AIC's 46th Annual Meeting

Imaging has been through the fashion cycle with a move away from the importance of materials analysis through microscopy, back to the current realization of how different imaging modalities complement each other. Spectral imaging utilizing controlled and modified modes of illumination provides a synergistic approach to materials analysis, while also mapping the spectral response of all materials across a document or object to augment the workflow for analysis by knowing what other regions on heritage materials require additional analytical techniques for characterization. The spectral imaging system at the Library of Congress was carefully customized to incorporate a number of imaging components that allow for multiple types of materials information within a single capture sequence. The base imaging sequence begins with reflected illumination throughout the visible and invisible range single LED illumination wavebands with a monochrome camera that assists with the full range of data in a high resolution 16-bit TIFF file. This is followed by raking (or side lighting at specified angle) illumination in both the blue and infrared (IR) regions of the spectrum to capture the topography of the object, and provides extremely useful information about production techniques, tool marks etc. and indentations remaining if ink or pigments have been eroded. The IR often shows the base substrate material without covering by inks and colorants. Raking at 90-degree angles or more essentially provides the information often associated with RTI. All image sequences are fully registered, greatly improving the productivity and workflow for processing of the image stack, as no re-registration is required since there is no filter used for the base sequence. Understanding materials refers not only to their characterization, but also to tracking change over time due to the impact of treatments, natural aging or specific environmental parameters. A standardized process for assessing any variation from the baseline has been invaluable for assisting the assessment of conservation treatments as well as change due to exposure to light or other factors, without the need for additional micro-fading. Additionally, z-plane imaging (focusing at multiple levels) adds a three-dimensional component and is enhanced within the workflow process by the inclusion of a laser for controlled layering. A fluorescent wheel incorporates multiple broadband filters to capture and enhance the fluorescent response for greater ease of characterization between similar colorants to assist pigment identification. Extending our understanding of materials is assisted with the addition of transmitted illumination, through a selected spectral range. The incorporation of transmitted imaging into the base sequence allows a combination of reflected and transmitted captures to be used in spectral processing and provides invaluable information about the impact of treatments on materials, disturbances with the base substrate, and imaging through treatments such as lamination. Overall, a structured standardized approach to integrated spectral imaging provides a thoughtful and nuanced approach to a better understanding of materials, while allowing for the potential of additional information to be captured from a diverse modality imaging methodology.