Chroma Shift and Gamut Shape: Going Beyond Average Color Fidelity and Gamut Area

Though sometimes referred to as a two-measure system for evaluating color rendition, IES TM-30-15 includes key components that go beyond the two high-level average values, Fidelity Index (IES R_f) and Gamut Index (IES R_g). This article focuses on the Color Vector Graphic and Local Chroma Shift (IES R_cs,_hj), discussing the calculation methods for these evaluation tools and providing context for the interpretation of the values. We illustrate why and how the Color Vector Graphic and Local Chroma Shift values capture information about color rendition that is impossible to describe with average measures (such as CIE R_a, IES R_f, or IES R_g) but that is pertinent to more completely quantifying color rendition and to understanding human evaluations of color quality in the built environment. We also present alternatives for quantifying the Color Vector Graphic and Local Chroma Shift values, which can inform the development of future measures.