This International Standard specifies a method for the calculation of transmittance based upon diffuse reflectance measurements. The use of the method is restricted to white and near-white translucent papers (see 3.9). If it is necessary to determine the transmittance of papers which contain fluorescent whitening agents, the fluorescence emission is eliminated using the prescribed UV cut-off filter. NOTE This means that, although this International Standard refers to ISO 2469, which permits the use of both filter colourimeters and abridged spectrophotometers, a filter colourimeter with no means of eliminating the emission of fluorescence is not suitable for this type of measurement if fluorescent whitening agents are present.