There are many types of liquid crystal displays, each with
unique properties. The most common LCD that is used for everyday
items like watches and calculators is called the twisted nematic
(TN) display. This device consists of a nematic liquid crystal
sandwiched between two plates of glass. A special surface
treatment is given to the glass such that the molecules are
homeotropic yet the director at the top of the sample is
perpendicular to the director at the bottom. This configuration
sets up a 90 degree twist into the bulk of the liquid crystal,
hence the name of the display. The twist is visible in the
following animation. This structure is similar to the
cholesteric state, and sometimes a small amount of a chiral
material is added to ensure a uniform twist. The underlying
principle in a TN display is the manipulation of polarized
light. When light enters the TN cell, the polarization state
twists with the director of the liquid crystal material. For
example, consider light polarized parallel to the direct