OPTICALLY VARIABLE INK (OVI)

An ink that changes its color depending on the angle of observation and illumination. The ink contains a nontransparent scaly pigment with a layered structure that includes a reflective metallic layer, a transparent dielectric layer, and a translucent metallic layer. White light is partially reflected from the translucent metallic layer (surface layer) and partially from the reflective metallic layer (bottom layer). Interference and multiple reflections of light beams by the two reflective layers result in selective absorption of the waves that make up the white light spectrum. The reflected light waves cause the ink color we observe. The ink does not contain coloring pigments. It has metallic sheen. The ink color depends on the thickness of the dielectric layer (MgF2), the angle of illumination and the angle of observation.

Scheme of OVI visualization. Structure of the ink pigment

a —  Paper substrate; 

b —  OVI: angle of illumination 90°, angle of observation 90°; 

c —  Change of color: angle of illumination 45°, angle of observation 45°; 

d —  Zoomed fragment. OVI pigments: metallic sheen, scaly shape, irregular outline, 100% opacity