The gas cap, mirror housing and bumpers of a car are painted where they are produced: in the respective supplier’s workshop. To ensure that these special parts exactly match the car body later on, their color shades and effects need to be accurately matched. To do this, the participating companies exchange painted metal color chips or product samples and have them visually matched by trained colorists – a time-consuming and error-prone procedure. Instead of these color sample plates, it would be simpler and faster to exchange purely electronic data – yet so far there is no way of reproducing color impressions accurately enough using physical color values. A further challenge is presented by the special-effect pigments mixed into the paint, whose metallic or pearl luster effect varies under different lighting and from different viewing angles, or even causes the color to change completely – an effect known as ‘flip-flop’ – such as from copper red to green.
In collaboration with the measuring device manufacturer ColorAIXperts, the Fraunhofer Institute for Manufacturing Engineering and Applied Materials Research IFAM has developed a means of distinctly characterizing even such complex color shades in the form of spectral data. This is done by characterizing each pixel not simply by the three conventional RGB values, but across its entire color spectrum. “This enables us to measure even inhomogeneous multicolored structures with glitter and flip-flop effects,” explains Dr. Volkmar Stenzel of the IFAM. Each color sample is registered from several different viewing angles. “We systematically tried out various combinations of viewing angle and illumination in a car manufacturer’s color matching booth, and chose seven that clearly characterize each shade,” Stenzel continues.
The spectral data thus acquired can be electronically forwarded, converted back into colored images and then compared with the manufacturer’s color specifications. A special software program was developed for the purpose by i2s industrielle information systems GmbH. A practical test of 47 popular vehicle paint colors proves that the new system is suitable for use in the automotive industry: Experienced colorists compared four samples of each particular shade against a reference standard – using both color chips in the conventional way, and spectral data on the computer monitor. The different color matching systems proved to be amazingly consistent: the colorists reached the same decision in 82 percent of all cases.