Calibration and Use of Photosensitive Materials for Light Monitoring in Museums: Blue Wool Standard 1 as a case study

Methodological aspects of the use of photosensitive materials as light dosimeters in museum monitoring arc investigated. As a case study, a spectroscopic investigation in the 400-700 nm range is developed for Blue Wool Standard No. 1 (BWS1), a colorimetric indicator traditionally used by conservators to obtain a qualitative indication of 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-quantitatively,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 call be evaluated.