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This part of ISO 5631 specifies a method for measuring the colour of paper and board by the diffuse reflectance method with the elimination of specular gloss.This part of ISO 5631 is primarily intended for measuring the colour of paper and board to be used in the graphic arts industry, where that industry specifies the measurement of colour under D50/2° conditions in accordance with ISO 13655. This method differs from ISO 13655, in that the UV content of the illumination is adjusted to a different level.The method can be used to determine the colour of papers or boards that contain fluorescent whitening agents, provided the UV content of the illumination on the test piece has been adjusted to conform to that in the CIE illuminant C, using a fluorescent reference standard that fulfils the requirements for international fluorescent reference standards of level 3 (IR3) as prescribed by ISO 2469 with an assigned ISO brightness value (C/2°) provided by an authorized laboratory, as described in ISO 2470-1.This part of ISO 5631 is not applicable to coloured papers or boards that incorporate fluorescent dyes or pigments.