It has long been known to print documents, to protect them from forgery, using fine patterns constituting a background for a main image, The more complex the fine pattern, the more difficult it is to reproduce.

The developments of xerography, laser printing and scanning have created new problems for the printer. Special measures have been devised to make security documents uncopiable by standard or even advanced state of the art copiers and scanners, EP 0920383 being an example of such in which an"object"is worked into a background pattern by linear components of the background being"hollowed", i. e., thick lines becoming slightly spaced thin lines. Such hollowed regions are not readily visible to the naked eye, so they do not mar the appearance of the major image, but they"fool"a photocopier, and are visible in a photocopy. The"object"in question can be a word, such, for example, as"FORGERY".

While this can be effective to prevent photocopying, it is possible that a copy can be made without photocopying or indeed any scanning technique that scans a document - a document sufficiently valuable could be reconstructed even from essentially first principles, by an artist re-drawing the document, possible with some degree of magnification to assist in copying the fine detail. The document may well be sufficiently good to fool the naked eye, and a photocopier may not be to hand in a situation where the document needs to be verified.

So it is desirable that a document can be detected as a forgery, even when a photocopier is not available, and it may even be required that the document does not betray the presence of a security measure by such a simple recourse as photocopying.

A security print proof against photocopying in which a primary image is rendered in a density modulation of pairs of lines, and a secondary image is applied to said primary image by altering the relationship between lines in a pair.

In one example of the invention, the relationship that is altered is spacing between lines in a pair.

In addition or alternatively, the relationship that is altered maybe frequency of crossing of the lines in a pair.

The lines may be straight or wavy. Lines of each pair may cross at the edges of secondary image features, and may cross repeatedly within secondary image features, remaining parallel outside, or vice versa.

The line spacing of the primary image may be such as to render it capable of being copied by a photocopier, but the line spacing in the secondary image such as to be capable of resolution by a photocopier. Such a print will, of course, reveal its hidden image in a photocopy.

The lines may be dotted.

The print may be a monochrome print or a colour print, in which lines are made up of colour segments. When the lines are dotted, the dots may be made up of RBG or CMYX components.

The invention comprises a security print as above described in combination with a detector in the form of a lenticular screen of which the lenslets correspond to the line spacing of the primary image. Placing the screen over the print in register with the lines of the print will reveal the hidden image because there the lenslets will be out of register with the lines.

A security print according to the invention will now be described with reference to the accompanying drawing, in which: Figure 1 is a small detail of a first security print; and Figure 2 is a detail of a second security print The print illustrated in Figure 1 has a primary image rendered as a density modulation of pairs of parallel wavy lines 11,12. A secondary image is applied to the primary image-in this case, the letter"C"of the word"copy", for example-by, within the confines of the"C", the spacing between the lines 11,12 reducing from"a"to"b".

Provided the line spacing"a"is sufficiently small, the eye perceives the print as a continuous image, and can detect no difference within the"C".

It may be arranged, however, that the line spacing"A"is such as to be capable of being copied by a photocopier or scanner, while the spacing"b"is beyond the resolution of such an instrument.

On the other hand, the image may have spacings"a"and"b"which can be copied on a photocopier or scanner, the hidden image being revealed by placing over it, in register with the wavy lines, a lenticular detector screen having cylindrical lenslets which correspond precisely to those lines. If a copy is made otherwise than by a photocopier or scanner, the aberration in the lines in the hidden image will not be reproduced, but using the detector screen will reveal that there is not hidden image where there should be one.

Figure 2 illustrates how parallel lines can convert to repeatedly crossing lines at the edges of hidden image features. In this case the hidden image is the word COPY (of which, in the detail, only the letters C and O are shown). The line pairs are parallel within the letters, cris-crossing outside.

Clearly, to avoid copies being made by a photocopier when such a reproducible hidden image is used, the print may have two hidden images, one capable of being copied by a photocopier and one not.