Goniochromism of Multicolor and Interference Pigments Under Varying Illumination Conditions

Color results from the interaction of objects with varying wavelengths of light and the human visual system’s perception under different illumination conditions. In this study, special emphasis was placed on examining how varying illumination conditions and measurement geometries affect the color appearance and optical properties of printed effect pigments. Two distinct groups of pigments were examined: three interference pigments (M-series) based on calcium–aluminum borosilicate substrates, and three multicolor pigments (C-series) based on silicon dioxide. To ensure comparability of the results, all pigments were printed using screen printing techniques onto black PVC film. Characterization involved using a multi-angle spectrophotometer to measure CIEL*a*b* values, chroma (C*), and hue (h*) under CIE standard illuminants D50, A2, and F2 at a fixed illumination angle of 45° and aspecular angles of −15°, 15°, 25°, 45°, 75°, and 110°. Furthermore, the research methodology included the evaluation of lightness difference (∆L*), color differences (∆E*), chroma difference (∆C*), and hue difference (∆H*), with the D50 illuminant chosen as the reference and A2 and F2 as sample illuminants. The flop index (FI), as the indicator of lightness change at different scattering angles, was calculated for all printed pigments under all three standard illuminations. This multidisciplinary approach provided a deeper understanding of the relationship between pigment structure, illumination conditions, and viewing angles in our visual perception of printed pigments, which is of great importance for the development and optimization of goniochromatic materials. The results showed that while A2 and F2 illuminants have a negligible impact on lightness differences across all pigments, they induce noticeable variations in color, chroma, and hue differences, particularly at near-specular angles (−15° and 15°). Conversely, these differences become negligible at far-aspecular angles (75° and 110°). Furthermore, flop index (FI) analysis revealed that despite the larger borosilicate flakes in the M-series, the silicon dioxide-based C-series pigments exhibited the highest overall flop effect, with pigment C1 maintaining consistently high FI values under all illuminants.