Methods are currently available for deploying a thin strip of clear material such as polyester film within paper during the papermaking process. If the film is used as a security thread and is first microprinted prior to dispensation during the papermaking process the resulting paper then contains a legible code that is legible in transmitted light. With papers of the substance and thickness used for currency and banknotes, the code is also visible under transmitted light. The purpose of printed security threads is to deter a potential forger since the coded indicia on the thread is buried within the paper and can not be duplicated by surface printing techniques. The authenticity of such a document can be readily verified by comparing the appearance of the security thread under reflected light against its appearance in transmitted light. It is the reflected light appearance that is most relied upon by the public when handling currency and banknotes in general circulation. The reflected light appearance, however, does not directly reveal the presence of the printing that is completely legible in transmitted light. In fact, the nondistinct muted line of the security thread that is conspicuous at the surface of the currency and banknotes becomes the distinctive feature in the eyes of the general public.
The inclusion of the security device within the body of the security paper now requires sophisticated papermaking machinery which is not available to forgers. One method of incorporating the security device is described within U.S. Pat. No. 3,880,706 to Williams wherein the security device is sandwiched between two layers of formed paper midway during the papermaking process.
An opaque item embedded within security paper is also described as early as the issue dates of U.S. Pat. Nos. 210,089; 964,014 and 1,929,828. It is believed that the two ply insertion approach disclosed within these patents detered feasibility for use within currency paper since the two ply papers could conceivably be replicated by laminating thin sheets of counterfeit paper without requiring a sophisticated papermaking machine. The simplicity in the configuration and design of the proposed security items were such that a forger could replicate them without a great deal of skill or expense.
Another method of incorporating the security device within the security paper comprises pressing the device within the wet paper fibers while the fibers are unconsolidated and pliable as taught by U.S. Pat. No. 4,534,398 to Timothy Crane which patent is incorporated herein for purposes of reference. In this method the security device is visible from one surface of the security paper for visual verification.
U.S. Pat. No. 4,552,617 also in the name of Timothy Crane describes a method of incorporating a security device within security paper by dissolving the security device carrier substrate and allowing the security device to be visually accessed from one side of the security paper.
All the aforementioned methods for incorporating security devices within the security paper allow for visible access to the presence of the security device by means of the unaided eye. The Williams approach could lead an observer to believe that a blurred line on the surface of the security paper, for example, is a sufficient indication of authenticity without requiring that the paper be held up to transmitted light for actual verification. The earlier security device also caused the security paper to exhibit an increased thickness in the vicinity of the device compared to the thickness of the paper itself. This increased thickness or "bulge" line can be duplicated by a skilled forger to lead the observer to believe that a security device is actually present, without further examination.
When an unprinted metalized plastic thread such as described within the aforementioned patent to Williams is used as a security thread, the optical and electrical properties of the thread can be used for automatic verification by determining optical variations across the paper as well as by standard electronic metal detection techniques. It has been determined, however, that the optical properties of such a metalized thread can be duplicated to some extent by means of a pale but opaque line printed on the surface of the paper in the vicinity of where the security thread would be located. A dull pencil line may also fool the unskilled observer as to the presence of a buried security thread.
For reliable verification, the currency or banknote must be held up to a light source to reveal the coded indicia on the security thread that make a security thread virtually impossible to forge. The nondistinctive line that is the visible surface manifestation of the embedded security thread under reflected light is easily simulated also by a variety of printing processes.
Earlier attempts at printing on plastic strips and embedding the strips within the paper have not heretofore proved feasible since the ink used to form the printed information that was legible under transmitted light also allowed the information to become legible under reflected light. The public could then rely upon the presence of the printed matter solely under reflected light, which printing is easily replicated by counterfeit means.
One way to insure that the public does not come to rely on such an easily simulated security thread characteristic is to manufacture currency and banknote paper containing a security thread that is virtually invisible under reflected light with no manifestation on the surface of the currency or banknote that such a security thread is present. This two-fold test of authenticification namely, legible under transmitted light and invisible under reflected light, has heretofore not been met by any of the earlier noted devices.
The purpose of this invention therefore, is to provide a method for incorporating a security thread bearing printed information within security paper that is easily readable in transmitted light but virtually undetectable when it is viewed under reflected light. A further purpose of this invention is to provide the printed information at a specific location within the security paper and to economically produce the paper at high speeds using modern manufacturing techniques.