The standard BS ISO 2471-2008 Determination of paper and Cardboard opacity (Cardboard backing), diffuse reflection method specifies the test method for the determination of paper and Cardboard opacity by diffuse reflection method. Opacity is an important indicator of the ability of paper and Cardboard to block the penetration of light rays and has a significant impact on their printability and performance. The standard describes in detail the test equipment, sample preparation, measurement procedures and calculation of results to ensure the accuracy and comparability of data. This method is suitable for mass evaluation of paper and Cardboard in different applications, provides key technical support for production control and Product Research & Development, and has important reference value for quality management and international trade in paper products industry.