CIE 2017 color fidelity index Rf: a better index to predict perceived color difference?

Cauwerts, Coralie [UCL] Jost, Sophie [ENTPE] Avouac, Pascale [ENTPE]

The CIE General Color Rendering Index Ra is currently widely and internationally used to assess the ability of white light sources to render colors. But over the past decades, there has been increasing evidence of its limitations. As a result of several years of scientific work, the Commission Internationale de l’Eclairage (CIE) published in April 2017 an updated calculation method. To reduce misinterpretation by users and make a clear distinction with the other aspects of color quality, the color rendering index was renamed color fidelity index (CIE-Rf). Before proposing this index as a new international standard for color fidelity prediction, there is a need for validation. This paper presents a psychophysical experiment designed to address this aim. Twelve observers evaluated color differences of 16 color samples under eight pairs of light sources. 3000K and 4000K light-emitting diodes and fluorescent sources with CIE-Rf values ranging from 64 to 94 were investigated. Results show that the prediction of color differences is significantly better in CAM02-UCS color space than in CIE U∗V∗W∗ or CIELAB, whatever the color matching functions. A high correlation was found between perceived color differences and CIE-Rf. This psychophysical experiment did not make it possible to detect significant differences with the other tested color fidelity indices (CIE-Ra, CQS-Qf, CRI2012, CRI-CAM02UCS, and IES-Rf). However, there are some signs of an improvement of the prediction by CIE-Rf in comparison to CIE-Ra. To increase, in further works, the chance to detect potential significant differences between color fidelity indices, some modifications of the experimental protocols are suggested.