The TAPPI T 519 standard specifies a method for measuring paper diffuse reflection opacity for white and near-white paper, formerly known as "print opacity". This method evaluates paper's ability to cover by measuring its diffuse reflection of light under d/0 geometric conditions. opacity is an important indicator for measuring the prevention of transparent printing of paper during the printing process, and has a direct impact on printing quality and paper performance. This standard provides the paper and printing industry with a scientific means of detection to help optimize paper mass and ensure its performance in practical applications.