WO1995023710A1

WO1995023710A1 - Light control material and method for making same

Info

Publication number: WO1995023710A1
Application number: PCT/US1995/002729
Authority: WO
Inventors: Richard A. Steenblik; Mark J. Hurt
Original assignee: Applied Physics Research LP
Current assignee: Applied Physics Research LP
Priority date: 1994-03-02
Filing date: 1995-03-02
Publication date: 1995-09-08
Anticipated expiration: 1996-09-02
Also published as: AU1939495A

Abstract

A light control material comprising a layer of photopolymer (131) having light traps (140) formed therein at selected locations. The photopolymer (131) layer constitutes a master from which embossments can be produced. The ligh traps (140) are formed in the master by reactively ion etching a layer of cured photopolymer (131) at selected locations to create fields of tapered structures (140). An embossment generated from the master contains fields of tapered structures (140) which correspond to those created in the master. The light control material is capable of operating in the reflective or transmissive modes. In the transmissive mode, the light traps (140) are coated with a reflective material, such as a reflective layer of metal. Light impinging on the light control material passes through the light control material except where the light enters the light trap (140). Light which enters the light trap (140) is absorbed. In the reflective mode, the entire surface of the light control material having the light traps (140) formed therein is coated with a reflective material such as metal. Light which enters the light traps (140) is reflected among the tapered structures (140) until substantially all of the light has been absorbed. Light which does not enter the light traps (140) is reflected. The method of the present invention can be used to produce color and black-and-white images as well as copy-resistant documents.

Description

LIGHTCONTROLMATERIALANDMETHODFORMAKINGSAME

The present invention relates to a method for selectively forming light traps in a light control material. The method can be used for creating black-and-white images by selectively forming light traps in a polymer material. In addition, the black-and- white images can be generated as embossments from a master. The method also enables high resolution color images to be generated by combining a high resolution black-and-white embossed image with a lower resolution color printed image.

In addition to creating color and black-and-white images, the method of the present invention can also be used to produce copy-resistant documents. In order to produce copy-resistant documents, the light traps are formed at selected locations in a polymer layer. The light traps and the surface of the polymer layer having the light traps formed therein are covered with a reflective material. Light impinging on the polymer layer from certain directions is absorbed while light impinging from other directions is reflected. The reflected light passes through a printed or copied image carried on a surface of the polymer layer. This allows the image to be viewed from certain directions while rendering the image dark when viewed from other directions.

The design of the surface of the polymer layer and the locations of the light traps are preselected so that the image is dark when viewed by a photosensor array from the angles normally used by photocopy machines and facsimile machines to sense and copy or transmit an image.

In general, the present invention utilizes photolithographic techniques to form light traps in a layer of material. The light traps are formed by reactively ion etching the material, which is preferably a hardened layer of photopolymer. It has been discovered that by reactively ion etching cured photopolymer with a reactive gas such as oxygen, tapered structures are created which have large height-to-width or aspect ratios. It is believed that these structures occur because of impurities or inclusions in the composition of the photopolymer. When the structures are covered with a reflective layer of metal, light which enters the light traps is reflected among the tapered structures until substantially all of the light has been absorbed. The high aspect ratios of the structures cause light entering the light traps to be reflected at very shallow angles. Depending on the reflective characteristics of the surfaces of the structures, as much as 40% of the light may be absorbed on each reflection. By using photolithographic techniques, these light traps can be selectively formed at precise locations in the cured layer of photopolymer.

Until the present invention, it was not known that images could be formed in a layer of material by reactively ion etching the material to form light traps in selected areas in the material. Although methods are known which utilize various techniques to replicate images, none of the known methods utilize the methods or concepts introduced by the present invention.

F.G. Yanes. U.S. Patent No. 2,374,910, discloses a method for making molds for printing surfaces. A plate is covered with a layer of emulsion which is exposed through an original half-tone image photographic negative. The plate is then etched such that areas which correspond to "whites" in the original image are not etched and areas which correspond to darker areas in the original image are etched more than others. A cast is then produced from the mold which, when inked and printed, will produce a half-tone image which corresponds to the original half-tone image.

