ASTM D523 defines the primary reference standard as a highly polished, flat black glass
ASTM D523 is the Standard Test Method for Specular Gloss. Developed by ASTM International (formerly the American Society for Testing and Materials), this standard provides a standardized methodology for measuring the glossy appearance of a surface. astm d523 pdf
Sometimes, industry groups (like the American Coatings Association or PCI) license ASTM standards for their members. Check if your trade organization provides D523 as a member benefit. ASTM D523 defines the primary reference standard as