CIELAB Hue‐Angle Anomalies at Low Tristimulus Ratios

Hue angles calculated using the CIELAB recommendation are usually dependent on the luminance factors of the stimulus if any tristimulus ratio is below the critical figure at which the Pauli linear function is applied. This can cause errors of up to 35° in the case of stimuli of low luminance lying on the spectrum locus or purple line. Errorrs are unlikely to occur in the case of surface colors, but in the case of transparent object colors their frequency and magnitude are such as to make CIELAB unsuitable as a color space or as a color‐difference formula whenever a tristimulus ratio is below the critical figure. Practical considerations suggest that in such cases the best solution is to use a modified Judd polynomial function, and this course is suggested for consideration by the CIE Colorimetry Committee and the committee responsible for developing an ISO Standard for the color measurement of plastics.