GB/T 26459-2011 "Paper, Board and Pulps — Determination of Yellowness Reversion" specifies the method for determining the yellowness reversion of paper, board, and pulps, applicable to all types of paper, board, and pulps. It employs a xenon lamp aging chamber to simulate light-aging conditions, measuring the blue light diffuse reflectance factor of the sample before and after aging. The yellowness reversion value is calculated to evaluate the yellowing resistance. The standard defines parameters such as aging time, temperature, and humidity, and standardizes instrument calibration, sample preparation, and testing procedures. Multiple parallel determinations are performed, and the average value is taken to ensure accurate and comparable results for yellowness reversion measurement.