This standard establishes a uniform test method for evaluating the whiteness level of opaque white or near-white powdered resins, sheet plastics, and opaque fluorescent whitened plastic products. Its core lies in using a specialized optical instrument known as a whiteness meter (also referred to as a whiteness tester or reflectance photometer) for measurement. By specifying particular illumination and viewing geometric conditions, the method measures the diffuse reflectance factor of materials to specific blue light wavelengths (approximately 457 nanometers). This quantifies color perception into a whiteness value, providing a scientific basis for the appearance quality control of plastic products, color consistency between different batches, and the evaluation of whitening effects in products containing fluorescent agents.