Kinney. U.S. Patent No. 3,607,273, discloses forming a black-and-white image by covering a black substrate with a thin layer of foaming agent and exposing the substrate and foaming agent to radiant energy through a photographic positive. The areas of the foaming agent which are subjected to the radiant energy swell and create light scattering surfaces which reflect light. The foaming agent is clear in the unfoamed state. The reflective areas correspond to white areas in the photographic positive and the areas in which the black substrate can be seen correspond to black areas in the photographic positive.

Fotland. et al.. U.S. Patent No. 4,920,039, discloses a method of providing images on the flat surface of a lenticular sheet. An image layer is formed on the flat surface of a lenticular sheet. The image layer consists of a cross-linkable polymer which undergoes a chemical change upon exposure to collimated radiation from an ultraviolet source. The image layer is selectively cured by exposing only certain portions of the image layer to ultraviolet light. The unexposed areas of the image layer are rinsed away. When the material is used as a package an image can be seen from a particular range of angles while from other angles the lenticular sheet is clear and the interior contents of the package may be seen.

The general idea of producing copy-resistant documents is not new.

Reinnagel. U.S. Patent No. 3,887,742, discloses several methods for producing copy resistant documents. For example, according to one embodiment, a transparent film having angularly oriented opaque planes formed therein is placed over an original document. The opaque planes are arranged such that when the document is viewed from a direction perpendicular to the document, the planes conceal the text of the document, thereby preventing photocopying. When the document is viewed at a particular angle with respect to the plane of the document, the opaque planes appear as end lines and the document is readable by the human eye. IBM Technical Disclosure Bulletin. Vo. 26, No. 5, October 1983, entitled

"Anti-Copy Paper", by Bealle and Rosen, discloses a transparent plastic material comprising a flat surface on one side and a reflective geometric pattern on the other. A document is rendered copy-resistant by overlaying the document with the plastic material. The geometric pattern is designed to reflect light which impinges on the geometric pattern from a direction generally perpendicular to the documents by total internal reflection, thereby preventing the image behind the pattern from being illuminated. When the document is illuminated at angles greater than the critical angle for total internal reflection the image is readable.

Foley et al.. U.S. Patent No. 4,791,449 discloses a system for preventing unauthorized photocopying of documents by providing a web which bears an image and which has phosphor particles uniformly distributed throughout its outer surface. During an electrostatic imaging cycle, the web is illuminated with ultraviolet light which causes the phosphor particles to emit visible light. The visible light emitted is detected by a sensing device. The exposure lamps are disabled in response to the detection of visible light by the sensing device to thereby halt copying of the document.

Gundjian. U.S. Patent No. 4,867,481 discloses paper which is resistant to photocopying or transmission by telefacsimile. The paper is provided with a colored pattern of at least two colors which repeats at a particular spatial frequency. When a photocopier attempts to read information printed on the paper, the photocopier fails to see any contrast at locations where the information falls on a particular color of the colored pattern, thereby preventing reproduction of that information. However, sufficient contrast is provided for the information to be readable by the human eye. It is evident from the prior art discussed above that numerous attempts have been made to produce a document which is readable by humans and which, at the same time, is incapable of being reproduced by copy machines or read and transmitted by facsimile. Not withstanding the attempts which have been made at achieving this objective, a need still exists for a copy-resistant document which is relatively simple and inexpensive to produce and which overcomes the deficiencies of the prior art.

For example, in Reinnagel. a pre-existing document may be treated in accordance with one of several embodiments to make the document copy resistant. Since these embodiments require the individual treatment of each and every document, making documents copy resistant by such treatment processes may become prohibitively expensive. Reinnagel also discloses creating an original copy-resistant document. However, this requires printing the document information on sloping surfaces of triangular shaped teeth formed in a substrate. Furthermore, black ink of a particular depth must be placed between the teeth. Therefore, generating an original copy-resistant document in accordance with this method may be considerably difficult.

The IBM Technical Disclosure refers to rendering a document copy-resistant by overlaying the document with the plastic material. Therefore, each document must be individually treated with the overlay to be made copy-resistant. In accordance with the present invention, an embossed light control material is the actual medium upon which images are copied or printed. Any information printed or copied on the embossments will be copy-resistant.

Foley et al. is limited in that it is required that the document be illuminated with ultraviolet light and that the copy machine be designed to halt the copying process in response to the detection of visible light during a copying cycle.

