METHOD AND APPARATUS FOR MEASUREMENT OF ABSOLUTE BIAXIAL BIREFRINGENCE IN MONOLAYER AND MULTILAYER FILMS, SHEETS AND SHAPES

摘要

A system and apparatus for the measurement of absolute biaxial birefringenceof plastic materials is described. The materials which must be at leastpartiallytransparent can be constituted of one or several similar or dissimilar layers.In thelatter case, the birefringence of each material can be determined. Thetechnique uses amultiwavelength white light source that provides at least two beams projectedatdifferent angles of incidence on the sample. The beams pass through firstpolarizersbefore they are incident on the sample and through second polarizers afterthey havepassed through the sample. The beams are then directed to a detector formeasuringeach beam intensity or transmittance as a function of wavelength for theincidentangles at different times, and are analyzed using nonlinear regressions todetermineretardation. By the knowledge of the thickness of the material, the absolutebiaxialbirefringences are calculated for any specific wavelength. The regressionprocedureincorporates the material dependent optical constants for the calculations andtodiscriminate between different materials for multilayer samples.