A “capsular” model for non-contact diffuse reflectance spectroscopy of two-layer medium by center-illuminated-area-detection

This study examines a forward model of total diffuse reflectance from a two-layer medium for non-contact dual-channel diffuse reflectance spectroscopy (DRS) using center-illuminated-area-detection (CIAD) [Piao et al, 2025, App. Opt.]. The model takes after a “capsular” approach initially demonstrated for radially resolved measurements from a solid organ of <1mm thick capsule when probed by contact DRS [Piao et al, 2018, J. Biomed. Opt.], which via radial integration is viable for DRS in CIAD of a two-layer medium. The integral operation of the model keeps the “capsular” essence to relax the diffuse reflectance measured from a two-layer medium to that from a mono-layer medium upon the vanishing of the inter-layer contrast. The resulting analytical model has been tested against Monte Carlo simulations of two-layer geometries with a 1-3mm top layer, an inter-layer two-way scattering contrast changing 2 orders of magnitude, an inter-layer two-way absorption contrast varying 4 orders of magnitude, and the radius of CIAD up to 15mm for 1” cage-applications. This “capsular” forward model for CIAD-based total diffuse reflectance of two-layer medium has been applied to experimental non-contact DRS engaging a center-accepted and center-blocked dual-channel configuration of CIAD. Applications to two-layer media having a 1.6mm thick layer of soft tissue and differing in the substrate base medium among aqueous phantoms of varied scattering and absorption, solid phantoms differing in absorption, and bulk avian tissues demonstrated the forward utility of the “capsular” model for non-contact assessment of below-surface medium presenting spectral absorption contrast with respect to a thick superficial layer.