William Christens-Barry, Equipoise Imagining LLC and Michael Toth, RB Toth Associates

Spectral imaging using optimized fluorescence for cultural heritage materials

Spectral imaging provides a powerful technique for assessment of material composition, condition, and content. It has enjoyed increased use in conservation studies of cultural heritage materials, most notably in studies of manuscripts and textual materials.

Techniques that utilize optical reflectance (e.g. infrared reflectography) have predominated, while recently developed fluorescence techniques are providing vital information that complements reflectography. Short wavelength illumination, which historically has been provided at ultraviolet (UV) wavelengths, is required to "excite" fluorescence emission at wavelengths visible to the human eye. Most often, light sources in the "UVA" portion of the ultraviolet spectrum are used, due to their widespread availability and their reduced potential to damage or alter the materials they illuminate relative to shorter UV wavelengths (e.g. 254 nm). The range of light sources that provide UV has been broadened in recent years through the introduction of high power LEDs that can match the power levels previously available only using lamps and tube-based devices.

While access to advanced UV light sources has been a boon to many spectral imaging investigations of textual materials, several features of UV must be considered. The potential for damage to or alteration (such as fading) of inks, colorants, and substrate materials (parchment, paper, papyrus), while minimal when properly addressed, requires careful evaluation of appropriate exposure levels and accumulated doses. Researchers and staff employing UV wavelengths are required to wear goggles and limit their exposure to UV light due to well understood safety concerns, and the need to provide and document the use of UV can complicate imaging protocols and applications for funding support. Finally, the capture of high quality images in UV wavelengths imposes technical requirements on image capture protocols, necessitates the use of specialized equipment that increases cost of the optical system, and lengthens the time required to complete image capture.

In recent work we have adapted existing fluorescence techniques to minimize the dose of UV light to which materials and personnel may be exposed during spectral imaging. We have also developed new, longer wavelength light sources (e.g. 405 nm) that can excite the emission of fluorescence from textual materials while relieving safety concerns and reducing the cost and complexity of the imaging system.

We describe methods of spectral imaging that employ UV and short wavelengths to produce fluorescence images that meet conservatorial standards of practice and care and that meet conservation goal. Theses include: factors that should be considered when using UV illumination; exposure and dose calculations; measures that can be employed to optimize the quality of captured images; practices that reduce the exposure of materials and materials to UV light; reduction of specular highlights; distinguishing surface features or damage from that within the writing substrate; distinguishing damage from image features; choice of filters to be used with either UV or short visible wavelengths to probe specific emission wavebands; modifications to protocols when using short visible wavelength illumination in place of UV wavelengths. We illustrate this discussion with images, data, and results from recent spectral imaging studies.