Recently, the demand for various smart devices such as smartphones, tablets, and smartwatches has increased significantly. Protective films are commonly applied to the display screens of these devices to enhance durability and improve visibility. Such protective films typically have a multilayer structure, with a three-layer configuration consisting of PET–adhesive–PET being widely used.
In particular, PET layers are manufactured through thermal extrusion and stretching processes, which result in thickness-dependent refractive indices as well as birefringent characteristics. Consequently, the refractive index of PET varies not only with thickness but also with polarization direction. Therefore, to ensure the optical performance of protective films, accurate measurement of the thickness and refractive index of each individual layer is required.
In this study, a spectral interferometry-based measurement method is proposed for the simultaneous layer-by-layer thickness measurement of a PET–adhesive–PET three-layer protective film. The refractive indices of the PET and adhesive layers are measured prior to lamination, and the obtained refractive index information is subsequently used to simultaneously determine the thickness of each layer in the final three-layer film sample.
Measurements were performed on three-layer film samples in which the upper and lower PET layers had different thicknesses, and reliable thickness and refractive index measurements were successfully achieved. Furthermore, for the PET layers, an inverse relationship between thickness and refractive index was confirmed. By rotating the sample with respect to the optical axis and measuring the refractive index in multiple orientations, the birefringent properties of PET and the thickness-dependent birefringence were also verified.
The proposed measurement technique is expected to improve inspection efficiency for protective films, which play a crucial role in enhancing the performance and lifetime of display devices.
