The use of decorative foils to enhance printed material is well known. Simple foil stamping is often used to apply a decorative, reflective foil to a particular part of a printed article. Foil stamping involves the use of a thin sheet of foil and a die. The die which is shaped in accordance with the area of foil to be applied presses the foil against the substrate. Heat and pressure is applied to make the foil adhere permanently to the substrate.

Flat foiling is the simplest form of foil stamping. Here a foil is applied to a flat substrate surface without any attempt to create any relief effect on the substrate.

Because of the applied pressure, there will be some depression of the substrate surface, but this is minimal.

Micro-embossing is a foil stamping technique where the stamping does produce a relief effect on the substrate. If the substrate is sufficiently thick compared with the surface relief pattern, then there is no deformation of the opposite surface of the substrate. The foil follows the contours of the relief effect to give different light reflection characteristics in the different areas. To carry out this technique, the face of the stamping die is formed with the desired relief pattern.

Deep embossing is a foil stamping technique where the

relief effect actually displaces part of the substrate out of its initial plane, so that the embossed effect can be seen on both the front and the back of the substrate.

I have now, surprisingly, found that it is possible to use a plate like a half-tone letterpress printing plate to apply micro embossed relief images to a reflective foil of a deformable reflective substrate and to produce a very high level of definition in a finished image.

Accordingly, the invention provides a method of generating an image on a deformable reflective surface using a stamping process in which a die is pressed onto the deformable reflective surface, wherein the die is a half- tone die that has a plurality of first areas raised above a plurality of second areas, the first areas being arranged in a grid pattern and having a common surface, and the second areas defining a lower surface, the first and second areas being of variable size in accordance with the image to be formed.

In a preferred embodiment of the invention, the deformable reflective surface is a reflective foil, and the reflective foil is stamped onto a supporting substrate in the stamping process.

The reflective surface may, however, be any reflective surface which may be deformed by the die, for example sheet aluminium, or a metalised substrate, such as a plastic material vacuum coated with aluminium, or a metal film coated paper.

The common upper surface of the first areas may, in the case of a flat die, be a plane. It is possible however, that the die may have another shape, which will be similarly reflected in the shape of the common upper surface. For example, the die may have a cylindrical shape.

In some applications, it may be desirable to produce the image with an overall or a bulk surface relief pattern, varying over a scale at least one order of magnitude larger than the spacing between adjacent second areas. For example, if the image is an image of a person's face, the bulk surface relief pattern may be a shallow or stylised duplicate of the shape of the person's face. In this case, the reflective surface, and any substrate, will be embossed with the overall or bulk surface relief pattern.

The second areas may be formed to a common depth, in which case the lower surfaces will lie on a common plane parallel with the common plane of the upper surface.

However, in a preferred embodiment of the invention, the second areas have variable depths, as well as variable sizes, which gives the image a greater contrast ratio or tonal range.

The invention also provides for the use of a die having the characteristics of a half-tone letterpress printing plate in a foil stamping process to produce a relief image.

The foil may be any type of thin metal or metalised material, such as a holographic colour foil, or a single

tone foil.

Half-tone letterpress ink printing (which produces pictures as opposed to text) is conventionally carried out using a plate etched or otherwise formed into an array of dots separated by troughs. When this plate is used for printing, ink adheres to the surface of the dots and is transferred from these dots to the final surface to be printed. To form an image, the size of the dots varies and the dots are larger in those areas of the final image which are to be dark, and are very small in those parts of the image which are to be very light. This produces a two- dimensional product.

Thus, in conventional half-tone letterpress printing it is only the tips of the dots (the printing surfaces) which are active in the printing process. The troughs between the dots only exist to provide a separation between the dots.

In conventional micro embossing, the designer aims to achieve sharp transitions between areas with different light reflecting characteristics. This is done by making edges between die surfaces and relief surfaces which are in the form of lines. The result is the conventional micro embossed/foiled surface where the appearance of light and dark areas changes suddenly and radically from one area to another, or when the image is moved relative to the ambient light.

We have now found that it is possible, using a foil stamping die which has the characteristics of a half-tone

letterpress plate, to stamp foil onto a substrate in such a way that the foil follows the contours of the die surfaces and relief surfaces, i. e., the foil extends over the dots and into the troughs, to produce an image which can be of near photographic quality. Making the foil follow the contours in this way progressively alters the reflective characteristics of the foil, so that an image is produced which may have a continuous variation in reflectivity, and this results in a (monochrome) image which is of near photographic quality. Larger dots in a particular area, the more the reflectivity of the foil is interrupted, producing a visible difference between areas where there are small dots and areas where there are equal areas of dots and troughs (tonal range 50%).

This technique produces a visible image through altering the reflective characteristics of the foil over the image area. The image is not the result of refraction or diffraction but merely varying the reflection of the light seen from the image.

