Final answer:
The Color Rendering Index (CRI), or Ra, is calculated using the color sample set R1-R8.
Step-by-step explanation:
The Color Rendering Index (CRI), or Ra, of a light source is calculated using the color samples R1-R8. This index measures the ability of a light source to accurately reproduce the colors of various objects in comparison with a natural light source. The sample set of eight colors (R1-R8) is commonly used to determine the average CRI rating of the light source, and it is central to the international standard method for CRI calculation.