This standard specifies the principles and methods for the indirect determination of the particle size of carbon black (furnace method) for rubber using a reflectometer. The core principle involves measuring the reflectivity of ink pastes prepared under identical conditions from the sample and a standard reference carbon black (such as SRB-5). By leveraging the optical property that smaller carbon black particles exhibit higher blackness, the method indirectly characterizes the particle size of the primary particles. Therefore, the measured result is not the size of individual carbon black particles but a relative value reflecting their tinting strength. The primary testing instrument specified in this standard is a reflectometer (such as certain blackness meters equipped with reflectivity measurement capabilities).