Cross References to Related Applications

This application claims the priority of Euro¬ pean patent applications 95117830.0 filed November 13, 1995 and 96105835.1 of April 13, 1996, the disclosures of which are incorporated herein by reference in their en- tirety.

Technical Field

The invention relates to security documents with security markings, m particular security markings formed by holes, as well as a method for producing same, according to the preamble of the independent claims.

Security documents can be documents repre¬ senting a value, such as cheques, shares, etc., espe- cially bank notes, but also documents of identification, such as passports, identity cards, access authorisation documents, etc.

Background Art

In order to make falsification and counter¬ feiting difficult, security documents are usually pro¬ vided with security markings, such as watermarks, kme- gra s, fluorescent marks, etc. Many conventional marks have, however, the disadvantage that they are difficult to produce or easy to forge and/or that they can hardly be verified without technical equipment by the layman.

WO 95/26274 describes a carrier representing a value provided with recognisable patterns formed by holes. The holes are created with a laser beam. The pres¬ ence of the holes can easily be detected by the eye. How¬ ever, it is still difficult to verify if the holes are

created by a laser or not. To allow verification, it is therefore proposed to make part of the hole patterns so fine that they can only be made visible by means of ap¬ pliances. This, however, has the disadvantage of requir- ing special equipment for verification.

Disclosure of the Invention

Hence, it is a general object of the inven¬ tion to provide a security document with security mark¬ ings of this type that are easily verified without spe¬ cial knowledge or equipment. It is also an object of the invention to provide a method for the production of such security documents.

This object is implemented by the document and method of the independent claims.

By using holes having a diameter chosen such that they are invisible when looking at them in reflec- tion while they are visible with the naked eye when viewed m transmission (i.e. when looking through them, e.g. by holding them in front of a source of light}, the security marking can easily be verified without special equipment. Holes with this properties preferably have diameters between 85 and 130 μm. In another embodiment, the diameter is between 85 and 115 μm, preferably between 100 and 110 μm.

Holes having these properties can be prepared with laser light (or other suitable radiation) as well as with discharge sparks while they can hardly be created with mechanical means.

The recognisability of the marking is in¬ creased when the holes are arranged in an absorbing, sub¬ stantially completely coloured, printed or coated area. In such an area, the transmission of the document is low and the holes are well visible when holding the document against a light source.

Preferably, the holes have an elongate cross section. Such holes can be produced with a higher rate because the document or the laser beam can be moved while burning the hole. In order to provide elongate holes that are difficult or impossible to recognise in reflection while they can easily be seen in transmission, the short¬ est diameter of the holes is preferably between 85 and 130 μm, while the largest diameter is preferably below 170 μm, preferably between 110 and 170 μm, in particular between 110 and 170 μm.

The security document can e.g. be a banknote, cheque, share, identification document, etc.

Other preferred features are described in the dependent claims.

Brief Description of the Drawings

The invention will be better understood and objects other than those set forth above will become ap¬ parent when consideration is given to the following de¬ tailed description thereof. Such description makes refer¬ ence to the annexed drawings, wherein:

Fig. 1 is a security document according to the invention, wherein the hole pattern, which is usually not or hardly visible in reflection, is represented by black dots of exaggerated size,

Fig. 2 is a schematic detailed view of the bank note of Fig. 1, Fig. 3 is a schematic representation of a method of production for the security document, and F g. 4 is a schematic detailed view of a second embodiment of the holes.

Modes for Carrying Out the Invention

In the following, the invention is shown in the embodiment of a bank note. However, it is not limited to bank notes and can be applied to any other type of se¬ curity document, preferably having a paper carrier, espe¬ cially documents representing a value and identification documents .

The design of a possible embodiment of the document or bank note 10, respectively, is shown in Figs. 1 and 2. It has been printed in conventional manner with various motifs 1 - 5. In addition to this, it comprises a security marking 6. This marking is represented by ex¬ aggerated black dots in Fig. 1. In reality it is practi- cally invisible for the inexperienced observer when viewed in reflection without optical aids. It consists of a two dimensional pattern of holes 11, as they are shown in Fig. 2. Each hole extends through bank note 10. When the bank note is held against the light, the holes 11 can be seen as bright spots with the naked eye, i.e. without the help of any technical equipment.

Since the pattern extends in two dimensions, it can form a sign familiar to the observer, such as a letter or a number. This makes a verification of the marking easier.

As described in detail below, production of the pattern is easier when the pattern is arranged in several, parallel rows 14. However, it is also possible to use a pattern with non parallel rows. To improve contrast, the whole security mark¬ ing 6 is arranged in a field 5 of the bank note that is substantially completely coloured, covered with printing or coated, i.e. the colour, printing or coating is either uniform or has a structure much finer than the holes. The optical transmission of the note in this area is there¬ fore low, such that the surroundings of the security

marking 6 appears dark when held against the light while the bright holes 11 are clearly visible.

