What does "FILM THICKNESS " mean ?
It is a term used in the DENTISTRY AREA .
A film is an extremely thin layer of anything. I don't know what it would be in dentistry, but I'm sure there are many things it could apply to.
Depending on what the film is, its thickness could be measured in microns or millimeters.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.