This part of ISO 5631 specifies a method for measuring the colour of paper and board by the diffusereflectance method with the elimination of specular gloss.This part of ISO 5631 is not applicable to coloured papers or boards which incorporate fluorescent dyesor pigments. It may be used to determine the colour of papers or boards which contain fluorescentwhitening agents provided the UV content of the illumination on the test piece has been adjustedto conform to that in the CIE illuminant C, using a fluorescent reference standard that fulfils therequirements for international fluorescent reference standards of level 3 (IR3) as prescribed byISO 2469 with an assigned ISO brightness value (C/2°) provided by an authorized laboratory, asdescribed in ISO 2470-1.