Calibration and Use of Photosensitive Materials for Light Monitoring in Museums

НазваниеCalibration and Use of Photosensitive Materials for Light Monitoring in Museums
Дата конвертации27.10.2012
Размер86.6 Kb.
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The use of photosensitive materials as tools for the quantitative evaluation of the environmental impact in terms of ELD has been considered through a case-study on samples of BWSl.

By means of a miniaturized fibre-optic spectro-photometer designed for automatic and periodic acquisition on selected samples, it has been possible to follow the spectral light-induced evolution of BWS1 during processes of artificial and natural aging. Two experiments were performed in a controlled laboratory setting and in an exhibition space of the Uffizi Gallery, adopting the same experimental procedure so that two comparable sets of spectral data were obtained.

The artificial aging experiment enabled us to investigate, in the visible range, the response of BWS 1 to light. Moreover, sensitivity of the material to the overall microclimatic conditions was evidenced. The spectral data relative to progressive stages of light-aging were used to build a calibration of the BWS1 in the 0-500 klux.h range. Three different calibration models were considered in order to illustrate the methodology of selecting the variable that provides the best parameters for light-induced spectral changes. In particular, a calibration model based on a local parameter, related to the absorption band of the BWS1, was compared with a model based on PCA analysis, which is intrinsically non-local since it refers to the whole spectral range. Moreover, since the BWS is traditionally used as a colorimetric indicator, yet a third model was considered, based on the colour variation of BWS 1.

The calibration curves obtained were used to estimate the alterations induced in BWS 1 samples exposed in situ, in terms of ELD. All the models predicted ELD values greater than the actual light-doses measured with a traditional lux-meter. This was interpreted as an indication of the active role of environmental agents other than light in contributing to the fading of the exposed sensor.

As for the quantitative estimation of ELD, comparison between the different models showed agreement between the spectral integral and the PCA models, indicating that such approaches can be considered reciprocally validated and equivalent for practical uses. On the other hand, the colorimetne approach described here underestimated the values of the ELD, presumably because of the com­pensating effects introduced in the colorimetric calculations by the spectral response functions, which are based on the response of the human eye to colour stimuli.

In addition to the characterization of the BWS1 system, this study was presented as a general inves­tigation on the methodological aspects involved in the use of photosensitive materials as a new class of dosimeters for light monitoring.



The following data were acquired by a datalogger model IrLog from Elsec. The values of the parameters were acquired continuously, every 10 minutes, throughout the duration of the tests. The data reported below are the mean values relative to the entire experiment.

Artificial aging experiment

Place: Laboratory of the Institute of Applied Physics 'N. Carrara' CNR, Florence (Italy). Exposure time: 68 hours. Temperature: 24.8 ± 0.5°C. Relative humidity: 50 ± 3%. Lighting: artificial. Source: quartz tungsten halogen lamps, Osram (150 W). Illuminance (mean value): 7.5 ± 0.4 klux.

Natural aging experiment

Place: Geographic Maps Room, Uffizi Gallery, Florence (Italy). Exposure time: 60 days (from 03/06/02 to 03/ 08/02). Lighting: natural. Illuminance: daylight. June. Temperature: 26 ± 2°C. Relative humidity: 44 ± 5%. Maximum illuminance value: I ,=1.3 klux.

Temperature: 30 ± 1°C. Relative humidity: 50 ± 5%. Maximum illuminance value: I , = 1.4 klux.


The construction of the instrumentation was partially funded by the Regione Toscana within the framework of the 'RIS+' Pilot Project. Thanks are expressed to Dr Anna Maria Petrioli Tofani, Director of the Uffizi Gallery in Florence, and to Dr Antonio Natali for their kind hospitality at the Uffizi.


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Mauro Bacci is director of research at the Institute of Applied Physics 'Nello Carrara' IFAC-CNR, Florence. His main interest is in spectroscopy, both theoretical and applied. For many years he has been developing totally non-destructive spectroscopic techniques for the study of works of art. Address: Institute of Applied Physics 'Nello Carrara' IFAC-CNR, Via Paudatklu 64, 50127 Bronze, Italy. Email:

COSTANZA CUCCI graduated in physics at the University of Florence in 1996. She has conducted research in condensed matter concerning applications to magnetic materials. Since 2000 she has worked at the Institute of Applied Physics-CNR, Florence, developing research on the field of environmental and cultural heritage control. Her current research interest is spectroscopic analysis of artists' materials and their light-induced degradation. Address as for Bacci. Email:

Andrea Azf.lio Mencaglia graduated in physics in 1987. Until 1996 he was a member of the Optical Fibres Group at IROE-CNR. His scientific activity during this period concerned optical fibre sensor systems for the measurement of physical parameters. After six months at the University of Strathclyde (UK), he became R&D head of Prodotec srl (Florence, Italy). Since 2002 he has been a researcher at the Institute of Applied Physics-CNR. Address as for Bacci. Email:

Anna Grazia Mignani graduated in physics from the University of Florence. Since 1983 she has held a permanent position at the Institute of Applied Physics 'Nello Carrara' (formerly IROE). Her research work includes fibre optic sensors and related passive guided-wave components, and fibre optic sensor networks. She chaired the 14th International Conference on Optical Fiber Sensors (OFS2000) and is a board member of the Italian Society of Optics and Photonics (SIOF) and the Italian Society of Sensors and Microsystems (A1SEM). Address as for Bacci. Email:

Simone Porcinai graduated in chemistry at the University of Florence in 1993, with a doctorate in science for conservation of cultural heritage. Since 2000 he has been a member of the Scientific Laboratory of the Opificio delle Pietre Dure. His main research interests are spectroscopy and multivariate analysis.

