Documents Containing Information Invisible to the Naked Eye

This invention refers to documents, and particularly to vital documents, contaning a marking which is not visible to the naked eye and which contains information.

In this instance, the term "vital documents" refers to bank notes, securities, credit cards, transportation tickets, identification cards, admission tickets, and the like. The high level of modern counterfeiting techniques makes it possible to imitate and to counterfeit such important papers. To prevent this or make it more difficult, vital documents are equipped with, for example, invisible markings that make it possible to test authenticity, using equipment that is for the most part expensive and complicated. The invisible marking is made with, for example, electrical, magnetic, and optical methods. In the optical realm, markings are known (DT-OS 2, 347, 836) that are made with two printing inks, which produce a clearly recognizable metamerism effect; that is, they look different under different illumination.

The purpose of this invention is to create a vital document fully protected from counterfeiting, the identifying information of which is not visible to the naked eye but can be made visible by a simple method.

The task is solved by the characteristics set forth in claim 1. Advantageous additional developments are defined in the remaining claims.

The vital documents pursuant to the invention, completely protected from counterfeiting, are described below in greater detail, with the help of two examples.

A vital document according to the first example contains or consists of an information support equipped with a "dye" that looks black in the range of the visible spectrum of wavelengths 400 to 700 nm, yet is transparent for infrared rays in wavelengths 700 to 1000 nm. The black color can be made, for example, by overprinting with three colors, such as cyan, magenta red, and yellow, or through a mixture of dyes whose absorption range covers the entire visible range. However, any other "dye" that is black in the visible spectral region and transparent for infrared rays can be utilized. On the first black layer of color made according to the manner described, an information-bearing marking is positioned using a black color, e.g., printer's black, which is transparent in varying degrees for infrared rays. The marking can be made in the form of a binary coding, in which the areas printed with the color last mentioned correspond to one binary configuration and the unprinted areas to the other binary configuration. For coding the information, a different degree of transparency or opacity can be used for infrared rays, in which the code is constituted by a series of areas with varying levels of transparency for infrared rays. The marking made in this way on the black supporting surface is invisible, since it is completely concealed by the black visible color. The marking that is transparent in varying degrees under infrared rays of wavelengths 700 to 1000 nm can therefore not be recognized with the naked eye, but only with the help of appropriate methods, for example, transmitting and receiving diodes for infrared rays, an infrared image converter, or similar equipment. The marking can be made first of all by covering it with the black color that is transparent to infrared rays. The information support of the vital paper can be transparent or non-transparent for infrared rays, depending on whether the irradiation method or the reflection method is used in making the marking visible.

In a vital document according to the second example, the black color or color mixture transparent under infrared rays is mixed with a color that is non-transparent in varying degrees for these rays, e.g., printing black, in a variable ratio, so that various mixtures are created. The marking bearing the information is made, for example, by the printing method using mixtures having varying mixture ratios, in which the mixture ratio, that is, the degree or level of transparency for infrared rays of wavelengths of 700 to 1000 nm, is used for coding the information.

In using colors that seem black and that, for other wavelengths outside the visible spectrum, are transparent, partially transparent, and non-transparent, the marking can be made visible using other rays that are not visible with the naked eye, e.g., ultra-violet rays, X-rays, and similar rays.

The vital documents described contain information identifying them, the contents of which cannot be discovered by the naked eye, as the observer sees only a black patch. The information is coded, so that unless one knows the code any information uncovered is difficult to counterfeit. The degree of security of the vital document described against counterfeiting is further increased by the fact that it is difficult to measure and imitate the level of transparency for the rays used. The marking can be simply made by the printing method.