Publications Sponsored by the Colour Measurement Committee-VIII

Optimizing Color-Difference Formulas for 3D-Printed Objects

Based on previous visual assessments of 440 color pairs of 3D-printed samples, we tested the performance of eight color-difference formulas (CIELAB, CIEDE2000, CAM02-LCD, CAM02-SCD, CAM02-UCS, CAM16-LCD, CAM16-SCD, and CAM16-UCS) using the standardized residual sum of squares (STRESS) index. For the whole set of 440 color pairs, the introduction of k_L (lightness parametric factor), b (exponent in total color difference), and k_L + b produced an average STRESS decrease of 2.6%, 26.9%, and 29.6%, respectively. In most cases, the CIELAB formula was significantly worse statistically than the remaining seven formulas, for which no statistically significant differences were found. Therefore, based on visual results using 3D-object colors with the specific shape, size, gloss, and magnitude of color differences considered here, we concluded that the CIEDE2000, CAM02-, and CAM16-based formulas were equivalent and thus cannot recommend only one of them. Disregarding CIELAB, the average STRESS decreases in the k_L + b-optimized formulas from changes in each one of the four analyzed parametric factors were not statistically significant and had the following values: 6.2 units changing from color pairs with less to more than 5.0 CIELAB units; 2.9 units changing the shape of the samples (lowest STRESS values for cylinders); 0.7 units changing from nearly-matte to high-gloss samples; and 0.5 units changing from 4 cm to 2 cm samples.

Optimization of the method for color measurement and color-difference calculation of holographic prints with light pillars

To determine the methods of color measurement and color-difference calculation for holographic prints with light pillars, 94 pairs of holographic prints constituted by 17 different products were collected. A set of color-difference comparison experiments was organized by 64 observers with normal color vision, and a total of 86 groups of visual judgments were gathered. The CIELAB and CIEDE2000 color-difference values were calculated on the basis of the analysis of the microstructures of gratings distributed on the holographic paper. The performances of the original formulas were evaluated in terms of the standardized residual sum of squares index, and then they were optimized considering the power function effects (a, b factors) together with a contribution from lightness (k_L factor). Meanwhile, the color-difference threshold of the holographic prints was estimated with a goal to minimize the number of wrong decision in the visual experiment; therefore, the values were set as 2.50 and 2.00 for the original CIELAB and CIEDE2000 with the consistency of 91.5% and 98.9%, respectively. The results can also provide guidance to evaluate the color quality of the holographic prints with light pillars in the packaging and printing industries.

Evaluation of threshold color differences using printed samples

The performances of uniform color spaces and color-difference formulae for predicting threshold color differences were investigated based on visual assessments of 893 pairs of printed color patches under a D65 source. The average ΔE(ab,10)* of the pairs was 1.1 units. A threshold psychophysical experiment was repeated three times by a panel of 16 observers with normal color vision. The experimental data were used to evaluate nine color-difference formulae and uniform color spaces using the standardized residual sum of squares (STRESS) measure. The results indicated that all formulae and spaces performed very similarly to each other, and outperformed CIELAB for threshold color differences. The chromaticity-discrimination ellipses were used to compare with previous results from small color differences [Color Res. Appl. (2011), doi:10.1002/col.20689], and they agreed with each other, except for the purple color center.