Two practical methods for implementing spectral imaging within the framework of museum studio photography were investigated. Imaging was carried out using a consumer RGB digital camera paired with either 1) colored glass filters and a broadband source or 2) optimized multichannel LED illumination, yielding five or six spectral image bands, respectively. Color targets were used to develop and verify profiles for transforming between the multiband camera signals and final color managed images. The filter-based and LED-based profiles were assessed quantitatively for color accuracy using color difference statistics, and several paintings were imaged and rendered using the profiles as a visual demonstration of the differences. While both were superior to conventional RGB imaging, the LED-based method outperformed the filter-based method for accurate reproduction of independent data. This supplements practicality and cost considerations that are informing the development of accessible spectral imaging strategies for highly color accurate museum studio photography.
The color accuracy of an LED-based multispectral imaging strategy has been evaluated with respect to the number of spectral bands used to build a color profile and render the final image. Images were captured under select illumination conditions provided by 10-channel LED light sources. First, the imaging system was characterized in its full 10-band capacity, in which an image was captured under illumination by each of the 10 LEDs in turn, and the full set used to derive a system profile. Then, the system was characterized in increasingly reduced capacities, obtained by reducing the number of bands in two ways. In one approach, image bands were systematically removed from the full 10-band set. In the other, images were captured under illumination by groups of several of the LEDs at once. For both approaches, the system was characterized using different combinations of image bands until the optimal set, giving the highest color accuracy, was determined when a total of only 9, 8, 7, or 6 bands was used to derive the profile. The results indicate that color accuracy is nearly equivalent when rendering images based on the optimal combination of anywhere from 6 to 10 spectral bands, and is maintained at a higher level than that of conventional RGB imaging. This information is a first step toward informing the development of practical LED-based multispectral imaging strategies that make spectral image capture simpler and more efficient for heritage digitization workflows.
A library cataloguing the electron paramagnetic resonance (EPR) spectra of artists’ pigments has been created. It contains spectral data collected using several spectrometers that operate at different frequencies for, currently, 51 pigments. The library is intended to serve as an open-access reference database for the scientific studies of cultural heritage objects that utilize this analytical technique. Furthermore, it is a living repository, in that entries will be added as more pigments found to have EPR signals at room temperature are studied. Because EPR is less well established in the field of heritage science than some other common spectroscopies, this companion paper serves as an educational supplement to the library. It focuses on first, describing the theory of EPR to the level necessary to understand the origins of spectral features and to utilize these for pigment identification, and then, on discussing the organization of the library to facilitate the navigation of its contents.
A software application for colorimetric and spectral processing of six-channel spectral images has recently been developed. The application, called Beyond RGB, takes as input two RAW RGB image sets (object/flatfield/dark current) captured under two different lighting or filtering conditions, and outputs 1.) a color managed RGB image with ancillary information about the accuracy of the color calibration and 2.) a spectral reflectance transform that enables the interactive estimation of reflectance curves from userselected regions of interest in the image. Beyond RGB was designed with considerations for form, function, and user friendliness, and is intended for use by cultural heritage imaging professionals. It is cross-platform compatible and is operated through an interactive graphical user interface. Beyond RGB is a living, updatable, opensource project, and is freely available for download from the project's public GitHub repository.
A portable, user-friendly multispectral imaging system assembled almost entirely of common photography equipment and open-source software has been developed. The system serves as an outreach and educational tool for demonstrating and promoting scientific imaging as a more routine practice in the contexts of cultural heritage digitization and photography. These efforts are aimed primarily at institutions where advanced imaging technologies are not already found, and where funding and expertise may limit access to commercial, bespoke multispectral imaging solutions that are currently available. The background and theory that were shared in tutorials given during the system’s initial testing campaign are detailed here. Testing was carried out in one-day on-site visits to six cooperating institutions of different sizes and collection types in the northeast USA. During these visits, the imaging system was presented, and the benefit of collecting spectral data using low barrier-to-entry capture and processing methods relative to conventional imaging methods was discussed. Imaging was conducted on site on selected collections objects to showcase the current capabilities of the system and to inform ongoing improvements to the setup and processing. This paper is a written companion piece to the visits, as a source of further detail and context for the two-light imaging system that was described and demonstrated.
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