Such a document is known from the international patent application PCT/NL95/00119.

In this prior art document the perforation pattern takes the form of a character or a number of characters.

This perforation pattern is herein applied in the form of round, rectangular, or character-like perforations. These perforation patterns are therefore always representations of a black and white image without intensity differentia- tion.

Although making these perforations already raises a considerable barrier for potential forgers, there exists a need for a security feature which is even more diffi- cult to forge.

This object is achieved in that the perforation pattern extends over a surface of the document and repre- sents an image comprising brightness tones.

The perforation pattern is herein formed such that, for instance when the thus treated document is held up to the light or placed on a light box, an image becomes visible at the location of the perforation pattern.

It will be apparent that arrangement of such an image representing brightness tones requires extremely advanced technologies. Such technologies are not easily accessible to potential forgers, so that documents thus provided with such a perforation pattern are very diffi- cult to forge.

The perforation pattern is preferably applied by means of laser light.

This measure has the advantage that at the position of the perforation the material, for instance paper, plastic or textile is completely removed, so that when

for instance the fingertips brush over the document no elevation or channel or burr can be felt at all.

If for instance a perforation pattern were to be applied by forgers by means of for instance conically formed needles, cup-shaped edges or burrs are always created which are easily discernible with the fingertips.

This therefore provides a good means of identifying possible forgeries.

According to yet another preferred embodiment, the perforation pattern is a representation of an image arranged on the document by means of a different tech- nique.

Owing to the possibility of visually comparing the image and the perforation, thus without the deployment of complicated assist means being required, a less than perfect forgery of just one of the two becomes immediate- ly recognizable. A high degree of security against fraud is thus obtained.

This measure requires that, in order to make such a document, there must be present means for making the perforation pattern and means for making the associated image with a different technique, respectively means for recording this image and putting it into digital form to enable control of the means necessary for making a perfo- ration pattern.

According to yet another preferred embodiment, the image arranged by means of a different technique is specific to the document. This provides the option of personalizing the document. It will be apparent that this results in an even higher level of security.

An important application of the present invention lies in the fact that the document is a passport and that the image is a passport photograph.

Other preferred embodiments are stated in the re- maining sub-claims. It is pointed out here that the invention also relates to a method for applying a securi- ty feature in the form of a perforation pattern in a document, wherein the perforation pattern is applied by a

laser device which is adapted to successively apply a perforation pattern in the document, wherein the size of the perforation holes or the density of the perforation holes is controlled by means of an electronic representa- tion of the image.

The present invention also relates to a device for performing such a method.

The present invention will be elucidated hereinbelow with reference to the annexed figures, in which: figure 1 shows a schematic perspective view of a device for manufacturing a document according to the present invention; figure 2 shows a detail view of a perforation pat- tern such as illustrated in figure 1; figure 3 is a schematic perspective view of a pass- port according to the present invention; figure 4 shows a view of a bank note provided with a pattern according to the present invention; and figure 5 shows a postage stamp provided with a perforation pattern according to another embodiment of the present invention.

Before discussion of the technique, it is pointed out: that in the present technique the making of bright- ness tones, as in the graphic art, is possible by means of perforations which are applied according to a fixed grid, wherein the size of the perforations is a measure for the intensity and that it is also possible to repro- duce brightness tones by making use of perforation holes of equal dimensions, wherein the density of these dimen- sions is a measure for the intensity. Both options can in principle also be combined.

It is pointed out here that in the graphic art the first option is equivalent to the manner in which black and white photographs are reproduced in newspapers and that an example of the second technique can be found in the series of Netherlands postage stamps in which the likeness of Her Majesty the Queen is represented by dots of varying density.

Shown in figure 1 is a video camera 1 which is directed at a passport photograph 2. Video camera 1 records the image of passport photograph 2, converts it into an electronic form and feeds the thus obtained signal to a computer 3 in which it is stored.

The device shown in figure 1 further comprises a laser beam generating means 4 which is controlled by computer 3. This control relates not only to the intensi- ty and focussing of the laser beam 5 transmitted by laser device 4, but also to the direction in which laser beam 5 is transmitted. It is possible to vary this direction in two planes to apply a perforation pattern 7 in a document 6.

It is pointed out here that such laser devices are known in the prior art; in order to change the laser light beam use is herein made of mirror systems not otherwise shown in figure 1.

It is also possible to have laser device 4 stand still and to cause a carrier on which document 6 is placed to move. It is also possible to cause the carrier to move in one direction and the laser beam in the other direction; the choice between the various possibilities depends on the technology used.

Essential is however that perforation pattern 7 comprises perforation holes of differing diameter, where- in the diameter is a function of the brightness to be represented in the image. Perforation holes of differing density can be made by causing the laser beam to generate more or fewer holes locally.

This is illustrated more clearly in figure 2, which shows a detail of perforation pattern 7. Herein can be seen that in the present embodiment the perforation pattern is formed by perforation holes 8 which are or- dered in a regular grid, for instance a rectangular grid.

The dimension of the holes is herein a measure for the brightness of the image represented by perforation pat- tern 7, in the present case the passport photograph 2. It is noted here that the dimensions of the perforation

holes can be adjusted continuously, thus in principle in analog manner; by processing with a digital computer a finite, yet large number of stages is however obtained.

Tests have demonstrated that it is nevertheless possible to obtain a representation of an image which forms an adequate rendering of the relevant image and can be easily compared therewith.

It will be apparent that in this manner a good auhenticity feature is obtained which is difficult to copy.

Shown in figure 3 is a passport 9 in which the photograph 2 is fixed, for instance by means of glue, tubular rivets or other manner of attachment. On the same page, adjacently thereof, a perforation pattern 7 repre- senting the relevant image is applied. A good comparison can be made by holding up the relevant page of the pass- port to the light.

It is otherwise also possible to apply the perfora- tion pattern on another page of the passport, provided a quick visual comparison is possible. This makes forgery more difficult since at least two different pages must be forged for this purpose. It is also possible to apply the image enlarged, reduced in size or modified in other manner.

Figure 4 shows a banknote 10 which is provided with a perforation pattern 11, in the present case in the form of an owl. This perforation pattern is not related to another image arranged on the banknote but forms exclu- sively a security feature per se; it is possible to provide banknotes with such a security feature. It is again pointed out that the different with the prior art lies in the fact that the image 11 represents different brightness tones, for instance grey tones. Use is other- wise made herein of a free grid, wherein the dimensions of the perforations are the same and the density of the perforations varies in order to represent the grey tones.

The same applies for the postage stamp 12 shown in figure 5 which is provided with a perforation pattern 13 in the form of a likeness of Her Majesty the Queen; both forms are herein combined, i.e a varying grid, wherein the dimensions of the perforations also differ.