Dr. Giovanni Verri Profile

Dr. Giovanni Verri

at British Museum

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Proceedings Article | 25 June 2009 Paper

The application of visible-induced luminescence imaging to the examination of museum objects

G. Verri

Proceedings Volume 7391, 739105 (2009) https://doi.org/10.1117/12.827331

KEYWORDS: Luminescence, Infrared radiation, Infrared imaging, Visible radiation, Reflectivity, Light emitting diodes, Manganese, Optical filters, Gold, Tungsten

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Visible-induced luminescence imaging in the near infrared range (800-1700 nm) can play a key role in the spatial characterization of certain historical blue pigments (principally Egyptian blue, Han blue and Han purple). All three of these pigments show a very strong infrared emission when excited across the visible range. The setup required for this type of measurement comprises a recording device with some sensitivity to near infrared radiation and an excitation source in the visible range. Inexpensive and widely available excitation radiation sources that can be used for this application include fluorescent lamps and LEDs. While visible LEDs do not usually emit infrared radiation, commercially available fluorescent tubes may emit some stray infrared radiation. Although the presence of such stray infrared radiation may in some cases be considered beneficial, allowing the user easily to locate the presence of the pigments under investigation within the composition of the piece, it can be easily removed using a simple subtraction method. This method, based on the measurement of the reflective properties of the surface under investigation in the emission range of the luminescent pigments, is described. The emission results obtained for Egyptian blue, Han blue and Han purple are compared to those of a set of representative historical and modern blue pigments, including manganese blue, which was the only other pigment found to show detectable luminescence properties. Examples of the application of visible-induced luminescence imaging to archaeological objects of the Antonine period (AD 100-200) in the collections of the British Museum are also presented.

Proceedings Article | 18 March 2008 Paper

Post-capture data analysis as an aid to the interpretation of ultraviolet-induced fluorescence images

Giovanni Verri, Daniela Comelli, Sharon Cather, David Saunders, Francesca Piqué

Proceedings Volume 6810, 681002 (2008) https://doi.org/10.1117/12.764463

KEYWORDS: Luminescence, Ultraviolet radiation, Photography, Absorption, Visible radiation, Imaging spectroscopy, Stray light, Multispectral imaging, Atmospheric particles, Optical filters

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Although digital multispectral imaging-particularly ultraviolet-induced fluorescence imaging-is a very common examination tool, its interpretation remains fraught with difficulties. Interpretation is strongly dependent on the capture methodology, requires an understanding of the physical and chemical characteristics and interactions among materials in artworks and is affected by data-analysis procedures. The present research, which began with imaging of paint materials of known composition and proceeded to a range of representative case studies, confirmed that fluorescence emissions by painting materials-such as organic binders or colorants-are generally severely affected by the presence of absorbing non-fluorescing materials, such as inorganic pigments. Application of a mathematical model based on the Kubelka-Munk theory, resulted in the possibility of distinguishing between real and apparent fluorescence emissions. Real emissions correspond to the presence of materials which de facto exhibit fluorescent properties (typically organic binders and colorants), while apparent emissions relate to the optical interactions among fluorescent materials and surrounding non-fluorescent materials (typically inorganic pigments). Correction for the 'pigment-binder interaction' can also provide useful information on the presence of materials whose fluorescence is almost obliterated by absorbing pigmented particles. Therefore, this image-processing methodology can be used to characterise and reveal emissions that are dimmed or altered by re-absorption. This capacity to reveal the presence of weakly fluorescing emitters has important conservation implications and informs the sampling strategy for further analytical investigations. Examples of the application of this data analysis to images made at the Grotto Site in Dunhuang, China, and at the British Museum are presented.

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