| Abstrakt: | SUMMARIES. Methodological aspects of the use of photosensitive materials as light dosimeters in museum monitoring are investigated. As a case study, a spectroscopic investigation in the 400-700 nm range is developed for Blue Wool Standard No. i (BWS1), a colorimetric indicator traditionally used by conservators to obtain a qualitative indication if the risk associated with lighting. Experiments using both artificial and natural light-aging were performed on BWS1 samples and a set of reflectance spectra was non-invasively collected during the aging process at increasing light-doses. A portable device based on fibre-optics technology, operating in real time, was used for the spectral acquisition. The problem of calibrating the BWS1 response to the light was investigated using the data-set of artificially light-aged samples, and different calibration models were compared. The calibration curves were then used to estimate the alteration, due to the possible synergistic action of light and other factors, which occurred in BWS1 samples naturally aged by exposure in situ. A method to account semi-quantitativcly for the cumulative action of light and other environmental agents is proposed, by introducing the concept of 'equivalent light-dose', in terms of which the overall risk factor can be evaluated.
CONCLUSION. 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. |
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