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Poly(ethyleneterephthalate) also known as PET, Mylar, Melinex and Estar is a commodity polyester widely used in a variety of applications: films, bottles or as spun fibers. It is typically quite birefringent in sheet form with distinct refractive indices in all 3 dimensions. As a rule of thumb the in-plane indices increase as the film (or foil) thickness decreases. (At the same time the out-of-plane index decreases.) This is due to the biaxial stretching process used to attain both a desired film thickness and specific sheet mechanical properties. Even formally describing PET film as having a x, y, and z refractive index is not truly sufficient as there is a slight index grading in the thickness direction*. Suffice it to say while this is a very common, everyday material, it is also a surprisingly complicated optical system.

*Elman J.F., Greener J., Herzinger C.M., and Johs B. "Characterization of Biaxially-Stretched Plastic Films by Generalized Ellipsometry" Thin Solid Films 1998, 313/314, 814-818.

For a typical sample of PET the and at are 1.6357 and 0. Below are files of complete refractive index and extinction coefficients. If the file is not available for download, you can request our proprietary file by clicking "Request".

Refractive Index Reference - Filmetrics composite

No guarantee of accuracy - use at your own risk.