Patent Publication Number: US-2009237758-A1

Title: Method for printing a binary hologram on a manufactured product, and optical lens with a binary hologram printed thereon

Description:
The invention relates to a method for printing a binary hologram on a manufactured product such as, for example an optical lens. It also relates to an optical lens with a binary hologram printed thereon. 
     Holograms are well known two-dimension optical objects. A hologram is based on a source image, and can be described as a phase record of a light reflected by the source image. When the hologram is subsequently lightened with a suitable light beam, it produces a reconstruction image which corresponds to the source image. 
     Binary holograms are digital holograms made up of pixels, each pixel being associated with an amplitude value selected among two possible values. 
     Printing a hologram on a manufactured product is well known, in particular for providing an authenticity proof of the product. This is used for credit cards and money bills, for example, because it makes it possible to detect counterfeiting rapidly. In fact, holograms are used as authenticity proving elements because they are difficult to reproduce, due to the technical means which are necessary for hologram generation and printing. 
     But hologram printing is generally expensive, and thus induces an increase in the cost of the manufactured product. Then, holograms cannot be used for many products, for which the market price is strongly constrained. It is so, for example, for optical lenses, and more particularly for optical lenses designed for eyewear, called ophthalmic lenses. In the case of ophthalmic lenses it is possible to print the said hologram on convex side or concave side. 
     Then, it is an object of the invention to provide a method for printing a hologram on a manufactured product, which is less expensive. 
     The invention thus proposes a method for printing a binary hologram on a receiving surface of a manufactured product, said receiving surface being coated with a metal layer, the method comprising the following steps:
         a—generating the binary hologram based on a source image;   b—providing a stamp with a pattern corresponding to the hologram on a stamping surface;   c—inking the stamping surface with a masking solution;   d—pressing the stamping surface against the receiving surface, so that masking solution is transferred onto the receiving surface according to the pattern; and   e—contacting the receiving surface with an etching agent, so that parts of the metal layer are etched if said metal layer parts are not covered with masking solution.       

     A hologram printing method according to the invention thus uses the printing process called “micro-contact printing”. Such process is well known, and corresponds to the steps b to e of the method. The micro-contact printing method is suitable for achieving prints with a submicronic accuracy. So it is compatible with the holography requirements for obtaining a reconstruction image which exhibits a good quality. In fact, the accuracy of a hologram printed by using the micro-contact printing process can be better than 0.1 μm (micrometer). Furthermore, holograms produced by micro-contact printing are cheap, and adapted for high production throughput. 
     However, a hologram printed according to the invention can be very difficult to reproduce if the stamp is not available. Then, it can be used as an authenticity proving element of the product. The difficulty for counterfeiting the hologram arises from the fact that the hologram pattern which is printed on the manufactured product may be complicated. If the hologram pattern is not correct, i.e. if it incorporates pixels or details which are different from those of the original pattern, then the reconstruction image is distorted or blurred. 
     Step a) of a method according to the invention can be achieved efficiently without inducing high costs by using computing means. Indeed, softwares now exist, which can produce binary computer-generated holograms rapidly, even for complicated source images. Such hologram generation method does not involve long nor expensive steps. 
     Advantageously, the hologram is designed so that the reconstruction image can be viewed upon lightening of the hologram printed on the manufactured product with a laser beam directed onto the receiving surface. In this case, the viewing of the reconstruction image can be performed easily. It only requires cheap equipment, such as a laser pen as currently available. Furthermore, the lightening of the hologram can be performed with any distance between the hologram and the laser, yielding to a very easy operation for viewing the reconstruction image. In another embodiment, a specific arrangement of lenses can be introduced on the laser path to optimize the resolution of the image at a given distance. 
     The hologram may comprise hologram pixels close to each other, each hologram pixel having a surface comprised between 0.2 μm 2  and 4.0 μm 2 . Preferably, the hologram comprises a great number of pixels, for example more than 10 000 hologram pixels, so that the reconstruction image exhibits a good quality. 
     The metal layer may be designed so that the reconstruction image can be viewed in reflection mode of a lightening beam directed onto the receiving surface. Alternatively, if the manufactured product is transparent, the hologram can be designed so that the reconstruction image can be viewed in transmission mode of the lightening beam. This applies, for example, to an optical lens. 
     The invention also provides an optical lens with a binary hologram printed on a surface of said lens. It may be, in particular, a lens adapted for eyewear. For reducing disturbance in the normal use of the lens caused by the hologram, the hologram can be printed near a peripheral edge of the lens. 
     In particular, the hologram may be printed using a method as described above. Indeed, such printing method is appropriate for manufactured products such as optical lenses, because of its low cost and its high throughput. Furthermore, the required production equipments can be easily combined with the existing manufacturing and distribution structures for optical lenses. 
     Optical lenses often have at least one pseudo-spherical surface. In the frame of the present invention, a pseudo-spherical surface refers to a continuous surface, i.e. without steps nor gaps, which is convex or concave. Then, a spherical surface is a particular case in which the curvature radii of the surface along two perpendicular directions are equal. Therefore, the expression of pseudo-spherical surfaces as used hereafter includes the case of spherical surfaces. When using improvements of the micro-contact printing process such as disclosed U.S. Pat. No. 5,817,242, the binary hologram may be printed on a surface of a lens which is pseudo-spherical. 
    
