The opacity of a material is an indication of how much light passes through the material. The higher the opacity, the lower the amount of light that can pass through the material. Generally, opacity is calculated from reflectance measurements of the material with a black backing and the material with a white backing.
Conditions for Measurement
Instrumental: Most HunterLab color measurement instruments. Availability may depend on the software package.
Standard Observer Function: 2 or 10 degree
Transmittance and/or Reflectance: Reflectance.
Opacity measurements are done on a variety of materials to determine the amount of light that passes through the materials.
(See attached pdf file for the complete article and formula)