Ordinal Scale Based Description of Colour Rendering

Perceived colour differences of 17 test colour samples (uniform standalone patches) were evaluated visually between a test and a reference light source on three visual scales. Two graphical rating scales (a greyscale-anchored colour difference scale and a similarity judgement scale) and a five-step ordinal rating scale (excellent, good, acceptable, not acceptable or very bad colour rendering) were used. The experimental setup included tungsten halogen, gas discharge, fluorescent, and white LED light sources at two correlated colour temperatures, 2700 and 4500 K. There was an inverse relationship between similarity judgement and visual colour difference results. Each category of the five-step ordinal rating scale had a characteristic mean visual colour difference value. Visual colour differences correlated best with the recently developed CIECAM02-UCS colour difference metric. Latter metric was used to predict the observers' ratings of visual colour differences on the above five-step ordinal rating scale. From the predicted ratings of 17 test-colour samples under the test light source, a new ordinal rating scale based colour rendering index (RCRI) was defined and compared with previous colour rendering indices. RCRI correlated well with both the mean visual colour differences and the mean similarity judgements. Despite the significant interobserver differences of the visual assessment of colour differences, the RCRI method showed an overall performance of 73% in terms of good predictions of the rating categories. Validation experiments with complex still life (tabletop) stimuli are currently underway. (C) 2010 Wiley Periodicals, Inc. Col Res Appl, 36, 272-285, 2011; Published online 7 October 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/col.20629