As I am working on a survey that includes all kinds of media, I recently
learned that one way to distinguish acetate film vs polyester film is to
hold the roll up to the light. I of course knew this technique from dealing
with audio tape. However, it seems that in film the rule is reversed: when
one can see through the roll, the film is polyester; when it appears
opaque, it is acetate. Does anyone know why this is so?