Changes of Refractive Index Ellipsoid by Biaxial Drawing of a PET Film

Abstract

The change of optical property on transverse drawing of a uniaxally oriented PET film was investigated. The birefringence in the plane of film decreased in proportion to transverse draw ratio, and come to zero in the optically balanced condition. The optical axis of uniaxally oriented film separated into two optical axes in the plane perpendicular to the transverse draw direction. At the optically balanced condition, the two separated optical axes joined into one optical axis perpendicular to the plane of films. These observations could be described well with a model based on the assumption that a refractive index in each principal direction changes in proportion to draw ratio.