    
     
       These and other aspects of the invention will become apparent from the non-limiting implementation described hereafter in reference to the following drawings: 
         FIG. 1  is a diagram of the main steps for a hologram printing method according to the invention; 
         FIG. 2  is a perspective view of a stamp adapted for printing a hologram according to the invention; 
         FIGS. 3   a  and  3   b  are enlarged views of a binary hologram printed according to the invention; 
         FIGS. 4   a  and  4   b  show ophthalmic lens blanks with holograms printed according to the invention; 
         FIG. 5  illustrates a reconstruction step for a hologram printed according to the invention; and 
         FIG. 6  shows an ophthalmic lens with a printed hologram according to an improvement of the invention. 
     
    
    
     In theses figures, same reference numbers refer to similar elements, or to elements with similar function. Furthermore, for clarity reason, the sizes of the represented elements do not correspond to sizes of actual elements. 
     According to  FIG. 1 , the hologram printing method starts by generating a source image (step  10 ). Various methods may be used therefore, such as photography based methods, capture of a printed paper by scanning, etc. According to a preferred implementation, the source image is computer generated. Then, it is recorded as a two-dimension table comprising luminous values respectively associated to pixels of the source image. In this manner, no physical support is required for the source image, such as paper sheet or display board. The source image may contain, for example, a branding logo such as a symbol or a group of alphanumeric characters. 
     A binary hologram is then computed (step  20 ), based on the source image. Various programs are available now, which perform calculations resulting in defining the hologram as another two-dimension table. This another table respectively associates amplitude values to hologram pixels. Each amplitude value is selected in a set of two possible values. These possible values may be 0 and 1, or −1 and +1, depending on the program used or on the program configuration parameters. The computation can take into account the specific shape and profile of the substrate to be printed, if it is necessary to reach high quality definition for the reconstructed image. In addition, if the substrate to be printed shows optical power, it is also possible to take this effect into account in the computation step, in order to optimize the reconstruction of the image under laser illumination. 
     A stamp is manufactured, with a pattern corresponding to the binary hologram (step  30 ).  FIG. 2  illustrates such stamp. It comprises two elastomer superimposed layers, respectively  101  and  102 , affixed to a rigid body  103 . The body  103  may be of any material which is stable in shape, such as metals, glass or hard plastics. Layer  102  is thicker than layer  101 , and both layers are made of a resilient material. For example, layers  101  and  102  are respectively 0.5 mm and 2 mm thick. Layer  102  is affixed to the rigid body  103  at its lower surface, and to layer  101  at its upper surface. Layer  101  is engraved at its upper surface  100  with a pattern P corresponding to the binary hologram. Thus, parts of the surface  100  respectively correspond to the hologram pixels, with a height level associated to the binary amplitude value of each hologram pixel. Two heights of the surface  100  are thus defined, with a difference of 1 μm, for example. The surface  100  forms the stamping surface of the stamp. Various processes may be used for manufacturing the stamp, which are supposed to be known and not described here. Among these processes, photolithography based processes are preferred, because they provide stamps with accurate hologram patterns. 
     The stamp is used for printing the pattern on an ophthalmic lens. The lens may be of any material, including organic materials such as polyethylene or acrylate based materials. It is initially coated with a metal layer. The metal may be gold, silver, palladium, platinum, aluminium or copper. The metal layer may be about 30 nm (nanometre) thick, for example. Known processes as evaporation or sputtering can be used for depositing the metal layer onto the receiving surface of the lens. The receiving surface of the ophthalmic lens may be 80 mm (millimetre) in diameter, and may be convex with a curvature radius greater than 55 mm, for example. 
     Ink is first transferred onto the stamping surface  100  by any known method, including direct inking, dipping into an ink tank and ink spraying (step  40  in  FIG. 1 ). The ink excess on the stamping surface  100  is eliminated by blowing filtered air or nitrogen against the stamping surface. 
