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Thin-film interference is commonly used for characterizing some physical properties of thin-film such as the thickness of it. Thin-film interference under broad-band light manifests spectrally varying periodicity, with the fringe becoming sparser towards longer wavelength, that informs the optical thickness of the thin-film. A non-scattering substrate of thin-film shall not alter the spectral periodicity of the thin-film interference due to no change to the pathlengths of interfering photons. A scattering substrate, however, may affect the fringe due to the contribution by photons of longer pathlengths than normal. And the proportion of the photons of longer pathlengths than normal may be affected by the diffusivity of the substrate. We observed that the spectrally varying periodicity of thin-film interference was affected by the substrate’s diffusivity. Spectral thin-film interference was acquired from regular household food-wrap (Bakers & Chefs) placed in good-contact with planar materials, by diffuse reflectance spectroscopy in a center-illuminatedcentral- acquired (CICA) geometry over 550-850nm. Spectral thin-film interference was compared among that acquired from film-attached Spectralon reflectance standards (40%, 60%, 80%, 99%) and film-covered solid tissue phantoms (near-identical reduced scattering with the absorption scaled 1:2:4). The variation of the spectral periodicity of thin-film interference can be associated with the scattering and absorption properties of the diffusive substrate. The effect of the diffusivity of the substrate on the spectral periodicity of thin-film interference may become a confounding issue for thin-film characterization but could provide information for probing thin-film covered materials towards applications including assessing surface and below-surface formation of metmyoglobin.
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