Caveats in the Use of DeltaH* and DeltaC* in Color Difference Analysis and Correction

The CIE uses deltaE* as an estimate of an overall color difference between two colors. However, it is desirable to segregate the overall difference into differences due to hue, chroma, and/or lightness. The CIE suggests the quantity (deltaE*)2 to be considered as the sum of (deltaL*)2, (deltaC*)2, and (deltaH*)2, where the correlates correspond to either the 1976 CIELAB or CIELUV color spaces. The quantity deltaH* is supposedly a psychometric of hue difference as defined by H*=[(deltaE*)2-(deltaL*)2-(deltaC*)2]1/2. Since deltaH* is defined as a remainder, this definition is valid only to the extent that deltaE* comprises exclusively deltaL*, deltaC*, and deltaH*, and that delta L*, deltaC* and delta H* are mutually independent compositionally, both psychophysically and psychometrically. It will be shown that the present definition of deltaH* lacks psychometric independence of chroma, and therefore deltaH* does not represent a difference due to only hue difference. Similarly, because C* is dependent on L*, a significant deltaL* can affect deltaC* so that deltaC* might not represent a difference due to only chroma difference. Such deficiencies can suggest an incorrect interpretation and subsequent adjustment to colorants.