Quote Originally Posted by David Grenet View Post
From the Provia datasheet: "Base Material ......... Cellulose Triacetate

It is unusual for any roll film to be coated on polyester. Kodak and Harman (Ilford) sheet films are, and certain specialty films have been in the past (HIE and tech pan spring to mind) but in general if it's 120 or 135 film it's on Cellulose Triacetate.

On a factory visit to Harman in, IIRC, 2009, it was stated that none of their current films were coated on polyester.

I believe that an advantage of polyester for aerial films is the dimensional stability, which enables accurate measurements to be taken from the processed negs or transparencies. It was also used for the Ilford "Motordrive" 35mm films in the 1980's for its strength in a thinner base, which allowed a 72 exp length to be squeezed into a standard cassette.