GB/T 39691 - 2020 is used to determine the Refractive index of plastics. The standard is based on the principle of light refraction, and stipulates that equipment such as Abbe refractometer can be used. Clear the requirements of instrument accuracy and Measurement range to ensure the accuracy of measurement. Detailed description of the plastic sample prepative method to ensure that it meets the test conditions. Standardize the operation process, including instrument calibrating, sample placement, Measuring temperature control and other links. Through this standard, the plastic Refractive index can be accurately measured, providing important data support for plastic product quality control, material research and development and performance evaluation, and promoting the standardized development of the plastics industry.