US Patent 9892585 Magnetic watermarking of a printed substrate by metameric rendering

A document may include a non-magnetic substrate, a first colorant mixture printed as a first image upon the substrate, the first colorant mixture including a magnetic ink, and a second colorant mixture printed as a second image upon the substrate in substantially close spatial proximity to the printed first colorant mixture. The second colorant mixture may consist essentially of one or more non-magnetic inks and exhibit properties of both low visual contrast and high magnetic contrast against the first colorant mixture, such that the resultant printed substrate does not reveal the first image to the human eye, but will reveal the first image to a magnetic image reader.