Effect pigments can provide fascinating optical effects on surfaces of solid objects. They consist of small flakes of metal, mica, polymer or other materials and are embedded in the lacquer layer. Based on optical reflection and interference effects due to varying illumination and observation angles, they can also cause changes of color, texture and gloss. Depending on the concentration of the pigments in the coating the lacquer appears either more homogeneous or more sparkling. Due to the effect pigments properties this can lead to the so-called flop effect where the dominant color switches completely or to the living sparkle effect where single eye-catching flakes seem to be twinkling in a dark but also in other lacquer surroundings. These effects are used, among other things, for functional purposes such as security printing and for many decorative applications as well such as plastics, cosmetics, printed products, coatings or car paints and even more applications in foods.
Automated goniometer at the Fraunhofer IOSB used for the measurements, © IOSB. (BRDF.mov)
The quantitative evaluation of effect pigment surfaces needs a methodic approach. So far there is no universally accepted standardization defined by a scientific committee or organization. Nevertheless, some suggestions and recommendations exist, and first gadgets to measure optical properties of effect pigments in layers have been presented by manufacturers of optical measurement devices.
Considering the manifold optical effects a lot of different aspects have to be taken into account and questions need to be resolved:
The Fraunhofer IOSB was asked to support a big manufacturer of effect pigments in finding suitable parameters and by testing different experimental conditions in order to capture the appearance of effect pigments in lacquers.
Object with effect pigment coating (© IOSB).
The technical conditions at Fraunhofer IOSB are excellent for extensive optical investigations as well as for basic research concerning the optical properties of reflection and the appearance of samples. The project aim was to capture the living sparkle effect as well as possible. In order to achieve this, a measurement setup had to be conceived and created that enabled subsequent image evaluation to deliver all the physical and statistical properties of the pigments mentioned above. The angle-dependencies were examined with the help of a robot-based automated goniometer. This allowed the light source and the detector to be moved separately and independently from each other around the measurement sample, covering a wide range of angles. Several digital cameras with different pixel numbers, detector sizes, dynamic range and sensitivity have been used for the test measurements. RGB-cameras were compared to a detection system consisting of a monochrome camera and an electrical switchable RGB-filter. Additionally, the influence of varying beam-spread angles on the appearance of the coating was tested on an optical bench. Since the observed flakes are so small (only a few microns in diameter), the detection unit is very sensitive to optical aberrations like defocus, spherical and chromatic aberration. As a result, some faults or flaws of the camera were exposed in the images. Therefore, the application of high quality optical components was crucial as well as careful adjustment during the measurements.
Image of a coating with effect pigments showing living sparkle effect, © Merck.
Several samples with living sparkle effect have been measured under different conditions. After comparing and evaluating a number of different measurement parameters, a setup consisting of a tungsten lamp with broadband spectrum, an electrical switchable RGB-filter and a monochromatic camera was recommended for the acquisition of high-quality images intended for subsequent image evaluation. The linearity of the selected camera with regard to exposure time and light intensity was verified. During the test series with varying illumination and observation angles it became apparent that the dynamic range of the camera was not sufficient in some situations. For this reason, the exposure times of the camera had to be adapted in order to avoid saturation of single sparkles and to guarantee an optimal dynamic range even at very small inclination angles. Furthermore, for noise reduction and calibration multiple measurements with a reference white panel were recommended as well.