Iridescent Ink
Semitransparent ink with pearl luster caused by an interference structure of a thin film. The ink contains scaly mica pigments covered by a thin film of titanium dioxide (TiO2), ferric oxide (Fe2O3) or other metal oxides. When falling on pigments, one part of light is reflected, the other part is transmitted into the underlying layers. As a result, multiple reflection and transmission of light by different layers occur and result in interference. The luster is almost unnoticeable at a right angle of observation and illumination. When changing the angle of observation or illumination, the intensity of luster and the hue slightly changes.
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Fig. 1. France. Emergency passport issued in 2006:
a — page 8. Paper substrate; b — the same. Zoomed fragment. Iridescent ink. View at a right angle of observation and illumination; c — the same. Zoomed fragment. Iridescent ink pigments
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Fig. 2. Japan. Travel document for return issued in 2013:
a — folded document. Front side. Paper substrate; b — the same. The letters JPN printed with iridescent ink. Oblique light; c — the same. Zoomed fragment. Iridescent ink pigments