The optical properties of modern OVI color shift ink

Optically variable ink is one of the security features that appears on various state licenses and is one of the harder features to evaluate without knowing what you're looking at. I want to explain the optical mechanism so that the check you do on a card is grounded in what you're actually observing.

OVI works through thin-film interference. The ink contains metallic flakes that are coated with extremely thin layers of material — each layer is only a few hundred nanometers thick. When light reflects off these thin layers, the wavelengths that constructively interfere depend on the viewing angle. As the angle changes, the wavelengths that constructively interfere change, and the color you perceive shifts. This is a physical optical phenomenon rather than a printing phenomenon — you can't achieve this effect by mixing pigments because the color shift depends on the physical thickness of the coating layers, not on pigment absorption.

What this means for evaluation: genuine OVI shifts through a specific color range as you tilt the card. The shift should be smooth across the range — not a single jump from one color to another, but a continuous transition. The specific colors in the shift range depend on how the ink was formulated. For the common gold-to-green shift used on many state licenses, you should see gold at one angle transitioning through a yellow-green intermediate to a green at the opposite extreme, with the transition being smooth and continuous as the angle changes.

Poor quality OVI imitations tend to produce a narrower color range — sometimes just a single color with a slight sheen rather than a genuine multi-color shift. They also often have a less smooth transition: you see the two extremes but the intermediate colors are absent or weak. The depth and richness of the color shift under a focused light source is what you're trying to assess, not just whether there's any shift at all.