A COLORIMETRIC LOOK AT THE RHS CHART - PERSPECTIVES FOR AN INSTRUMENTAL DETERMINATION OF COLOR CODES

From their special reflectance curves, the colours of the 808 chips in the RHS colour Chart have been measured in the CIELab system using the L*C*h notation (Lightness, Chroma, hue) with respect to the CIE D65/10 degrees Illuminant/Observer condition; three spectrocolorimeters and two copies of the Chart were used. Standardized conditions for visual observation are proposed, matching those used for colour calculations in the perspective of an objective instrumental assessment of colour codes. An important heterogeneity in colour, reaching an impairing level for a reproductible RHS codification was detected over the surface of all the chips printed by the half-tone screen process. For most of those printed in solid colours the non-uniformity of colour remains below the threshold of visual perceptibility colour in a single copy. Colour variations between patches of two copies (one recent and brand new, the other seven years old but rarely used) are more important but, in most cases, cannot be considered large enough to vitiate colour code determination. Regarding the organization of the Chart, especially the basic parameter of hue, many discrepancies were detected by colour measurement: different colour groups (purple-violet, purple and red-purple for instance) comprised chips sharing a largely common hue range, with mismatched hues in many leaves. The important inconstancy of colour steps between closest colours or closest colours displayed in different leaves or even different fans are additional factors causing practical difficulties or wrong decisions in visual codification. Using colour measurement coupled with an automatic search of RHS codes using advanced colour differences calculations (CMC model) handled by a computer database management system, is helpful for overcoming these difficulties. Such a system is also efficient in rearranging patches sorted according to homogeneous hues then to variations of Chroma and Lightness: an example with the patches originally placed in the yellow group (no. 1-13) is discussed. Finally as a reference for plant colour codification, the CIELab scale based N2S system is considered as a potential substitute for the RHS Chart.