An Early Warning System for Light Damage in Museums and Exhibitions

Works of art cannot be appreciated without light, but light is also one of the most powerful parameters in altering colour and so causing irreversible damage. Therefore, a compromise has to be found that can balance visitor's requirements and object's safety. It is known that the damage increases with the length of exposure and the irradiance. Rather than limiting the time of exposure for each object, it is advisable to monitor the lighting conditions on site. A continuous monitoring program for a number of objects with data loggers, besides being aesthetically unacceptable, would be rather expensive, and applicable only for selected examples. So, the dream of each conservator would be to have at her/his disposal a low cost and non-invasive tool, which allows a quick and easy monitoring of the entire collection. Now such a dream has been realized thanks to a project partly funded by the European Commission (Key action "The City of Tomorrow and Cultural Heritage", reference EVK4-CT2000-00016). In fact, new light dosimeters, LightCheckÒ Sensitive (LCS) and LightCheckÒ Ultra (LCU), have been proposed as an early warning system for light damage. These dosimeters can fill the present gap created by the lack of suitable materials to detect low to medium values of light doses. Actually, the well-known Blue Wool Standard, in its most sensitive formulation n. 1, is suitable only for light doses of the order of several hundred of Kluxh. The two dosimeters consist of photosensitive dyes, applied on a substrate with a polymer binder. Their response to light is cumulative, as they undergo a progressive variation in colour with increasing exposure. Clearly discernible colour steps are reached depending on the dose of light received, so that the dosimeters can be used for a first, instrumentation free, easy and fast estimation of the luminous exposure that an object receives in a given environment during a given period. The working principle is quite analogous to the one of pH indicator papers: the colour of the exposed dosimeter is compared to the one of a properly calibrated scale so as to get a semi-quantitative information on the total light dose received during the exposure period. The exhaustive work carried out for the elaboration and characterisation of LightCheck® dosimeters led to the definition of their respective field of application. In fact, LCU and LCS can be used in a complementary manner because the former is especially suitable for the monitoring of very-light sensitive and fugitive objects, usually exhibited under low levels of light or for short exposure periods, while the latter is better suited for controlling the lighting of more durable objects or for longer exposure time.

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