Crossed Polarizers Test

Polyester film can be positively identified with a simple test using polarized filters. Polarized plastic sheeting (similar to the lenses in polarized sunglasses) can be obtained from a scientific or photographic supply house. Position a transparent section of a film sample between two polarizing sheets. Holding the sheets and film above or below a light source, twist or rotate one of the sheets. If the film is polyester, you will see rainbow-like interference colors not unlike those formed by oil slicks on water. This phenomenon, known as birefringence, only occurs with polyester film. If the film you are testing has an acetate or nitrate base, you will not see these bands of color.