The present subject matter at least provides an apparatus for inspecting a slab of material including a passivation layer. The apparatus includes a frequency-domain optical-coherence tomography (OCT) probe configured to irradiate the slab of material, and detect radiation reflected from the slab of material. The apparatus also includes a spectral-analysis module configured to analyze at least an interference pattern with respect to the OCT probe to thereby determine a thickness of the slab of the material. The apparatus also includes a thin-film gauge configure to determine a thickness of the passivation layer such that the determined thickness of the slab of material may be adjusted baes on the thickness of the passivation layer.