Living Sparkle

Effect pigments can create 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 paint layer. Based on optical reflection and interference effects caused by different lighting and viewing angles, they can also lead to changes in colour, texture and gloss. Depending on the concentration of pigments in the coating, the coating appears more or less homogeneous or sparkling. Because of the effect pigment properties, this can lead to the so-called flop effect, in which the dominant colour changes completely, or to a vivid glitter effect, in which individual conspicuous flakes appear to sparkle in the dark, but also under other brightness conditions. These effects are used for functional purposes such as security printing and for many decorative applications such as plastics, cosmetics, printed products, coatings, car paints and other applications such as food.

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The quantitative evaluation of effect pigment surfaces requires a methodical approach. So far there is no generally accepted standardisation defined by a scientific committee or organisation. Nevertheless, there are some suggestions and recommendations, and the first devices for measuring the optical properties of effect pigments in layers have been presented by manufacturers of optical measuring instruments.

In view of the many different visual effects, many different aspects must be considered and questions must be answered:

• Illumination and observation angles: fixed angle pairs or movement, angle range
  
• definition of the light source (spectrum, light intensity, diffuse or directional lighting)
  
• Specification of the optical detector (single sensor, digital RGB camera or spectrometer, spectral sensitivity and resolution, number of pixels and dynamic range, optical magnification, viewing area, sharpness and depth of field when viewed at an angle, etc.)
  
• Analysis, processing and presentation of the measured data, definition of characteristic and significant parameters and quantities (density, thickness, number, colour of sparkle, granularity, visibility range, etc.
  

The Fraunhofer IOSB supported a major manufacturer of effect pigments in finding suitable parameters and testing various test conditions to determine the appearance of effect pigments in coatings.

The technical conditions at the Fraunhofer IOSB are excellent both for extensive optical investigations and for basic research into the optical properties of reflection and the appearance of samples. The aim of the project was to capture the vivid glitter effect as well as possible. In order to achieve this, a measurement set-up had to be designed and created that would allow for subsequent image evaluation, providing all physical and statistical properties of the above mentioned pigments. The angle dependencies were investigated using a robot-based automatic goniometer. This allowed the light source and the detector to be moved separately and independently around the measurement sample and cover a wide angular range. Several digital cameras with different pixel numbers, detector sizes, dynamic range and sensitivity were used for the test measurements. RGB cameras were compared with a detection system consisting of a monochrome camera and an electrically switchable RGB filter. Additionally, the influence of different divergence angles on the appearance of the coating was tested on an optical bench. Because the observed flakes are so small (only a few micrometers in diameter), the detection unit is very sensitive to optical aberrations such as defocusing, spherical and chromatic aberration. In the course of the measurements some defects of the cameras were found in the images. Therefore, the use of high quality optical components and careful adjustment during the measurements was crucial.

Several samples with lively glitter effect were measured under different conditions. After comparing and evaluating different measurement parameters, a setup consisting of a tungsten lamp with a broadband spectrum, an electrically switchable RGB filter and a monochromatic camera was recommended for taking high-quality images for later image evaluation. The linearity of the selected camera in terms of exposure time and light intensity was checked. During the series of tests with different angles of illumination and observation, 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 adjusted to avoid saturation in order to guarantee an optimal dynamic range even at very small tilt angles. For noise reduction and calibration purposes, several measurements can also be made with a white reference panel.