Patent ID: 7420631

Claim:
High contrast, wide viewing angle LCD light switching element, in particular light shutter, comprising two crossed polarizers (P, A), an electrically controlled optical birefringent LC cell including a layer of highly twisted liquid crystal ( 4 ) enclosed between two boundary glass plates ( 1 a a , 1 a b ), said plates ( 1 a a , 1 a b ) being on an inner side covered by transparent electrodes ( 1 b a , 1 b b ) and alignment layers ( 1 c a , 1 c b ), and a negative-birefringent compensating layer (R) built-in between the boundary glass plates ( 1 a a , 1 a b ) and either one of the crossed polarizers (P, A), characterized in that the layer of highly twisted liquid crystal ( 4 ) is formed with a twist angle of at least nearly 180° and that the crossed polarizers are oriented with respect to a liquid crystal ( 4 ) surface alignment direction determined by the alignment layers ( 1 c a , 1 c b ) so that only a single mode of light propagation is selected and that the optical axis of the negative-birefringent layer (R) is oriented parallel to the normal to the LC cell boundary glass plates ( 1 a a , 1 a b ), an absolute value of the birefringence of said layer (R) being at least nearly equal to a difference between an absolute value of a positive-bireflingence of a homeotropically aligned part ( 3 ) of the layer of highly twisted liquid crystal ( 4 ) formed in an optically closed state of the LC cell and an absolute value of a negative-birefringence of the crossed polarizers (P, A) in order to compensate for an angular dependence of the homeotropically aligned part ( 3 ) in the LC cell, wherein the optically closed state is achieved by driving the LC cell with a driving voltage exceeding 2× a switching threshold voltage V th of the LC cell, preferably with a driving voltage of at least 5×V th .