Polymeric films effective as optical phase retarders and polarising assemblies.

摘要

A thermoplastic polymer, preferably unoriented and amorphous, e.g. of polyester, polysulfone, polyvinyl chloride, polyacrylonitrile or acrylonitrile, is molded into a sheet or film and uniaxially stretched, preferably by tentering. The stretched film has (a) a low optical color unevenness expressed by its color difference ( DELTA E*) being such that the highest measured value thereof is not more than 30 as measured with a spectrophotometer on usually 10 locations of the sample whilst illuminated between crossed parallel polarizers with the sample optical axis at 45 DEG to the phases of polarization, (b) a retardation value (R), i.e. the product of the thickness of the film and its birefringence, of 30 - 1200 nm, preferably 200 - 1000 nm, and (c) an average visible light transmittance of at least 50%. The polymer preferably has a defined spectral alpha -value to sodium light of at least 1.00. The film has a fixed optical axis and a retardation value suitable for its use as an optical phase retarder. The film can be placed against one of a pair of polarizing sheets in a liquid crystal cell; e.g. for TN or STN type of liquid crystal displays; the displays have good image quality with white background.