Lighting quantity indexes for lighting paintings in museums

In museum lighting, paintings are the artworks with the highest light sensitivity. They are very vulnerable to irreversible permanent photochemical damage such as fading and discolouration. The basis of effective preventive conservation is to determine the two lighting quantity indexes of illumination and annual exposure. At present, there are some problems such as inconsistent standards, lack of accuracy, and failure to consider protection and visual needs comprehensively. In this study, halogen lamp was used as experimental light source, and 25 pigment specimens were irradiated for 1440 h at four illumination levels. The colour parameters were tested every 240 h and the colour difference was calculated. Based on the data analysis, three-dimensional visual curved surface plots of colour difference Delta E of 25 pigments varying with illuminance I and time t were established, which revealed the colour damage law of different pigments caused by illumination and time. Then, the curved surfaces were fitted into the colour damage evaluation equation set Delta E = f(n) (I, t) (where n = 1, 2, 3, ..., 25) to establish the damage evaluation model of paintings, which described the damage law of pigments mathematically. Through the above model, the general recommended values of illumination and annual exposure for general painting lighting can be calculated, and the method for obtaining the special recommended values of illumination and annual exposure for precious paintings lighting was given.