Gundjian requires that each sheet of paper be uniformly colored with a first color and then overprinted with a second color in a grid-like pattern. Furthermore, a specially configured printing plate is needed to form the grid-like pattern. In contrast, the present invention provides for the creation of one master from which embossments can be easily and relatively inexpensively generated. An image can be copied or printed directly onto the surface of the embossment which will serve as the copy- resistant document.

SUMMARY OF THE INVENTION

In accordance with the present invention, photolithographic techniques are used to expose selected areas in the polymer layer and then the exposed areas of the polymer layer are reactively ion etched to form light traps in the selected areas. The present invention is capable of operating in the transmissive or reflective modes. In the transmissive mode, only the light traps are covered with a reflective material.

Light impinging on the light control material passes through the light control material except where the light enters the light traps. Light entering the light traps is absorbed.

In the reflective mode, the surface of the polymer layer having the light traps formed therein and the light traps themselves are covered with a reflective material, such as a reflective layer of metal. In the reflective mode, the surface areas having the light traps formed therein absorb light whereas the other surface areas reflect light. The method of the present invention can be used to create color images, black-and-white images and copy-resistant documents. Once the method of the present invention has been used to create an original, the original can be used as a master from which embossments can be generated by various embossing techniques.

Accordingly, it is an object of the present invention to create an embossed light control material which displays black-and-white or color images. The embossed light control material is produced from a master which has light traps selectively formed therein by reactive ion etching.

It is another object of the present invention to provide a new and useful method for reproducing photographic images in a light control material from which embossments can be generated which also display the images.

It is another object of the present invention to provide a new and useful method for converting black-and-white images into color images.

It is yet another object of the present invention to provide a light control material which displays a replicate of a photographic image with enhanced brightness. It is a further object of the present invention to provide an inexpensive and convenient means for replicating photographic images.

It is an even further object of the present invention to provide a method for creating an image in a light control material in accordance with a computer generated mask. It is another object of the present invention to produce high resolution black- and-white and color images.

It is another object of the present invention to create embossed light control materials for displaying black-and-white or color images which operate in the reflective or transmissive modes. It is yet another object of the present invention to provide a light control material upon which an image can be copied or printed wherein the light control material controls light reflected therefrom such that the image is dark and unreadable when viewed from particular directions and bright when viewed from other directions.

It is another object of the present invention to provide a light control material which is an embossment generated from a master.

It is another object of the present invention to provide a document which cannot be reproduced by a copy machine or read and transmitted by a facsimile machine but which can be read by a person.

It is yet another object of the present invention to provide a copy-resistant document which is inexpensive to produce. It is yet another object of the present invention to provide a light control material which has predetermined viewing and non-viewing angles.

It is a further object of the present invention to provide a method for producing an embossed light control material upon which an image can be copied or printed in order to produce an original copy-resistant document which light control material is thin enough to use in conventional printing equipment.

It is yet another object of the present invention to provide a method for preventing the counterfeit reproduction of holograms.

It is yet another object of the present invention to provide a method for enhancing the contrast of a hologram.

These and other objects of the present invention will become apparent from the detailed description of the invention provided below.

BRIEF DESCRIPTION OF THE INVENTION Figs, la-le illustrate a preferred method of the present invention for creating light traps in a master and for generating an embossment from the master;

Figs. 2a-2g illustrate one embodiment for creating a master having a continuous pattern of domed-shaped elements formed therein and for generating an embossment from the master;

Fig. 3 illustrates a top view of hexagonal elements which have arcuate-shaped top surfaces and which may be used to create the light control material of the present invention.

Fig. 4 illustrates an isometric view of truncated conical-shaped elements which may be used to create the light control material of the present invention.

Figs. 5a-5c illustrate cross-sectional views of the light control material of the present invention having light traps formed at selected locations in the material;

Fig. 6 is a cross-sectional illustration of the tapered structures of the light traps of the present invention;

Fig. 7 illustrates a cross-sectional view of the light control material of the present invention in which the light control material has light traps selectively formed therein such that the material displays a black-and-white image; Figs. 8a and 8b illustrate one embodiment for creating an embossed light control material which displays a black-and-white image;

Figs. 9a and 9b illustrate an alternative embodiment for creating an embossed light control material which displays a black-and-white image; Figs. 10a and 10b illustrate an alternative embodiment for creating an embossed light control material which displays a black-and-white image; and

Fig. 11 illustrates a cross-sectional view of the embossed light control material of the present invention which has a black-and-white image formed therein. The black-and-white image is converted into a color image by overlaying the embossment with additive or subtractive color transparencies.