The area 5 should extend at least over part of security marking 6 (i.e. several holes 11), preferably the whole security marking 6, for providing a good visi¬ bility of the marking.

The diameter a of the holes 11 is preferably m the range of 85 to 130 μm, more preferably between 85 and 115 μm. When using smaller diameters the holes are difficult to recognise and irregularities m the margins of the holes lead to an irregular hole size. When using larger diameters, the holes become visible m reflection. Best results are achieved with diameters around 100 to 110 μm. The mutual distance d between the holes should be at least 700 μm. A smaller distance d weakens the paper and can lead to tears between the holes .

For producing security marking 6, the bank note 10 is e.g. hit simultaneously by several pulsed la- ser beams 12, such as it is shown schematically in Fig.3. The laser beams are arranged in a row beside each other. The fully printed bank notes 10 are moved individually or in groups on sheets through the beams into a direction 13. Direction 13 is perpendicular to the row of laser beams. The number of laser beams depends on the desired width or numbers of rows 14 of the security marking. By suitable pulse modulation of the laser beams the hole security marking 6 can be generated in a single pass.

The laser source 15 can e.g. be an arrange- ment of one or more Nd:YAG lasers, because these lasers allow the generation of short pulses and therefore high rates of production.

For applying the security marking 6 to a bank note 10 of pure cotton paper with a thickness between 80 and 95 μm pulses with an energy of 3 - 4 j and a dura¬ tion of 80 nS with a wavelength of 1.064 μm can be used. The laser beams are focused to a theoretical diameter of

20 - 22 μm. The effective Gaussian beam diameter is difficult to measure directly - it is expected to be between 10 and 30 μm.

The holes generated m this way have a typi- cal diameter between 100 and 110 μm independently of any printing present on the note. They are well visible in transmission with the naked eye while they are invisible m reflection. No sharp edges or burs are formed around the holes. The paper's resistance to tearing is not af- fected.

The security marking according to the inven¬ tion can be combined with further conventional markings, such as water marks, kmegrams, etc.

For further security, the holes 11 can also be aligned to marks printed on the bank note.

In the above examples, a bank note on a paper carrier has been described. However, the invention can also be used with other carriers, such as plastic carri¬ ers . In the examples shown so far, the holes have a round cross section. However, it is also possible to use holes with a non-round cross section, such as holes with elongate or polygonal cross section.

A preferred embodiment of elongate holes is shown m Fig. 4. The holes shown here have roughly ellip¬ soidal shape with a shortest diameter dl and a longest diameter d2. dl is preferably m the range of 85 - 130 μm, d2 is smaller than 170 μm, preferably 110 - 170 μm. Holes having such a size are barely visible m reflection while being clearly visible in transmission.

The elongate holes 11 are arranged parallel to each other.

Holes of the type shown m Fig. 4 can be gen¬ erated in an efficient manner by aligning their longitu- dmal axis with the direction of the relative movement between the notes and the laser beam(s) . By moving the notes or beams during application of the laser pulses,

elongate holes are formed automatically. This allows a continues movement of the notes or laser beam(s) during the application of the laser pulses and therefore a larger production rate for a given pulse length. According to a further embodiment of the in¬ vention, a single laser beam is used for creating the perforations, which beam is deflected appropriately for generating a desired pattern, which also allows the pro¬ duction of non-parallel rows of holes. It is preferred to move the paper in one direction while deflecting the la¬ ser beam in a direction transversally thereto, such that a similar situation as shown in Fig. 3 results, with the individual beams of Fig. 3 corresponding to different positions of a single laser beam. The deflection allows the generation of any arbitrary pattern, which can be more complex than a simple row or rows of holes. The laser can e.g. be a CO2 laser with a wavelength of 10.6 μm. An acousto-optic deflector with high acoustic band width can be used for beam deflection. Furthermore, a fo- cusmg lens is used, the focal width of which is deter¬ mined in known manner by the diameter of the holes and the properties of the laser beam (beam diameter, beam quality) . The speed of the note movement and the pulse repetition rate are adjusted for generating the desired pattern. Preferably the perforation occurs under a pro¬ tecting gas, such as nitrogen, which prevents a burning of the notes. The protecting gas can also extend to the focusing lens for preventing contaminations thereon. Preferably, a detector is arranged on the side of the paper facing away from the laser, which de¬ tects the laser beam and therefore immediately recognises if a hole has been perforated or not. The sensor is con¬ nected to a control unit, which controls the generation of the perforations.

While there are shown and described presently preferred embodiments of the invention, it is to be dis¬ tinctly understood that the invention is not limited thereto but may be otherwise variously embodied and prac- tised within the scope of the following claims.