Polymer sheets are currently used worldwide in a wide
range of applications. The manufacturing process of these sheets involves
extruding machines that stretch the material in both lateral and
longitudinal directions with respect to the machine direction, thus inducing
birefringence. In most cases, the film obtained is optically biaxial.
Polarimetric spectroscopy (Ellipsometry and Mueller Matrix) combined with
conoscopic observation are the methods of choice to study these properties.
In this work we report an analysis of commercially available polyethylene
terephtalate (PET) films used as substrate for food packaging as well as for
embedded electronic devices such as solar cells or flexible displays.
Initial observation of these films through polarizing microscope in
conoscopic mode reveals first the trace of optical axis plane with respect
to the film surface and second, whether the optical axis is acute or not.
This preliminary study allows optimal sample positioning for further
polarimetric studies. The measurements and modelling are done in both
reflection and transmission mode on several spectroscopic polarimetric
setups from UV to NIR. The models give as a main result, the dielectric
tensor of the film as well as its orientation with respect to the laboratory
reference frame.