The 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 Sometimes the Rd, a, b color scale is shown as LRd, aRd, bRd.In general, the Hunter Rd, a, b color scale has been replaced by either Hunter L, a, b or CIE L*, a*, b* in most applications with the exception of the Rd value being used as a single-value to quantify the overall brightness of reflectance neutral limestone-based products. In addition, bRd is sometimes used as a companion metric with Rd to quantify the slight yellowness associated with trace metal contamination (iron, copper) of limestone-based powders.
A limestone powder or pressed plaque of the best color quality is associated with a high Rd value and + bRd yellowness value close to 0.
For more information, see AN1016 Hunter Rd, a, b Color Scale.
Mr. Philips has spent the last 30 years in product development and management, technical sales, marketing, and business development in several industries. Today, he is the global market development manager for HunterLab, focused on understanding customer needs, providing appropriate solutions and education, and helping to solve customer color challenges across these industries and cultures.