The optical properties of twisted nematic liquid crystal structures with twist angles ⩽90 degrees

Liquid-crystal twisted nematic structures can be made with twist angles other than the conventional 90 degrees . It is shown that theoretically the transmission of such a structure when placed between two polarizers aligned parallel and perpendicular to the respective surface molecular directors is given by T=(1+u2)-1 sin2( theta (1+u2)12/) where u= pi d Delta n/ theta lambda , and theta is the twist angle, d being the cell thickness, Delta n the refractive index anisotropy of the nematic material, and lambda the wavelength of the transmitted radiation. In the idealized case T is taken to be zero. The departures from this, predicted by the above expression, are due to the fact that plane-polarized light becomes elliptically polarized as it passes through the nematic structure. Experiments confirm the validity of the theoretical model.