The new CIE 2017 colour fidelity index Rf: a better index to predict perceived colour difference?

The CIE General Colour Rendering Index Ra is currently widely and internationally used to assess the ability of white light sources to render colours. 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 colour quality, the colour rendering index was renamed colour fidelity index (CIE-Rf). Before proposing this index as a new international standard for colour fidelity prediction, there is a need for validation. This paper presents a psychovisual experiment designed to address this aim. Twelve observers evaluated colour differences of 16 colour samples under eight pairs of light sources. 3000K and 4000K LEDs and fluorescent sources with CIE-Rf values ranging from 64 to 94 were investigated. Results show that the prediction of colour differences is significantly better in CIECAM02-UCS colour space than in CIE U*V*W* or CIELAB, whatever the colour matching functions (CMFs). A high correlation was found between perceived colour differences and CIE-Rf. This psychovisual experiment did not make possible to detect significant differences with the other tested colour 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 colour fidelity indices, some modifications of the experimental protocol are suggested.