Quantifying Appearance Attributes and Monitoring Appearance Changes

The amount of research investigating the risks of light damage to fine art materials is very small, but the amount directed at ethnographic and natural history materials is even smaller. More might have been done if the fundamental properties of many of these products were easier to characterize.
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Feathers of a great horned owl (Bubo Virginianus). Under visible light (left) and ultraviolet illumination (right), porphyrins—pigments found primarily in new feathers—show up as intense pink. Photo: Renee Riedler

Ethnographic Materials with Biocolorants and Structural Colors

Unlike paints and dyes, biological colors are divided into those created by biopigments and those produced by a handful of physical phenomena that constructively and/or destructively interfere with reflected light. Structural colors are notoriously difficult to measure in a repeatable manner. Iridescence, for example, causes an angular dependence to color that gives butterfly wings and peacock feathers their enormous range of appearance effect.
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It was not until recently that the main parameters that specify and define the geometry of color measurement –the angle between illumination and observation axes (Θ), the angle between the measured surface plane and the illumination/observation plane (Φ), and the orientation of the surface in regard to the axis around which the sample surface can rotate(ω) – were introduced systematically into feather color measurement.

One technique that has been exploited by biologists to measure the color of birds employs the bidirectional reflectance distribution function (BRDF). In this technique, overall color appearance can be described by a series of incident and reflected light beams with small solid angles. Measurement of BRDF at different incident wavelengths is thus exceedingly burdensome and to gain realistic experimental insights into light damage the approach needs to be simplified
In a feather, this can be achieved using a limited multi-angle series of incident and reflectance measurements, but since the organization of feather structure also includes large macro-scale features (e.g., feather barbs and barbules), rotational freedom about those features also need to be incorporated into any function.

It becomes an important decision which subset of solid angles and barbule orientations imbed the best discrimination between structural and biopigment changes when estimating overall light sensitivity. This question has never been resolved in conservation research and is part of the project’s goals. The current recommendations on feather ISO blue wool sensitivities are, at best, conjectural. Some of the literature even permits a skewed interpretation based upon a few uniquely sensitive members of the superset of feather colorant systems that are possibly atypical and unique.

The overall goal is to better identify the light-sensitivity of anthropological, fine art, and natural history artifacts composed in part, or totally, of feathers.
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Adapting techniques from ornithology, surface reflectance of feathers were measured with multi-angle spectrometry using a fiber optic illuminant and a detector within an adaptable angle fiber holder. Photo: Cristal Pesme   Reflectance spectra of a peacock feather under various geometrical conditions of measurement
The GCI's partner in this area of research is Ellen Pearlstein of the UCLA/Getty Master's Program in Conservation of Ethnographic and Archaeological Materials. This newly developed expertise on surface appearance complexity will be applied to better understand and quantify the appearance of various challenging artistic materials such as plastic. Also it will be used in combination with implemented methods of colorimetric analysis in an attempt to quantify the respective impact of light exposure on the different surface attributes- i.e color, gloss, texture and translucency- and to refine the understanding of the effects of light exposure on different aspects of artifact appearance.