Paper making and fiber liberation – Processes and products – Non-uniform – irregular or configured web or sheet
162103, 162108, 162116, 162126, 162127, 162129, 162133, 162140, 162146, D21H 2126
THE FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to a method of producing visible, preferably transparent or translucent, continuous streaks and/or delimited fields in paper in conjunction with forming a paper web in a paper machine. The invention also relates to valuable paper, particularly banknotes, produced from such paper. More specifically, the invention relates to the manufacture of a continuous paper web containing delimited fields and/or streaks of varying size and in varying positions, and to valuable paper produced from such webs. The fields and/or streaks differ from the surrounding paper with regard to material, colour, opacity and/or transparency or with regard to some other readily identified property, preferably an optical property. The incorporated fields and/or streaks are preferably transparent or translucent.
The rapid development of reproduction methods and of fourcolour copiers in particular, places increasingly higher demands on the manufacturers of security paper, cheque paper and banknotes. These demands have led to the development of new methods, materials and products which are intended to prevent forgery or at least make successful forgery more difficult to achieve. Many of the methods proposed hitherto are complicated and expensive in practice and the products produced by these methods are encumbered with drawbacks, such as unsatisfactory durability.
There is thus a need for methods which will enable security paper that is difficult to forge by reproduction with the aid of colour copying and which can be printed in conventional printing presses with conventional printing inks intended for security print to be produced in a relatively simple and inexpensive manner.
PRIOR ART TECHNIQUES
In order to prevent an image or text on a basic material, such as paper, from being reproduced with the aid of simple fourcolour copying on a copier, it is desirable that parts of the irradiated surface will emit and/or reflect incident light back to the sensors or like devices provided in the copier, in a manner which differs from the reflection caused by the remaining surface of the basic material and thus capable of being discerned by an observer without the aid of technical auxiliaries, i.e. can be seen with the naked eye. Alternatively, the surface of the basic material may be treated so as to obtain a very low reflectance from the surface, which can be achieved either with high absorption or by the transmission of incident light.
These known phenomena have earlier been utilized to make forgery difficult by colour copying, for instance, different methods of obtaining discernible reflection or interference have been proposed. In the case of a distinguishing or spectacular reflection, light is reflected in parallel from a mirror image, meaning that the sensor of a photocopier will receive no signal because the reflected light lies outside its measuring range, i.e. the measured surface or area, or because the luminosity is too weak to be determined. The best known example of a distinguishing reflection is "a partially embedded security filament or thread", proposed by Portals Ltd., see for instance EP 0059056, and by The Bank of England, see SE-C 415,214. According to these known methods, there is partially embedded in the paper a thread/strip which contains a layer or coating which produces reflectance, for instance a strip containing, e.g., a metallic mirror-reflective coating. The thread/strip is exposed at mutually spaced locations. When photocopying a valuable paper in which such a thread of reflective material is partially embedded, the reflective surfaces appear as dark fields on the photocopies.
Another earlier proposed method utilizes interference between reflective light of different wavelengths. When viewing a surface that is provided with an interference material, the visible colour impression varies with the angle at which the surface is viewed. This phenomenon cannot be reproduced in a copied image or picture. Interference effects can be achieved, for ins
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AB Tumba Bruk
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