A Technique for Online Biaxial Birefringence Measurements and Its Application

In this study, we discuss the applications of a birefringence technique for online or off-line quantitative measurement of biaxial orientation in transparent films, sheets, bottles, etc. Absolute values of biaxial birefringence are measured in two directions using a technique based on an incident multiwavelength double beam and photodiode array assemblies, combined with in-house developed software. Both machine and transverse direction birefringences (relative to the normal direction) were measured simultaneously. Film and sheet of different thicknesses can be tested and birefringence values from 0.0005 to 0.25 can be measured. The results reported here are on the use of this technique for online birefringence measurements in two different cases: orientation and relaxation of birefringence during and after deformation of polyethylene terephthalate sheet; and online biaxial orientation of polystyrene sheet monitoring in an industrial process plant.