Fig. 12 illustrates an alternative embodiment for converting a black-and-white image into a color image.

Figs. 13a-13c illustrate an alternative embodiment for creating the light control material of the present invention wherein the light control material is intended to operate in the transmissive mode.

Fig. 14 illustrates a light control material for displaying color images which operates in the transmissive mode.

Fig. 15 illustrates a cross-sectional view of a first copy-resistant document produced in accordance with the method of the present. Fig. 16 illustrates a cross-sectional view of a second copy-resistant document produced in accordance with the method of the present invention.

Fig. 17 illustrates a cross-sectional view of one type of surface created in accordance with the method of the present invention which can have an image copied or printed thereon to produce a copy-resistant document. Fig. 18 illustrates a cross-sectional view of another type of surface created in accordance with the method of the present invention which can have an image copied or printed thereon to produce a copy-resistant document.

Figs. 19a- 19c illustrate cross-sectional views of a light control material of the present invention having arcuate-shaped reflective surfaces with light absorbing areas formed therein. Figs. 19d and 19e illustrate enlarged views of the light control materials depicted in Figs. 19b and 19c, respectively.

Figs. 20a-21 illustrate another embodiment of the light control material of the present invention wherein a hologram is selected to provide the desired viewing angles of the copy-resistant document.

Figs. 22a-22d illustrate a method in accordance with the present invention for making a hologram counterfeit proof.

DETAILED DESCRIPTION OF THE INVENTION

Figs, la-lc illustrate the prefeπed method for creating the light traps in the master. A cured layer of photopolymer 131 is bonded to substrate 130, such as, for example, glass or quartz. Photopolymer 131 is coated with a thin layer of metal such as chrome 132. The layer of chrome 132 is coated with a layer of photoresist 133. As shown in Fig. lb, the photoresist layer 133 is exposed to ultraviolet light through a mask 135. The exposed photoresist is then developed away such that the layer of chrome 132 is exposed in the position where the photoresist has been removed. The chrome is then etched by an acid bath to expose the cured photopolymer 131. The photopolymer is then etched in a reactive ion etcher which preferably utilizes oxygen as the reactive gas.

It has been discovered that due to the impurities in the composition of the cured photopolymer, the reactive ion etching process creates stalagtite-type or tapered structures 140 in the photopolymer which have high height-to- width, or aspect ratios.

The dimensions of the tapered structures are typically on the order of 0.5μ to 4μ in height and O.lμ to 2μ in width. The high aspect ratios cause light which enters the light traps to be reflected at very shallow angles. Approximately 40% of the light which enters the light trap may be absorbed on initial impact. Each time light which enters the light trap is reflected, approximately 10% of the light may be absorbed.

Due to the shape of the structures, light is reflected many times before there is a chance for the light to be reflected back out of the light trap. Since approximately

10% of the light may be absorbed each time the light is reflected within the light trap, virtually all of the light that enters the light trap is absorbed. Fig. 6 illustrates how the light trap works. As light 170 enters the light trap, it is reflected by the tapered structures 169 until all of the light has been absorbed. It takes very few reflections before substantially all of the light has been absorbed.

As shown in Fig. lc, once the light trap has been created, the photoresist layer is completely removed, leaving a layer of chrome covering everything except the light trap 140. The purpose of the chrome is to protect the photopolymer during the reactive ion etching process in areas where it is undesirable to create light traps. Once the light trap has been created, the device shown in Fig. lc can be used as a master for making embossments. Fig. Id illustrates how an embossment is made from the master by using a soft embossing procedure. A glass substrate 136 is placed in contact with liquid photopolymer 135 which is in contact with the master of Fig. lc. Ultraviolet light is then projected onto the entire structure. The photopolymer becomes hardened or cured by the ultraviolet light. The result is the embossment shown in Fig. le. The embossment is then preferably coated with a reflective metal such as aluminum, preferably by evaporative coating.

