Heat setting optical films

Abstract

A method of making an optical film includes providing a film, substantially uniaxially orienting the film, and heat setting the oriented film. The film includes a polymeric material capable of developing birefringence.

1 . A multilayer reflective polarizer comprising a plurality of polyester-based polymeric layers, the film defining in-plane orthogonal x- and y-directions and a z-direction orthogonal to the x- and y-directions, the reflective polarizer stretched along the x-direction, each polyester-based polymeric layer having refractive indices nx, ny and nz along the respective x-, y- and z-directions, the reflective polarizer heat set at a temperature above a glass transition temperature and below a melting point of each of the polyester-based polymeric layers, wherein for at least one polyester-based polymeric layer, the heat setting results in a decrease in nx and |ny−nz|/|nx−nu|, and an increase in ny and nz, wherein nu is an average of ny and nz.