Patent ID: 7433075

Claim:
An invisible information recording method comprising: extracting a location of at least one blank area of a page image of a sheet of paper, wherein said location of said at least one blank area is different from a location of an image, which is visible to the naked eye, of said page image of said sheet of paper; and recording a digital image on said location of said at least one blank area on said sheet of paper; wherein said digital image comprises information in the form of pixels so sized as to be invisible to a naked eye and at a print density invisible to the naked eye, wherein said extracting comprises extracting a plurality of locations of blank areas from said page image of said sheet of paper, wherein said plurality of locations of blank areas extracted from said page image of said sheet of paper include at least one blank area which is located in a position on said page image of said sheet of paper other than a location of a margin of said sheet of paper, wherein no part of said digital image is recorded in said location of said image of said page image of the sheet of paper, and wherein a print density invisible to the naked eye is such that the pixels each so sized as to be invisible to the naked eye are coarsely distributed and an image density is 0.1 or less.