Address: Opificio delle Pietre Dure, Via degli Alfani 78, SO 121 Firenze, Italy. Email:

Resume On a étudié les aspects méthodologiques de l'utilisation de matériaux photosensibles comme indicateurs de dose de lumière dans le contexte du controle de l'environnement des musées. Comme étude de cas, on présente une étude spectrométrique dans la région 400—700 nm du standard de laine bleue No 1 (Blue Wool Standard no í, BWSÍ), un indicateur colorimétriquc traditionnellement utilisé par les restaurateurs pour obtenir une indication du risque associé à la lumière. Des expériences de vieillissement à la lumière artificielle aussi bien que naturelle ont été menées sur des échantillons de BIVSÍ, et des spectres de réflectance ont été mesurés pendant le processus de vieillissement à des doses d'éclairage croissantes. Un instrument portable basé sur la technologie des fibres optiques fonctionnant en temps réel a été utilisé pour l'acquisition des spectres. Le problème de la calibration de la réponse du BWS1 à la lumière a été étudié en utilisant les données d'échantillons artificiellement vieillis à la lumière, et on a pu comparer différents modèles de calibration. Les courbes de calibratiou ont ensuite été utilisées pour estimer l'altération due à une possible action synergique de la lumière et d'autres facteurs sur des échantillons de BWS l vieillis à la lumière naturelle par exposition in situ. Ou propose une méthode d'estimation semi-quantitative de l'action cumulée de la lumièn et autres facteurs environnementaux en introduisant le concept d' « équivalent dose-lumière », grâce auquel le facteur de risque peut être évalué dans son ensemble.

Zusammenfassung Photosensitive Materialien, wie sie als Lichtdosimcter in Museen vielfach genutzt werden, wurden methodologisch untersucht. Als Beispiel wurde eine Methode zur Untersuchung des Blue Wool Standard No. 1 (BWSI) im Bereich von 400-700 um entwickelt. Dieser colorimctrische Standard wird seit langem von Restauratoren zur qualitativen Abschätzung des durch Umgebungslicht verursachten Risikos genutzt. Die Experimente zur lichtiuduzierten Alterung des BWSI wurden sowohl in künstlichem wie in natürlichem Licht durchgeführt, wobei wahrend des gemäß der ansteigenden Lichtdosis fortschreitenden Alterungsprozesses zerstörungsfrei Reflcktionsspektren aufgenommen wurden. Dazu diente eine tragbare auf Glasfaseroptik basierende Einrichtung, die die Spektren in Echtzeit aufnahm. Zur Kalibrierung des BWSI dienten die Daten der in künstlichem Licht gealterten Proben. Dabei wurden verschiedene Kalibricrungsmodelle miteinander verglichen. Die Kalibrierungskurven wurden dann dazu genutzt, den Einfluß des Lichtes in Kombination mit anderen Faktoren abzuschätzen, wie er am BWSI Standard bei natürlicher in situ Alterung auftreten kann. Eine Methode zur halbquantitativen Berechnung des kumulativen Einflusses von Licht und anderen Umweltfaktoren wird vorgeschlagen. Dabei wird das Konzept der äquivalenten Lichtdosis vorgestellt, durch welches der Gesamtrisikofaktor abgeschätzt werden kann.

Resumen En este trabajo son investigados los aspectos metodológicos del uso de materiales fotosensibles como indicadores de dosis de luz en monitorizaáones de museos. Como ejemplo clave fue desarrollada una investigación espertroscópica en un rango entre 400 y 700 nm para el Blue Wool Standard No. í (BWSÍ), un indicador colorimétrico usado tradicionalmentc por los conservadores para obtener indicaciones cualitativas del riesgo asociado a la iluminación ambiental. Los experimentos, usando tanto envejecimiento natural (in situ) como artificial, se realizaron en muestras de BWSÍ recogiéndose, no invasivamente, un i¡rupo de espectros de reflectando durante los procesos de envejecimiento, con dosis que iban aumentando progresivamente. Para la obtención de los espectros se usó un aparato portátil basado en tecnología de fibra óptica operando en tiempo real. El problema de calibrar la respuesta del BWSÎ a la luz se estudió utilizando el grupo de datos obtenidos de las muestras envejecidas artificialmente; también fueron comparados otros diferentes modelos de calibración. Las curvas de calibración fueron después utilizadas para estimar el nivel de alteración debido a la posible acción sincrgica de la luz y de otros factores, los cuales ocurren cu las muestras de BWSÍ envejecidas naturalmente por exposición in situ. Se propone además un método para contabilizar semi-cuautitativameute la acción acumulativa de la luz y de otros agentes ambientales, introduciéndose el concepto de 'dosis equivalente de luz', de tal manera que el factor de riesgo general puede ser evaluado.
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