Instrumental color measurements of automotive samples with convex cylindrical curvatures under diffuse lighting

Although car bodies often have curved surfaces sprayed with solid or effect colors, instrumental color measurements of these surfaces are usually carried out on flat samples with multi-angle spectrophotometers. We used a DigiEye® instrument, equipped with diffuse lighting, to measure color in different areas on the surface of 22 car body samples (105 mm wide x 148 mm high), 10 with solid colors and 12 with effect colors. Each measured sample had 5 convex cylindrical curvatures with radii ranging from 50.4 mm to infinity (flat sample). On the average, color variations are 3-4 times greater in the effect samples than in the solid samples, as deduced from measurements in 27 mm x 36 mm rectangles and circular areas of 8 mm in diameter 32 mm apart. For solid color pairs, the color differences between analogous regions of 8 mm diameter were very similar (standard deviations below 0.2 CIELAB units). For samples pairs with solid colors there was a high linear correlation (r>0.95) between the color differences measured with DigiEye® at 8 mm diameter analogous regions and BYK-mac measurements with 45°/110° geometry, but for color pairs with effect colors the linear correlation between measurements with these two instruments was always very low (r<0.5).