The present invention is capable of operating in the reflective or transmissive modes, as discussed in further detail below. In the reflective mode, which is the preferred embodiment, the embossment is coated with a reflective layer of metal. The reflective layer can be put down by chemical reduction of a metal such as silver. In the transmissive mode, it is unnecessary to coat the embossment with a reflective layer of metal. However, when the embossment is intended to operate in the transmissive mode, it is possible to coat only the light traps with a reflective layer of metal in order to enhance their light-absorbing characteristics. Figs. 2a-2g show a prefeπed embodiment for creating a master. The structure is designed to enhance the brightness of an image which has been formed therein by reactively ion etching selected areas as discussed below with respect to Figs. 3a-3c. The structure for enhancing brightness comprises a plurality of tiny focusing elements 163 as shown in Fig. 2g. The device is capable of maximizing the brightness reflected from the inside of the structure while controlling the amount of specular and diffuse reflection off of the surface. However, the shapes of the focusing elements are not limited to the arcuate shapes shown in Fig. 2g. Other shapes may also be used. For example, the method discussed with respect to Figs. 2a-2f may be used to create the hexagonal-shaped focusing elements shown in Fig. 3 or the truncated conical-shaped focusing elements shown in Figs. 2g, 3 and 4 which are all arcuate in shape across their top surfaces. Each of these shapes will enhance the brightness of an image formed in the embossment over a particular range of viewing angles.

The arcuate-shaped focusing elements "re-image" the source of the light impinging thereon and provide a minimum field of view which is independent of the angular extent of the source of the light. The overall field of view provided by a particular focusing element is dependent upon the geometry of the focusing element and the angular extent of the light source. By controlling the minimum field of view of the focusing elements, the overall field of view can be controlled. This allows light to be reflected and concentrated over a particular range of viewing angles which in turn allows the black-and-white or color image to be enhanced over the range of viewing angles.

The preferred embodiment for creating the master will now be described with respect to Figs. 2a-2g. Fig. 2a illustrates a substrate 150 which is covered by a layer of chrome 151. On top of the layer of chrome 151 is a layer of photoresist 152. A mask 155 is placed over the structure as shown in Fig. 2b. The structure is then exposed to ultraviolet light. Clear areas in mask 155 correspond to areas in the photoresist which will be exposed to the ultraviolet light and subsequently developed away. This creates pads of photoresist 153 on top of the chrome layer 151. An acid bath is then used to etch away the chrome in positions where the chrome is not covered by the photoresist pads 153. The result is the structure shown in Fig. 2c. The chrome pads 154 act as an adhesion promoter between the photoresist pads 153 and the substrate 150. Photoresist heat flowing is then used to cause the photoresist pads 153 to flow, thereby creating dome shapes on top of the chrome pads as shown in Fig. 2d. As shown in Fig. 2e, a glass substrate 156 is placed in contact with fiquid photopolymer 157 which is in contact with the structure of Fig. 2d. The structure is then exposed to ultraviolet light which causes inverted dome shapes to be created in the cured photopolymer 157. The master is comprised of layers 156 and 157 shown in Fig. 2f. Fig. 2f also shows how an embossment can be made from the master by using the soft embossing procedure described above with respect to Figs. Id and le. A piece of glass 160 is placed in contact with liquid photopolymer 161 which is in contact with layer 157. This structure is then exposed to ultraviolet light thereby causing photopolymer 161 to harden. The master and the embossment are then separated and the result is the embossment 162 shown in Fig. 2g.

Once the embossment is made, the light trap can be formed directly therein by using the photolithographic techniques discussed above with respect to Figs, la-lc to expose selected areas of the photopolymer embossment and then reactively ion etching the exposed areas to obtain a result such as that shown in Fig. 5a. Fig. 5b illustrates what an embossment would look like if the structure in Fig. 5a was used as the master. Fig. 5c illustrates the effect of reactive ion etching an inverted domed structure such as the master consisting of layers 156 and 157 in Fig. 2f.

Fig. 7 illustrates one embodiment for creating a light control material which displays a black-and-white image in accordance with the present invention. The photolithographic techniques described above with reference to Figs, la-lc are used to expose selected areas in the photopolymer and reactive ion etching is used to create light traps 174 at the selected locations in photopolymer layer 172. The structure of Fig. 7 may then be used as a master from which embossments can be generated by using the soft embossing procedure discussed above with respect to Figs. Id and le and 2e and 2f.

