A fallacy in the definition of Δ H *

When a color differs from the reference, it is desirable to ascribe the difference to differences in the perceptual attributes of hue, chroma, and/or lightness through psychometric correlates of these attributes. To this end, the CIE has recommended the quantity Δ H * as a psychometric correlate of hue as defined by Δ H * = [(Δ E *) 2 ‐ (Δ L *) 2 ‐ (Δ C *) 2 ] 1/2 , where the correlates correspond to either the 1976 CIELAB or CIELUV color spaces. Since Δ H * is defined as a “leftover,” this definition is valid only to the extent that Δ E * comprises exclusively Δ L *, Δ C *, and Δ H * and that Δ L *, Δ C *, and Δ H * are mutually independent compositionally, both psychophysically and psychometrically. It will be shown that as now defined Δ H * lacks psychometric independence of chroma and always leads to incorrect hue difference determination. Such a deficiency causes problems, especially in the halftone color printing industry, since it can suggest an incorrect adjustment for the hue of the inks. A revised definition herein of Δ H * provides a psychometric hue difference independent of chroma, valid for large and small psychometric color differences regardless of chroma. However, for small chromas, the seldom used metric Δ C might be a better color difference metric than Δ H * because complex appearance effects make the perceptual discrimination of lightness, chroma, and hue components more difficult than for high chromas.