in which a metameric match is easily degraded by a slight change in conditions, such as a change in the illuminant. Measuring metamerism The best-known measure of metamerism is the color rendering index (CRI), which is a linear function of the mean Euclidean distance between the test and reference spectral reflectance vectors in the CIE 1964 color space. A newer measure, for daylight simulators, is the MI, the CIE metamerism index, which is derived by calculating the mean color difference of eight metamers (five in the visible spectrum and three in the ultraviolet range) in CIELAB or CIELUV. The salient difference between CRI and MI is the color space used to calculate the color difference, the one used in CRI being obsolete and not perceptually uniform. MI can be decomposed into MIvis and MIUV if only part of the spectrum is being considered. The numerical result can be interpreted by rounding into one of five letter categories: Metamerism and industry Using materials that are metameric color matches rather than spectral color matches is a significant problem in industries where color matching or color tolerances are important. Automobile industry A classic example is the automobile industry: the colorants used for interior fabrics, plastics and paints may be chosen to provide a good color match under a cool white fluorescent source, but the matches can disappear under different light sources (e.g. daylight or tungsten source). Furthermore, because of the differences in colorants, spectral matches are infrequent and metamerism often occurs. Textile industry Color matching in the textile dyeing industry is essential. In this branch, three types of metamerism are commonly encountered: illuminant metamerism, observer metamerism and field-size metamerism. Due to the wide range of different illuminants we are exposed to in daily life, textile color matching is hard to ensure. Metamerism on large textile items can be resolved by using different light sources when comparing colors. However, metamerism in smaller items such as textile fibers, is more difficult to be solved. This difficulty arises due to the necessity of a microscope, which has one single illumination source, to observe these small fibers. Therefore, metameric fibres cannot be distinguished neither macroscopically nor microscopically. A method which can solve metamerism in fibres combines microscopy and spectroscopy, and is called microspectroscopy. Paint industry Color matches made in the paint industry are often aimed at achieving a spectral color match rather than just a tristimulus (metameric)