Patent ID: 6865001

Claim:
A security device comprising: a pattern formed from an array of cells having at least two substructure regions within each cell and displaying an unencoded component visible to the unaided eye of a viewer and an encoded covert image component not readily discernable to the unaided eye of a viewer from any viewing angle without a separate decoder; said substructure regions for image cells and for non-image cells having the same geometry but different contrast; the image cells having a contrast that is selected from the set: (i) diffractive regions, (ii) modulated-diffractive regions, and (iii) non-diffractive regions; the non-image cells having a contrast selected from the same set but different from the contrast of the image cells; said modulated-diffractive regions having at least one diffractive characteristic different than a diffractive characteristic of said diffractive regions; each cell and the at least two substructure regions within each cell being small enough to be not readily discernable to the unaided eye of a human viewer; each cell having at least one substructure that is a diffractive region and at least one substructure region that is a non-diffractive or modulated-diffractive region, the ratio of the area of the diffractive region to the area of the non-diffractive or modulated-diffractive region of the at least two substructure regions in each cell of the encoded or unencoded cells being substantially identical; the encoded component not visible to an unaided eye being encoded into said pattern by substantially reversing the contrast of each cell forming a portion of the encoded covert image component to change diffractive portions to non-diffractive or modulated-diffractive portions, and to change non-diffractive or modulated-diffractive portions to diffractive portions, the brightness within each cell remaining substantially unchanged as a result of the contrast reversal by maintaining a substantially constant ratio of substantially 1-to-1 between the diffractive area to the non-diffractive or modulated-diffractive areas among the substructure regions within the cells so that the same amount of light is diffracted to the viewing location from the cells; and the encoded component not readily visible to an unaided eye of said viewer being rendered visible to the unaided eye of said viewer when viewed through an external decoding device.