For measuring the brightness of some materials, some dated industrial methods cite a Magnesium Carbonate block standard to be set to Y = 100% in reflectance. This is a very old top-of-scale standard and HunterLab does not recommend using it.
FAQ: What is a Y Brightness/Luminance?
FAQ: What is a Beta (ß) Reflectance or Luminance Factor for Brightness?
FAQ: What is a Reflection Coefficient (0-1)?