Schadt-Helfrich-effect

Cholesteric liquid crystals, e.g., those of cholesteroylnonaoate (see Sec. 3.2), produce a Bragg-type scattering, which depends on temperature and angles of incidence and observation. Either total reflection or total transmission of circular polarized light is observed, which effect provides the basis of the dark-bright liquid crystal display in the Schadt-Helfrich cell (Fig. 3.5.3) as well as color reflection. 

Figure 3.5.3 Chiral liquid crystals without a chromophore may reflect colors because of light-scattering effects in helices of 400-700 nm pitches. Models of the cholesteric phase and the Schadt-Helfrich cell for liquid crystal displays are given. Two perpendicular polarization filters let light pass only if its direction of polarization has been rotated by the liquid crystals. If the hquid crystals are destroyed by an electric field, no light is transmitted, because the crossed polarizers quench it.

The publication of the twisted nematic effect by Schadt and Helfrich in the February 1971 issue of Applied Physics Letters [14] spurred a strong interest of the scientific community in the new effect and in the development of positive... 

With the help of Fig. 3.18, we are now in a position to illustrate briefly how a twisted nematic display operates. For more details on this and other effects related to liquid crystal displays the reader is referred to the review articles by Schadt [242] and Scheuble [243]. This type of device, based on the experiments involving electric fields carried out by Schadt and Helfrich [241], was patented for technological applications by these authors [125] in 1970 and by Fergason [84] in 1971. Fergason also went on to develop other patents on twisted nematic displays, most notably one issued in 1973 [85]. 

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