Liquid Crystal Cell:

The liquid crystals are one of the most fascinating material systems in nature, have properties of liquids as well as of a solid crystal. The terms liquid crystal refers
to that these compounds have a crystalline arrangement of molecules, yet they flow    like  liquid.  Liquid  crystal  displays  do  not  emit  or  generate  light,  but  rather  alter external generated illumination. Their ability to modulate light when electrical signal
is applied made them very useful in flat panel display technology.

The crystal is made up of organic molecules which are rod-like in shape with a length  of  —20°A  -  l00°A.  The  orientation  of  the  rod  like  molecule  defines  the ‘director”  of  liquid  crystal.  The  different  arrangements  of  these  rod-like  molecules lead to three m categories of liquid crystals.

1. Smectic

2. Nematic

3. Cholesteric

Smectic

Fig.  shows  smectic  structure  of  liquid  crystals.  In  this  structure  the  rod  like
molecules are arranged in layers, and within each layer there is orientational order
over  a  long  range.  Thus  in  a  given  layer,  the  rods  are  all  oriented  in  the  same direction  Also,  in  the  smectic  liquid  crystals  the  molecules  of  different  layers  are ordered.  Thus  both  orientation  order  and  positional  order  is  present  in  the  smectic crystals.

Nematic

Fig shows nematic structure of liquid crystals. In the nematic structure the
positional  order  between  layer  of  molecules  is  lost,  but  the  orientation  order  is maintained.


Cholesteric

Fig shows cholesteric structure of liquid crystals. In these crystals the rod-like
molecules   in   each   layer   are   oriented   in   a   different   angle   within   each   layer. Orientation order is maintained in each layer. The cholesteric liquid crystal is related
to  the  nematic  crystal,  with  the  difference  being  the  twist  of  the  molecules  as  one goes from one layer to another.

The optical activity of the crystal depends upon the orientation and the twist of the molecules as one goes from one layer to another.

There  are  two  types  of  liquid  crystal  displays  (LCDs)  according  to  the  theory  of operation

1. Dynamic scattering LCD

2. Field effect LCD