Conventional inked half-tone letterpress printing uses a tonal range from 5% of the area being inked (in the lightest parts of the image) to 95% of the area being inked (in the darkest parts of the image).

In the present invention, the reflection characteristics produced in a large area where there are no or substantially no dots would be the same as in a large area where there are no or substantially no troughs. Thus the best results with the present invention will be obtained from a stamping die which has the characteristics of a

half-tone letterpress plate but a tonal range of from 0% to 50%, ie the die surfaces occupy between only 0 and 50% on average over any particular area of the die face and the relief surfaces then occupy between 50 and 100% of that area. Obviously it could be the die surface which occupies 50 to 100% and the relief surfaces which occupy 0 to 50%.

The invention thus differs from a conventional letterpress printing plate in including areas where there are no dots/troughs (uninterrupted foil reflectivity), ie 0% tonal range.

The material of the substrate needs to be chosen so that it can accept the necessary relief between the dots and the troughs.

The invention also provides a foil stamping die having die surfaces and die relief areas arranged on a regular grid pattern, with the areas of the die surfaces varying across the die face.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic plan view of part of a half- tone printing plate; Figure 2 is a cross section taken on the line II-II through the plate of Figure 1;

Figure 3 shows the plate, foil and substrate ready for stamping; and Figure 4 is a cross section through a stamped substrate.

In all the figures, the dimensions shown are very much magnified, to assist in explanation.

The cross-section through a printing plate 10 shown in Figure 2 is of a half-tone plate, prepared for printing an image. The figure illustrates the form of the plate with a relief pattern of craters 12 and unetched surface 14. The size of the craters 12 varies across the plate, but the centre of each crater is located on a regular grid pattern. The craters are therefore arranged on a rectangular or a square array.

In letterpress printing, ink is applied to the unetched surface 14. When the inked plate is pressed against a surface to be printed, ink is transferred from the plate to the surface to form a corresponding image on the surface. In Figure 1, the larger craters, e. g. 12a, 12b are in the parts of the plate where the area of the unetched, ink-carrying, surface 14 is smallest and thus where the printed image is to be lightest.

In contrast to letterpress printing, where the size of the craters varies from almost nothing (5% of local area) to almost the whole of the surface (95% of local area), the present invention uses a tonal range of 0% to 50%. This means that the area occupied by craters will vary between

nothing, ie no craters at all, to regions where half the surface area is represented by craters. However it must be noted that the invention can be applied equally effectively with a tonal range of 50% to 100%. In this case, instead of craters (which are areas below the mean surface level), we refer to dots (above the mean surface level).

The plate 10 is formed by a photoetching technique. If the original image is a continuous tone photograph, then this is imaged by conventional reprographic processes to create a screened negative or a screened positive (referred to hereinafter as a screened film) of the image. If the original image is a digital file, then conventional computer technology in conjunction with conventional reprographic processes may be used to create the screened film.

The half-tone image should have a tonal range of 0% to 50%, and in this range (as can be seen in Figure 1) the largest dots are not so large that they join up with adjacent dots. A resist is laid down on the surface of a copper plate. This resist is exposed to light through the screened film, and the unexposed areas are subsequently developed and washed away leaving some areas still protected by the resist. The unprotected areas are then etched away. This results in a surface configuration of the type shown in Figure 2. The method of the invention works best starting from a high quality image which has good, even contrast.

Alternatives to photoetching are known. It is possible for

example to use a laser etching processes or CNC engraving processes.

In carrying out the invention, the plate 10 is used as shown in Figure 3, to emboss a substrate 16 with an interposed reflective foil 18. Applying heat and pressure between the plate (die) 10 and the substrate 16 results in a finished product having a cross-section as shown in Figure 4, with the foil following the contours of the craters 12 and unetched surface 14.

The reflective nature of the foil, and the application of this foil to the embossed surfaces on the substrate can result in a visual image which has such fine definition that it is of almost photographic quality (monochrome), and thus is eminently suitable for displaying portrait images such as are required, for example, on identity documents.

The fineness of the image will depend on the spacing of the grid pattern on which the dots are arranged. However stamping foil can now follow very fine contours, and grid patterns as fine as about 3.9 to 11.8 dots per millimetre (about 100 to 300 dots per inch) can be used in this process. This is however below the fineness of holographic images.

Any suitable method can be used for producing the half- tone letterpress plate.

The die should be of a material which will accept the desired level of definition and (especially for continuous

production) be capable of withstanding wear. Copper plates are particularly suitable.

The substrate should be chosen so that it can accept the emboss, and preferably so that any recovery of the material after the emboss does not noticeably reduce the fineness of the image. The process can be used with a wide range of materials. For example, the reflective surface can be reflectively coated plastic or paper, or even aluminium foil.