GB/T 15972.21-2008 "Specification for optical fiber test methods, Part 21: measurement methods and test procedures for dimensional parameters, coating geometric parameters", specifies the measurement methods and test procedures for the geometric parameters of optical fiber coatings, covering the coating layer diameter, concentricity and other key parameters. Clarify the test principle, equipment, sample preparation and operation steps to provide a unified standard for accurate measurement of the geometric dimensions of optical fiber coatings, ensure the accuracy and standardization of mass detection of optical fiber coatings, and help optical fiber production and quality control.