What is the Hunter Rd, a, b Color Scale?

Hunter Rd, a, b Color ScaleThe Hunter Rdab opponent-color scale, developed in 1943, was an early version of the Hunter L, a, b scale. 

Instead of L, Rd is used to designate lightness and is identical to the CIE Y Brightness/Luminosity value.  Sometimes the Rd, a, b color scale is shown as LRd, aRd, bRd. Continue reading