     Then, the stamping surface  100  is pressed against the receiving surface of the lens, so that ink is deposited on the metal layer at locations corresponding to the pattern P (step  50 ). During this stamping operation, the stamping surface  100  accommodates the curvature of the receiving surface of the lens, thanks to the deformation capability of the layers  101  and  102  of the stamp. In fact, layer  102  acts as a soft back support for layer  101 . For this reason, layer  102  should be soft enough, whereas layer  101  should be hard enough so that the pattern P experiences minimal distortions during printing. In a simpler embodiment, layers  101  and  102  could be reduced to one single layer of the same material if pattern stability requirements are not too stringent. For example, the commercial product named Sylgard® 184 and supplied by Dow Corning may be used. 
     The ink is designed for protecting the metal layer from etching. Preferably, the ink is designed for forming a self-assembled molecular monolayer on the metal layer. Such ink is very efficient for protecting against etching because of the high density of the deposited ink layer. It comprises molecules which have a functional group that binds to the metal material of the layer. A remainder part of each molecule, for example a long-chained hydrocarbon, interacts with neighbouring molecules to form a dense structure which is impenetrable by the etching agent. For example, hexadecanethiol (C 16 H 34 S) based ink, supplied by Sigma Aldrich, is appropriate with a gold layer deposited on the receiving surface of the lens. 
     After the stamp has been removed from the lens, the receiving surface  100  of the lens is contacted with an etching agent, so that the metal layer is etched at locations in the receiving surface where no ink has been deposited during the stamping operation (step  60  in  FIG. 1 ). Known processes as wet etching can be used, depending on the material of the metal layer. In particular, a solution containing ferric nitrate (Fe(NO 3 ) 3 ) at 40 mMole/litre and thiourea (CH 4 N 2 S) at 60 mMole/litre is appropriate for etching a gold layer at ambient temperature. It is also possible to use a pH-controlled cyanide solution as etching solution. After etching, metal layer parts remain only at locations in the receiving surface where ink has been deposited by the stamp, according to the pattern P. 
       FIGS. 3   a  and  3   b  represent a portion of a binary hologram  3  printed on the lens. The lens is referenced  1  and  FIG. 3   b  is a cross sectional view of the lens in the plane indicated by the arrows in  FIG. 3   a . The hologram  3  is made up of a pixel matrix, for example 400×400 pixels. Each pixel may be square-shaped, with 1 μm side length, for example. Then, the hologram  3  appears as a little diffusing reflective square 0.4 mm×0.4 mm in size. Each hologram pixel corresponds either to a remaining portion of the metal layer  4 , or to a uncovered portion  5  of the lens  1 . The thickness of the remaining metal parts  4  is indicated by e. It is equal to the initial thickness of the metal layer. According to the hologram general principles, the vertical arrangement of the metal parts  4  in the hologram of  FIG. 3   a  reveals that the source image has a structure which is more pronounced parallel to the horizontal direction than vertically. 
       FIGS. 4   a  and  4   b  each represent a eyeglass blank  1 . In a known manner, the eyeglass  2  is obtained by eliminating a peripheral portion of the blank, according to a contour drawn on the blank. The contour is indicated in broken line. The hologram  3  can be printed on the blank  1  either inside the contour ( FIG. 4   a ) or outside the contour ( FIG. 4   b ). The printing of the hologram  3  inside the contour is appropriate when the hologram is to remain on the final eyeglass. Then it is possible to authenticate the eyeglass, for example, by checking the hologram content. 
       FIG. 5  illustrates a reconstruction step for checking the hologram content. The eyeglass  2  is lightened using a laser pen  110  of limited power, for example of red colour. The laser beam  111  is directed onto the hologram  3 . Any lightening distance may be arranged between the laser pen  110  and the hologram  3 . The laser beam  111  is diffracted by the hologram  3 , so that it is divided into two transmitted beams  112  and  113 . Each transmitted beam  112 ,  113  produce a reconstruction image, respectively  115  and  116 , both corresponding to the source image. The reconstruction images  115  and  116  are formed remotely from the hologram  3 , at a distance comprised between 20 cm and 50 cm (centimetres), for example. Each reconstruction image can be revealed by