The standard ISO 2471:2008 Diffuse reflection method for the determination of paper and Cardboard opacity (paper substrate) specifies a method for measuring paper and Cardboard opacity using the principle of diffuse reflection. Opacity is an important indicator for evaluating the shading performance of paper and Cardboard, which directly affects its application in printing, Encasement and other fields. The standard describes in detail the measuring instrument, sample preparation, test procedures and calculation methods to ensure the accuracy and conformity of the results. This method is suitable for all kinds of paper and Cardboard, which provides a scientific basis for production quality control, product development and technical exchange in trade, and helps to improve the industry standardization level and product competitiveness.