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A Kiessig fringe is an oscillation in reflected intensity caused by wave interference between contributions from the two sides of a thin film. For specular neutron scattering at grazing angle $\theta$ and wavelength $\lambda$, the normal wave vector transfer is $q_z=4\pi \sin \theta /\lambda$. Well above the critical edge, a film of thickness $d$ contributes a relative wave phase approximately $q_{z}d$, giving fringe spacingThus closely spaced fringes indicate a thicker film. As a calculated example, spacing $0.0628\mathrm{n}{\mathrm{m}}^{-1}$ corresponds to about $100\mathrm{nm}$. Interface surface roughness, contrast, additional layers and instrumental resolution affect the fringe pattern; thickness is inferred with an appropriate reflectivity model, especially near the critical edge.^[ref]