Color shifting pigments and colorants have been used in numerous applications, ranging from automobile paints to anti-counterfeiting inks for security documents and currency. Such pigments and colorants exhibit a property of changing color upon variation of an angle of incident light, or upon variation of a viewing angle of the observer. Color shifting pigments and foils are taught in U.S. Pat. No. 5,135,812 by Phillips et al., and color shifting polymeric stacks are taught in U.S. Pat. No. 6,797,366 by Hanson et al., both of which are incorporated herein by reference. Using color-shifting inks and foils in optical security devices is advantageous because they are difficult to counterfeit.
At present, many anti-counterfeiting devices include holograms and other diffractive optical devices. A dove image on a Visa credit card is one example of such a hologram. Diffractive security devices are taught in U.S. Pat. No. 5,825,547 by Lee, which is incorporated herein by reference. Unfortunately, the holographic devices can be counterfeited by shining a laser beam on the device and recording the diffracted image, or simply by mechanically replicating the holographic surface relief pattern.
Other types of micro-optic security devices are based on microlens arrays. These devices, providing a so called “floating image” effect, are taught in U.S. Pat. No. 7,333,268, U.S. Pat. No. 7,006,294, and US Patent Application Publication 2008/0165423, all by Steenblick et al., and U.S. Pat. No. 7,068,434 by Florczak et al., all of which are incorporated herein by reference. A major drawback of the microlens arrays devices for a banknote security application is that they tend to be thicker than required 1 mil, or 25 microns thickness. The device of Florczak, for instance, is about 2 mils, or 50 microns thick. Incorporating thick security devices into a banknote causes it to be slightly thicker on one end than on the other, which causes a stack of such banknotes to tilt towards one side, eventually causing the stack to fall. The color shifting optical security devices of this invention are generally free from the drawback of the increased thickness.
Color shifting foil and inks exhibit a shift of perceived color due to a phenomenon of light interference in thin films. Reflective thin film stacks are particularly useful as they exhibit high chromaticity at only a few thin film layers in the stack. When such a film is incorporated into an image of a printed document, the image changes color as the document is tilted back and forth. This phenomenon is observed because when the document is tilted, the light path differences in light reflecting from the various surfaces of the thin film optical stack change. These changes causes constructive and destructive light interference at different visible wavelengths. These spectral changes result in different colors to the human eye. When the document is rotated in its own plane, the observed color is constant and does not change because the angle of view of the image stays constant relative to the observer.
In U.S. Pat. No. 3,338,730 by Slade et al., which is incorporated herein by reference, a multi-hued reflective surface is disclosed. The multi-hued reflective surface is formed from a multiplicity of sharply defined regions, wherein the surface of each region has a plurality of parallel planes at an angle to the surface, the angles of the planes varying from region to region. The areas of the parallel planes and their positional angles in any one of the regions are different from that of the region or adjacent regions and are randomly oriented with respect to one another. To the viewer, one observes the surface as brilliant and multi-hued. Each region is coated with a reflective layer of aluminum, a spacer dielectric layer, and an optical metal (Ag, Cu or Au) to complete an optical interference filter. Each area is formed of grooves having a saw-tooth-like cross section consisting of triangles or squares.
When the orientation of surface of Slade is changed, multi-hued changing colors are observed in those sharply defined regions that are larger than human eye resolution, which is about 100 microns. The regions that are less than 100 microns is size appear to have a uniform color that does not change upon rotation, because various colors would be integrated by the eye into a region of uniform color.
The prior art lacks an optical device that exhibits a controllable and identifiable change of color upon rotation of the device, for example a controlled rotation of a banknote or security document such as a passport or lottery ticket placed on a flat surface for an authenticity examination. Such a device is highly advantageous and desirable because it provides an easily identifiable and difficult to counterfeit security feature.
It is an object of this invention to utilize known color shifting coatings in a novel and inventive manner so as to produce an optical device which exhibits varying color by rotation of the device, for use in optical security and identification devices, as a decorative element, and for numerous other purposes.