Schadt-Helfrich cell

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.

Lehmann postulates 1971 Schadt and Helfrich describe LCs as the 4th state the twisted nematic (TN) cell... 

C. B. Burckhardt, M. Schadt, and W. Helfrich, Holographic Recording with an Electro-Optic Liquid Crystal Cell, Appl. Optics, Vol. 10, p. 2196 (1971). 

If the axes of the two polarisers were mutually parallel then the cell would appear opaque for H < He or V < Vc, and transparent for H > He oi V > Ve. Schadt and Helfrich [241] carried out the electric field experiment on a twisted nematic cell having 20o = parallel polarisers for two liquid crystal substances having positive dielectric anisotropy Sa. The cell depth d was 10 fim and both dc and ac voltages were applied. One substance, a (racemic) mixture, was in the nematic phase for temperatures in the range 20-94°C. For this mixed nematic at around room temperature they found that Vc 3V at IkHz with around a 90% transparency when V was increased to about 6V. For further details the reader is naturally referred to their paper [241], while for a brief physically based review of the details the reader may consult Chandrasekhar [38]. 

---