Exploiting the penetration depth of XRF and NIR radiation: from 2D to 3D spectral imaging

The present review examines the fundamental mechanisms governing the penetration of X-ray and near-infrared (NIR) radiation under sample surface – a feature that is often disregarded in analytical applications, especially in the spectral imaging implementations, which are usually considered as surface analytical techniques. The impact of material composition and geometry, scattering effects, as well as instrumental factors are thoroughly described and critically discussed. A particular focus is placed on data processing techniques, from first-principle equations to data-driven multivariate models, implemented to estimate/assess the extent of penetration. Applications in several areas, including food, forensic, material and cultural heritage sciences, are comprehensively reviewed. The potential for exploiting penetration of electromagnetic radiation is highlighted, paving the way for the development of 3D-resolved X-ray fluorescence (XRF) and NIR imaging approaches able to characterize multilayer samples in a non-invasive way.