Correction of Ultraviolet-Induced Fluorescence Spectra for the Examination of Polychromy

Abstract

Ultraviolet-induced fluorescence spectroscopy is a commonly used technique for the characterization and identification of painting materials, such as organic binders and colorants. Its interpretation is strictly connected to both the experimental setup and an understanding of the physical and chemical interactions among materials in paint layers, which are commonly composed of a fluorescent organic binder and a pigment. When irradiated with ultraviolet radiation, the light emitted by fluorophores present in the organic binder undergoes several types of interactions, in particular scattering and absorption by neighboring pigmented particles and auto-absorption. As a result of scattering and absorption phenomena, the emission spectrum is deformed according to the physical properties of the surrounding pigmented particles. This can lead to shifts of the emission maxima and/or to the formation of apparent new emission bands. The extent of the modifications to the emission spectra, caused by auto-absorption and selective absorption phenomena, may lead to the erroneous characterization or identification of the fluorescent materials. As a consequence, the interpretation of the emission signal can be greatly compromised. A correction based on the Kubelka-Munk theory is proposed to evaluate the extent of the spectral distortion and is assessed on modern replicas of wall paintings of known composition. Although the model cannot be applied to all cases, qualitative distinctions between real and apparent emissions are achieved.

Practical notes on ultraviolet technical photography for art examination

“…Nevertheless, it must be taken into account that the spectral emission of commonly fluorescent artists' materials can be highly modified by the presence of other surrounding materials. Optical emitted radiation can interact with surrounding matter, giving rise to selective absorption and scattering phenomena [12].…”

Time-Resolved Photoluminescence Spectroscopy and Imaging: New Approaches to the Analysis of Cultural Heritage and Its Degradation

“…The modification of the fluorescence of organic materials has been reported and related to general and more specific molecular changes, including those related to oxidation phenomena: for example, the photooxidation of protein-based binders [12], oils and varnishes [2], or the oxidation of modern polymers and plastics [13]. It is recognized, however, that the discrimination of materials on the basis of fluorescence spectra is often impossible—subtle spectral differences, which may arise from chemical modifications of materials or differences in molecular properties, may be masked by competing effects, auto-absorption phenomena [14], or scattering [15], for example. Fluorescence emissions may also be weak and thus spectra may be difficult to detect.…”

A technical step forward in the integration of visible-induced luminescence imaging methods for the study of ancient polychromy

“…For madder lakes this is centred at c. 455-540 nm, depending on their preparation procedure [38], the blue LED (λ max = 465 nm) is an optimal excitation wavelength for these coloured species whilst avoiding the excitation of binders, varnishes or resins that preferentially absorb UV wavelengths. This not only increases the emission signal from the species of interest but the narrow emission band also minimises the effects of distortion of the emission features, such as self-absorption, multiple scattering and inner filter effects, produced by interference from other excited species [21,39,40]. Evidence of this can be seen by comparing the area of pink pigment under the belly of the duck in the UVL and VIVL (Lee No 21 filter) images of Terracotta A (Fig.…”