Detection of color shifting elements using sequenced illumination
AssigneeDe La Rue North America Inc.
InventorRonald Bruce Blair
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Abstract
The present invention provides a method and apparatus for determining the presence of Color Shifting Elements (CSE) such as optically variable inks and foils on documents such as bank notes. The invention comprises passing a document past an image sensor such as a line scan camera while sequentially illuminating the document from at least two alternating azimuths. The light source at each azimuth alternates between different colors, producing an image that is interleaved according to color and azimuth of illumination. The invention calculates a reflected color value for each azimuth and compares the color values of the different azimuths to each other. A difference in color between azimuths of illumination indicates the presence of a CSE on the document.
Claims17
A method for determining the presence of color shifting elements on documents, the method comprising the steps of:
- (a) passing a document past an image sensor;
- (b) sequentially illuminating said document using at least two alternating azimuths of illumination, wherein the document is divided into discrete successive sections and a predetermined azimuth of illumination is applied to each section as the document passes said image sensor until a predetermined area of the document is imaged, producing an interleaved, multi-azimuth image of said area of the document, wherein a first one of the at least two azimuths of illumination is associated with a first light source at a first azimuth relative to the document, wherein a second one of the at least two azimuths of illumination is associated with a second light source at a second azimuth relative to the document, and wherein the first azimuth is different from the second azimuth;
- (c) applying a transformation function to said interleaved, multi-azimuth image to generate color values for each azimuth of illumination;
- and
- (d) comparing said color values, wherein variation in color value between azimuths of illumination indicates the presence of a color shifting element on the document.
An apparatus for determining the presence of color shifting elements on documents, the apparatus comprising:
- (a) an image sensor;
- (b) two light sources positioned at different azimuths relative to a common focal point;
- (c) means for synchronizing said image sensor with said light sources to sequentially illuminate a document passing in front of the image sensor using alternating azimuths of illumination, wherein the document is divided into discrete successive sections and a predetermined azimuth of illumination is applied to each section as the document passes said image sensor until a predetermined area of the document imaged, producing an interleaved, multi-azimuth image of said area of the document;
- (d) means for applying a transformation function to said interleaved, multi-azimuth image to generate color values for each azimuth of illumination;
- and
- (e) means for comparing said color values, wherein variation in color value between azimuths of illumination indicates presence of a color shifting element on the document.
Dependencies are inferred from the claim text.