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This Part of EN 13523 defines terms of the procedure for determining the metamerism of a colour match of an organic coating on a metallic substrate. When two colour specimens have identical spectral reflection curves, they are matching under any illuminant irrespective of its spectral characteristics. This is termed a "spectral match". It is also possible for two colour specimens having different spectral reflection curves to match visually under a given light source but not to match under another light source with different spectral characteristics; such matches are termed "metameric". One quantitative description of metamerism is the so-called "metamerism index". The information of the metamerism index is of limited value where ?E (instrumental colour difference for a given illuminant, see EN 13523-3:2001) is > 0,5. The metamerism index is not suited for determining the absolute colour difference or colour constancy of a given specimen at change of illuminant. The colour difference under the reference illuminant is to be measured in colour coordinates L*, a* and b* (see EN 13523-3:2001). Excluded from this method are organic coatings producing fluorescence and/or which are multicoloured, pearlescent or metallic.