GB/T 42674-2023 "Optical Functional Films—Test Method for Microstructure Thickness" specifies the method for determining the cross-sectional microstructure thickness of optical functional films using a scanning electron microscope (SEM). This standard is applicable to single-layer or multi-layer structured films with a thickness not less than 50 nm. The test requires the use of an SEM with a resolution of less than 50 nm, calibrated for magnification using a stage micrometer with an uncertainty error of less than 5%. After sample pretreatment such as cutting, grinding, and polishing, the cross-section is observed via SEM. EDS line scanning or backscatter techniques are employed to assist in distinguishing interfaces. Measurements are conducted at specified magnifications in accordance with the standard, providing accurate data for film quality control and performance evaluation.