arranging a screen  114  on the path of the corresponding transmitted beam  112 ,  113 . Any object may be used as a screen, because the lightening beam is of laser type. The reconstructed source images  115 ,  116  are several centimetres in size, so that the content of the image source appears clearly and can be viewed by several people at the same time. The reconstruction images  115  and  116  may correspond to opposite diffraction orders, such as +1 and −1, for example, so that the reconstruction images  115  and  116  are in mirroring relation. For illustrative purpose of  FIG. 5 , the source image is supposed to contain the character “a”. Instead of the a-character, the source image may comprise branding elements for authenticating the lens. 
     A transparent hard coat  6  ( FIG. 3   b ) may be arranged on the surface of the lens  1  above the hologram. Such hard coat protects the hologram against accidental scratches or dirty depositions which would cause a distortion or a reduction in the visibility of the reconstruction images  115 ,  116 . Such hard coat could be a polymerizable coating. 
     According to a preferred embodiment of the invention, the hologram is printed at multiple locations in the receiving surface of the manufactured product. Because the laser beam  111  is about 1 mm 2  in cross section, several hologram prints can be lightened simultaneously. Therefore, each hologram print participates to the formation of the reconstruction images, so that these images are more luminous, substantially in proportion with the number of hologram prints which are lightened. Advantageously, some of the multiple prints of the hologram  3  on the lens  1  are close one to another. Then, the contrast of the reconstruction images  115  and  116  is increased. 
     The pattern P corresponding to the hologram may be reproduced at multiple locations in the stamping surface  100  of the stamp in step b). The locations of the hologram prints  3  in the receiving surface of the lens are then determined by the pattern locations in the stamping surface  100  when step d) is performed so that the whole stamping surface is pressed against the receiving surface. Thus, the time necessary for implementing the invention is substantially independent of the number of hologram prints achieved. 
     Optionally, the locations in the receiving surface of the lens are selected so that the hologram prints form respective pixels of a macroscopic pattern directly visible on the receiving surface.  FIG. 6  and its enlarged view illustrate such embodiment. In the example illustrated, the hologram prints  3  are located on the lens so as to form another character “a” of one to several millimetres in size. Thus, the macroscopic pattern itself on the receiving surface is similar to the source image, although there is no relation between the source image and said macroscopic pattern on the receiving surface of the lens. 
     A binary hologram printing method according to the invention has numerous advantages, and the following ones can be listed:
         the method is compatible with the transparency requirement of an optical lens;   the method is compatible with aesthetic requirements of an ophthalmic lens;   a branding element contained in the source image can be viewed easily by performing an image reconstruction operation. Such operation is simple, rapid and costless;   no contact with the lens surfaces is necessary during the image reconstruction operation, so that the risk for scratching the lens is reduced.       

     Finally, the hologram can be printed on the lens so that at least one of the reconstruction images corresponds to the source image positively or negatively. This depends on the rendering of a luminous point in the source image, as a luminous point or as a dark point in the reconstruction image. 
     Furthermore, it is clear that the invention described in details may be adapted or modified in order to comply with special applications. For example, the pattern may be printed positively or negatively, in a manner that can be immediately derived from the implementation described. 
     In particular, it is straightforward to implement the invention by using the technique so called “Positive Micro contact Printing”, an example of such being described in the article “Positive Microcontact Printing”, in the Journal of the American Chemical Society, volume 124, page 3834 (2002) 
     The invention can also be implemented with the technique so called “Electroless deposition” by using specific catalyst molecules as ink on the stamp, an example of such being described in the journal article “Patterned Electroless Deposition of Copper by Microcontact Printing Palladium (II) complexes on Titanium-covered surfaces”, Langmuir volume 16, page 6367 (2000).