Watermark encoding and detection using narrow band illumination
AssigneeXerox Corporation
InventorsKaren M. Braun, Raja Bala, Robert P. Loce
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Abstract
A system is employed to reveal a watermark in a document. A watermark generator is utilized to select the placement and at least one colorant combination of an image and at least one colorant combination for a watermark on a document, where the at least one colorant combination of the image and the watermark form a metameric pair. A printing system receives data from the watermark generator and places the image and the watermark on the document. A decoder comprising a narrow band illumination element is selected or tuned to a wavelength corresponding to the colorant combinations utilized by the printing system to reveal the watermark placed thereon.
Claims20
A system to reveal a watermark in a document, comprising:
- a watermark generator that is utilized to select the placement and at least one colorant combination of an image and at least one colorant combination for a watermark on a document, where the at least one colorant combination of the image and the watermark form a metameric pair;
- a printing system that receives data from the watermark generator and places the image and the watermark on the document;
- and
- a decoder comprising a narrow band illumination element that is selected or tuned to a wavelength corresponding to the colorant combinations utilized by the printing system to reveal the watermark placed thereon.
A method to decode a printed watermark, comprising:
- deriving at least one colorant combination of an image and at least one colorant combination for a watermark on a document, where the at least one colorant combination of the image and the watermark form a metameric pair;
- selecting a visible narrow band illumination whose spectral power distribution maximizes the visual distinction between the image and the document;
- and,
- utilizing said visible narrow band illumination to decode a printed watermark generated from the image.
The method according to claim 13, wherein the narrow band illumination is at least one of a light emitting diode, a laser, a fluorescent light source, a light source filtered to be narrow band by a filter, grating, or prism, a scanner or a color sensor.
The method according to claim 19, wherein the narrow band illumination is an LED scanner, the peak wavelengths of the metameric pair have opposing polarities, wherein at least one of a sum and a difference of the peak wavelengths are used to increase strength of signal associated with the decoded watermark.
The method according to claim 19, wherein the watermark strength for the metameric pair is calculated as |Y1−Y2|, where Y1=∫R1(λ)×S(λ)×V(λ)dλ and Y2=∫R2(λ)×S(λ)×V(λ)dλ, where R1 and R2 are the spectral reflectances of the metameric pair, S is the spectral power distribution of the narrow band illumination, and V(λ) is the spectral luminous efficiency function.
A method for generating a printed watermark, comprising:
- selecting at least one colorant mixture/pattern for printing;
- deriving a second colorant mixture/pattern with the property of appearing substantially similar to the first mixture/pattern under broadband illumination, but distinct from the first mixture/pattern under a selected visible narrow band illumination;
- and
- printing the two colorant mixtures/patterns in close spatial proximity to produce a watermark that is concealed under broadband illumination and revealed under the visible narrow band illumination.
Dependencies are inferred from the claim text.