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)?
ASTM E284 Standard Terminology of Appearance defines reflectance factor as:
Reflectance Factor, n – ratio of the flux reflected from the specimen relative to the flux reflected from the perfect reflecting diffuser under the same geometric and spectral conditions of measurement.
The most common metric used to measure the brightness of materials is CIE Y Brightness or Luminance. The CIE Y metric is scaled between 0 (representing a Perfect Black of 0% reflectance across the visible spectrum) and 100 (representing a Perfect White of 100% reflectance across the visible spectrum).
The “ß (beta) reflectance or luminance factor” is the same as Y Brightness or Luminance, expressed as a ratio relative to base 1 instead of 100.
Another metric called “Reflection Coefficient, Visible (0-1)” is the same as ß (beta) reflectance factor: scaling the reflectance of the material between 0 for a perfect black and 1 for a perfect white.
In all cases the illuminant/observer combination used for calculation has been C/2° historically, but measurements of typical white or neutral materials, made under D65/10° will be very close.
Here is a table comparing Y Brightness/Beta reflectance and Reflection coefficient for typical white colors.