The TAPPI T 425 standard specifies the measurement method of paper opacity, which is tested under the conditions of 15/d geometry, Light source A/2 °, 89% reflection backing and paper backing. Opacity is an important indicator to measure the covering ability of paper, which affects the printing effect and the visual performance of paper. The standard applies to all kinds of paper and Cardboard, and helps to evaluate the performance of paper in printing, writing and other applications by accurately measuring the reflection and transmission characteristics of paper to light. It is widely used in the paper industry for product quality control, raw material evaluation and product development, providing a scientific basis for optimizing paper performance.