The standard GB/T 33051-2016 Optical inspection functional films, method for determination of film thickness of surface hardened films hardened layer specifies a method for measurement of hardened film coating thickness using reflection interference Light spectrum method. The standard is mainly applicable to the determination of film thickness of polyester films based on polyethylene terephthalate (PET), whose surface is coated with a single transparent hardened layer. This method provides accurate Film thickness measurement by analyzing the optical path difference and phase difference of coherent light. Miscellaneous material substrates can also refer to this method for transparent coating thickness measurement.