted nematic liquid-crystal display and (b) display set-up containing a color polarizing filter (64).

Liquid crystal displays (LCDs) have many advantages over other display types. They are flat and compact, possess extremely low-power consumption (microwatts per square centimeter in the case of the twisted nematic effect), their color and contrast does not fade with an increase in the illumination intensity, they work both in transmissive and reflective modes in a wide operating temperature range and with a long lifetime. Besides that, LCDs are the most economically produced flat display systems. LCDs have... 

Schematic description of a simple LCD display. The colored arrows represent the direction of propagation of the light, while the black arrows represent the direction of polarization. A. With the twisted nematic between the two perpendicular polarizers, the polarization induced by the liquid crystal allows the light to pass. B. When an external electric field aligns the liquid crystal parallel to the first polarizer (indicated by the dipole arrow) the light cannot pass through the second polarizer and so there is no reflected light.

More advanced LCDs take advantage of so-called super-twisted nematic displays, in which the director rotates 270° between the two polarizers. This leads to a crisper distinction between on and off states. Color LCDs simply include appropriate dyes to make red, green, and blue (RGB) pixels—simple in principle but a fairly complex technology. 

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