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This part of ISO 12625 specifies testing procedures for the instrumental determination of opacity paper backing of tissue paper and tissue products by diffuse reflectance. This part of ISO 12625 also gives specific instructions for the preparation of test pieces (single-ply, multi-ply products), where special precautions may be necessary. It can be used to determine the opacity of tissue paper and tissue products which contain fluorescent whitening agents, provided the UV content of the radiation incident on the test piece has been adjusted to conform to that in the CIE illuminant C using a fluorescent reference standard provided by an ISO/TC 6 authorized laboratory as described in ISO 2470-1. This International Standard is not applicable to coloured tissue paper and tissue products which incorporate fluorescent dyes or pigments.