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One of the chief challenges of the finite-difference time-domain (FDTD) subgridding algorithm is developing a field-coupling scheme to avoid late-time instability resulting from spatial interface between regions with different meshes. This paper introduces the concept of the nonuniform grid method to appropriately handle the tangential electric component on the spatial interface of the two meshes. This study analyzed shielding effectiveness of metallic enclosures with apertures. A novel subgridding algorithm was employed to locally refine the region containing a thin perfect electric conducting (PEC) film with apertures through FDTD simulation. Image theory was implemented to obtain an interpolation scheme of the magnetic fields as the sub-boundary surface when a thin PEC film traverses the subgridding interface. The numerical accuracy and stability of the proposed method were verified by comparing with results from other established methods. [ABSTRACT FROM PUBLISHER]