Optical Variable Ink (OVI) is a specialized security ink that changes color when viewed from different angles. It is one of the most effective and difficult-to-counterfeit features used on high-value documents, banknotes, and certificates.
The defining property of OVI is its dynamic color shift.
Color Shift Effect: When you tilt the printed item, the ink's color visibly and distinctly changes. For example, it might shift from green to blue, gold to green, or magenta to gold. This is not a subtle sheen or iridescence; it's a clear, dramatic change between two distinct colors.
Non-Metallic Appearance: Unlike metallic inks, which simply look shiny, OVI has a high-quality, opaque, and often "liquid" or deep appearance. It does not contain metallic flakes like standard pearlescent inks.
The magic of OVI lies in its physics, not chemistry. It works through light interference, similar to the way a soap bubble or an oil slick creates shifting colors.
The ink contains microscopic multi-layer flakes (often made of a magnesium fluoride core coated with semi-transparent layers of chromium or aluminum).
These layers create a precise optical filter. When light hits them, some wavelengths are reflected while others pass through and are reflected by lower layers.
Depending on the viewing angle, the light waves reflecting from different layers either constructively interfere (making a specific color very bright) or destructively interfere (canceling that color out).
As you tilt the ink, the effective distance the light travels changes, altering which wavelengths interfere constructively. This results in the perception of a different color.
This complex physical structure is extremely difficult and expensive for counterfeiters to replicate accurately.
OVI is reserved for items requiring a high level of security to prevent forgery:
Banknotes: This is the most common use. For example, the numeral on the lower-right front of recent US $100 bills uses green-to-black color-shifting ink (a specific type of OVI). Many currencies (Euros, British Pounds, Swiss Francs) use more dramatic color shifts.
Passports: Often found on the passport's data page, on the cover, or on visa stickers.
ID Cards and Driver's Licenses: Used on high-security national ID cards.
Certificates: For university degrees, stock certificates, and other important documents.
Brand Protection: On high-end products (e.g., luxury goods, pharmaceuticals, software) to guarantee authenticity.
Impossible to Photocopy or Scan: A flat scanner or copier can only capture the ink from one angle, producing a static color that looks obviously wrong compared to the real item.
Difficult to Manufacture: The ink formulation and the precise flake structure are proprietary and tightly controlled by a few specialized companies (e.g., SICPA in Switzerland, Gleitsmann Security Inks in Germany).
Easy for the Public to Verify: No special tool is needed to check it. The "tilt test" is immediate and intuitive. This makes it a powerful public security feature.
In summary, Optical Variable Ink is a high-security printing feature that uses the physics of light interference to create a dramatic, visible color shift, making it an extremely effective tool in the fight against counterfeiting.
