This document assesses the long-term susceptibility to light of materials and treatments used in conservation of porous inorganic materials. Examples may include materials which have been treated in the course of conservation (e.g. cleaning, consolidation, water repellents, coatings or biocides) or materials which may exhibit colour changes following exposure (e.g. mortars and small number of stones). The procedure can be used to evaluate the impact and longevity/durability of treatments against untreated materials or unexposed materials. NOTE 1 Porous inorganic materials are both natural stones (rocks) and artificial stone materials (mortars, stuccoes, bricks, ceramic materials, etc.). NOTE 2 Treated materials are those on which one of the following treatments has been applied: cleaning, application of water repellent, consolidating materials, coatings or biocides and artificial ageing.