| 摘要 |
An object of this invention is to provide a method of producing printing with a hidden image which cannot be observed with the unaided eye but which can be observed by use of a tool to determine the authenticity of a document. In accordance with the invention, the lithographic printing process is used in conjunction with tints or halftones to produce a printed document having an image therein which is not detectable with the unaided eye, but which may be observed when the print is viewed through a tint screen. The hidden image is produced by making a composite negative comprising an exposure through a negative containing the image and a tint oriented at a first angle onto a piece of film, and then making a second exposure through a positive containing the image and a tint oriented at a second angle onto the same piece of film. The composite negative is enlarged and dots in the area of the intersection of the image and background are blended to hide the image. Thereafter, the print is reduced to render the image invisible to the unaided eye.
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