AIC's 47th Annual Meeting

Hyperspectral imaging has become an increasingly ubiquitous tool for the technical study of cultural heritage objects. It has been used to non-invasively characterize pigments and binding media on painted surfaces, as well as a variety of other object types from manuscripts to wood, over a wide field of view. However, the interpretation of hyperspectral data cubes can be complicated by the nonlinear mixing response of two or more colorants present below the resolution limits of the camera. Consequently, complimentary point analysis techniques, such as Raman or FTIR spectroscopy, are usually employed to obtain ground truth detailed material information. When possible, samples are removed and mounted in resin for further elemental and molecular characterization. However, some challenges are encountered with traditional analytical techniques when used to map molecular phases in these samples, notably the contaminating fluorescence background which can swamp the signal in Raman microspectrometry or the low signal-to-noise ratio in FTIR which can necessitate long integration times prohibitive for imaging. Avoiding these obstacles, a hyperspectral imaging technique with high spatial resolution would offer advantages for characterizing at the sub-micron range, with little additional sample prep or need to access more expensive equipment such as electron microscopy. As of yet, hyperspectral microscope configurations are rarely applied outside of the biological sciences. However, a recent hyperspectral optical microscopy experiment demonstrated excellent spatial resolution for the detection of single silver nanoparticles down to 100 nm diameters using a high numerical aperture objective, indicating the technique’s potential for material studies across many disciplines. This work presents the design and development of a simple dark field, reflectance hyperspectral microscope system for the purpose of extracting high spatially- and spectrally-resolved information, particularly for pigmented samples. Using a tunable light source to illuminate monochromatically over a range of visible to Near Infrared wavelengths, diffusely reflected light was collected with a long working distance, 20x objective. The challenges of fully automating and mechanizing the experimental construction of the hyperspectral data cube will be discussed as well as best practices determined for normalizing the acquired reflectance spectra as the surface texture of reflectance standards can become problematic under magnification. The developed microscope was used to characterize the pigment distribution in the stratigraphy of painting cross sections as well as other materials systems such as red opaque glass.