also pearlescent effect, pearl effect, rainbow effect
Printing inks, pearlescent pigments which are mixed are called iridescent colors because of their optical effect. They therefore set themselves apart due to their reflective properties of the conventional pigments.
The iridescent effect or pearl effect is caused by interference in nature e.g. occurs when viewing the beads of a shell. Depending on the viewing angle of a color spectrum of the light source is intensified, which leads to a spectral profile over a certain length ("rainbow effect").
In a paint as a medium tiny, coated with film mica platelets are incorporated. These small mirror particles throwing incident or already remitted by the subject light differently depending on the angle. This leads to the fact that parts of the light are multiply reflected within the paint layer before they can escape from it. This path difference has a path difference of the light waves to each other result, thereby causing interference at certain wavelengths.
The interference effect affects not only one spectral color, but will have a certain length. For the viewer a gradient in the vicinity of a determined color is visible. Since these change slightly with each change in viewing angle, the viewer's attention is drawn to the surface so refined.
Such colors are particularly well suited for striking decoration of food and cigarette packaging. You give the more conventional offset printing products an iridescent sheen.
Pearl colors behave similarly to dispersion coatings. They dry quickly by absorption or evaporation of the water component. For them, therefore, the same limits apply.
Sources and additional literature
 Murvai, Geza; Ober, Juliane: Inline-Veredelung im Bogen- und Endlosdruck. Bundesverband Druck und Medien, 2007
 Pfennig, Nicole; Steinwandel, Viktoria: Diplomarbeit. Freie Hochschule für Grafik-Design & Bildende Kunst Freiburg e. V.
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