This Gulf International Standard specifies a method for the determination of the opacity (paper backing) of paper by diffuse reflectance. It can be used to determine the opacity of papers or boards 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 papers or boards which incorporate fluorescent dyes or pigments.