An imaging, differential optical sectioning interference microscopy (DOSIM) system and method for measuring refractive indices and thicknesses of transparent thin-films. The refractive index and thickness are calculated from two interferometric images of the sample transparent thin-film having a vertical offset that falls within the linear region of an axial response curve of optically sectioning microscopy. Here, the images are formed by a microscope objective in the normal direction, i.e., in the direction perpendicular to the latitudinal surface of the thin-film. As a result, the lateral resolution of the transparent thin-film is estimated based on the Rayleigh criterion, 0.61.lamda./NA.