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 birefringence of plastic materials is described. The materials which must be at least partially transparent can be constituted of one or several similar or dissimilar layers. In the latter case, the birefringence of each material can be determined. The technique uses a multiwavelength white light source that provides at least two beams projected at different angles of incidence on the sample. The beams pass through first polarizers before they are incident on the sample and through second polarizers after they have passed through the sample. The beams are then directed to a detector for measuring each beam intensity or transmittance as a function of wavelength for the incident angles at different times, and are analyzed using nonlinear regressions to determine retardation. By the knowledge of the thickness of the material, the absolute biaxial birefringences are calculated for any specific wavelength. The regression procedure incorporates the material dependent optical constants for the calculations and to