A shifting, rainbow-like colour effect that changes depending on the viewing angle.
Usually produced by structural colour — thin-film interference or diffraction from microscopic surface structures — rather than pigment, so the perceived hue changes with viewing and lighting angle.
Iridescence is not the same as a simple metallic paint finish, though metallic finishes can look superficially similar.
Soap bubbles, some beetle shells and certain butterfly wings show iridescence.
Iridescent finishes and structural colour behave differently under changing light than flat pigment colours — see Colour in Nature for real examples.
An iridescent surface is measured with a multi-angle spectrophotometer, which records reflectance at several fixed angles away from the mirror direction, because no single reading describes it. For a thin film the physics is compact: the wavelengths that reinforce depend on the optical path through the film, twice its thickness times its refractive index times the cosine of the angle inside it. Tilting the surface shortens that path, so the reflected colour moves towards blue.
An iridescent or colour-shifting finish is specified with a single colour swatch or hex value.
Supplier and client each picture a different angle, and the delivered part is 'wrong' from where the client stands. Effect finishes are approved from physical master panels viewed at agreed angles.
A product with an iridescent coating is photographed for an online shop.
Each photograph freezes one angle and one light, so the listing shows a blue item and the customer unpacks a purple one. Video or a turntable sequence is the only fair representation.