Dr. Ranjith Ramanathan Profile

Dr. Ranjith Ramanathan

at Oklahoma State Univ

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Publications (2)

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Proceedings Article | 13 March 2024 Poster + Paper

Spectral thin-film interference observed with diffuse reflectance in a center-illuminated-central-acquired geometry indicates potential effect of the substrate scattering and absorption on the interference fringe

Daqing Piao, Nafiseh Farahzadi, Ranjith Ramanathan

Proceedings Volume 12836, 128360E (2024) https://doi.org/10.1117/12.2692687

KEYWORDS: Thin films, Diffuse reflectance spectroscopy, Scattering, Film thickness, Reflection, Absorption, Reflectivity, Visibility, Light scattering

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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.

Proceedings Article | 13 March 2024 Presentation + Paper

Novel dual-channel configuration of non-contact diffuse reflectance spectroscopy towards assessing the spectral absorption at a depth below 1mm

Nafiseh Farahzadi, Daqing Piao, Ranjith Ramanathan

Proceedings Volume 12836, 1283608 (2024) https://doi.org/10.1117/12.3001559

KEYWORDS: Diffuse reflectance spectroscopy, Simulations, Light scattering, Light absorption, Titanium dioxide

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It is challenging to assess the spectral absorption of sub-surface medium using non-contact diffuse reflectance spectroscopy (DRS). A demanding application of such is assessing myoglobin oxygenation at a depth of >1mm to inform beef discoloration. Common to the broad-band DRS and especially challenging to non-contact DRS conducted in continuous-wave, probing the sub-surface medium needs to deal with several geometry-dictated limitations in the measurements. For example, photons of longer pathlength are to be acquired to probe the deeper medium. However, the information associated with the photons with paths specific to only the sub-surface layer of the deep-probing path cannot be easily isolated in the measurement when additional information specific to only the shallower layer is absent. Another challenge to instrumentation relates to the substantially different scales of the magnitudes of the shallower probing and deeper probing DRS signals when originating from the same source of illumination at a similar timeframe. We demonstrate a novel dual-channel non-contact DRS for the assessment of spectral absorption at a depth of below 1mm. The dual-channel non-contact DRS combines a center-illuminated-central- acquired (CIC-A) geometry and a center-illuminated-central-blocked (CIC-B) geometry that are co-centric with respect to the same point-of-illumination (POI). The CIC-A geometry enables probing the surface layer of less than 1mm deep. And the CIC-B geometry acquires deep-probing photons while rejecting the short-path-shallow-probing- only components that would confound otherwise. The combination of CIC-A and CIC-B geometries allows assessing the below-surface spectral absorption. The principle is demonstrated by measurements from phantoms and tissues conforming to a two-layer geometry.

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