To provide a releasing polyester film for polarizing plate protection or retardation plate protection in which the mechanical axis direction and optical axis direction of a film highly precisely are matched with each other and the optical axis direction of the film is uniform in the film surface, to provide a base film of the polyester film and to provide a manufacturing method of the polyester film.
A biaxially-oriented polyester film which is used by being attached to a polarizing plate or a retardation plate and is used as a base material of the releasing film supplied for an inspection according to a Crossed Nicols method is characterized in that (1) the polyester film has a molecular orientation main axis in the film longitudinal direction and (2) a difference between refractive index of the film longitudinal direction and refractive index of the film width direction is in the range of 0.03 to 0.10.
SASAKI YASUSHI
INAGAKI JUN
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