Maximizing the microscope: instrument design and data processing strategies for hyperspectral imaging of cross-sectional cultural heritage samples

Abstract

The use of non-invasive hyperspectral imaging techniques has become standard practice in the materials analysis and study of precious cultural heritage objects such as drawings, paintings, murals and more. However, the non-linear mixing of spectral signatures from complex and heterogenous objects with multiple colorants present below the resolution limits of the camera can complicate material identification. Consequently, ground truth measurements are still usually obtained from microscopic samples removed and embedded to expose stratigraphy and obtain sub-surface information about the artist’s material choices and technique. This work considers a microscopic spectral imaging technique capable of mapping molecular information in such micro samples at high spatial and spectral resolution while avoiding some of the challenges of complimentary techniques, such as swamping fluorescence in Raman spectroscopy or long integration times using FT-IR spectroscopy. Construction of a dark field hyperspectral microscope for cultural heritage samples is described using a tunable light source to illuminate the sample monochromatically from the visible to near infrared wavelengths, with the diffusely reflected light collected from the specimen with a long working distance, 20x objective. The illumination and detection arms were decoupled to better focus the power of the tunable light source across the tunable range through Köhler illumination optics. By mounting the optical train on a rotating arm, we can achieve multiple angles of illumination and optimize lighting conditions. The sample is also rotated in order to reconstruct an even distribution of light across the field of view. This multi-axis movement capability also provides exciting opportunities to leverage more than simple spectral information from an image series such as surface topography and differential phase contrast information. The developed microscope was used create a library of spectral signatures for comparison to painting cross sections, and the ability of the microscope to identify and examine individual pigment particles was tested.