In creating the light control material shown in Fig. 7, either a photographic negative or a computer-generated mask is used as the mask during exposure of the photoresist. When a photographic negative is used, the photoresist will be exposed in areas which correspond to clear areas in the negative (i.e., dark areas in the photographic positive). In areas in which the photoresist has been developed away, the chrome layer is etched with an acid bath thereby leaving the photopolymer layer exposed in certain locations in preparation for the reactive ion etching step. Reactive ion etching is then used to create light traps in the photopolymer layer. During reactive ion etching, the chrome layer serves as a mask. After reactive ion etching the remainder of the chrome layer is stripped away. The photopolymer layer having light traps formed therein constitutes a master from which embossments can be produced. An embossment can be generated from the master by the soft embossing procedure discussed above. The surface of the embossment having light traps selectively formed therein is coated with a reflective layer of metal such as aluminum to give it the desired reflective and light absorbing characteristics. The embossment displays a black-and-white image in accordance with the photographic negative or computer- generated mask, whichever was used in creating the master.

Fig. 8a illustrates an alternative embodiment for creating an embossment which displays a black-and-white image. The master may be comprised of a glass substrate 175 and cured photopolymer 176, which has been reactively ion etched such that tapered structures 168 are formed throughout its entire surface. The master comprised of layers 175 and 176 is placed in contact with liquid photopolymer 177 which is in contact with a substrate 178 such as glass. The liquid photopolymer 177 is then exposed to ultraviolet light through mask 180 which is a photographic negative or computer-generated mask. The master is then separated from the embossment and the uncured photopolymer is washed away. The result is the embossment shown in Fig. 8b. The surface of the embossment comprising the light traps is preferably coated with a reflective layer of metal such as aluminum to provide the embossment with the desired reflective and light-absorbing characteristics.

Fig. 9a shows an alternative embodiment for creating an embossment which displays a black-and-white image. A master 185 having inverted dome shapes formed therein is placed in contact with an embossment comprised of glass substrate 178 and a cured photopolymer layer having tapered structures 177 formed throughout its surface. Liquid photopolymer is disposed in the domed spaces between the master and the embossment. The liquid photopolymer is then exposed to ultraviolet light through mask 180 which may be a photographic positive or a computer-generated mask. Wherever the photopolymer is exposed to ultraviolet light, domed-shaped structures will be formed on the embossment such that, in the exposed areas, the light traps are covered by the domed-shaped structures. The resulting embossment is shown in Fig. 9b. When the surface of photopolymer layer 177 is covered with a reflective layer of metal such as aluminum, the domed or arcuate shaped elements will reflect light and the light traps will absorb light, thereby providing a black-and-white image.

Fig. 10a illustrates another embodiment for creating an embossment which displays a black-and-white image. This embodiment is a variation of the embodiment shown in Figs. 9a and 9b. In this embodiment, the master is comprised of substrate 175 and photopolymer layer 176 having the tapered structures formed throughout its surface. The master is placed in proximity to an embossment comprised of a glass substrate 187 which is bonded to photopolymer layer 188 having domed shapes formed therein. Liquid photopolymer is placed between the master and the embossment and exposed to ultraviolet light through mask 180 which can be a photographic negative or a computer-generated mask. In areas in which the liquid photopolymer is exposed to the ultraviolet light, a light trap will be formed on the domed shaped reflectors of the embossment, as illustrated in Fig. 10b. The embossment shown in Fig. 10b is coated with a reflective layer of metal on its upper surface 186. Fig. 11 illustrates an alternative embodiment in which an embossment which displays a black-and-white image can be converted to display a color image. The embossment which carries the black-and-white image can be generated in accordance with any of the embodiments discussed above. The substrate 190 of the embossment can be overlaid with color transparencies 192 and 193 which can be additive or subtractive. The black-and-white image, formed by generating light traps 189 in polymer layer 197, in combination with the color transparencies produces a color image.

In accordance with the present invention, high resolution black-and-white images can be created. Generally, the present invention allows a black-and-white image to be created with a resolution of at least 2000 dots per inch. By overprinting the high resolution black-and-white image with a low resolution transparent color ink image, a high resolution color image is produced. As shown in Fig. 12, an embossment 201 has a high resolution black-and-white image formed therein as indicated by light traps 207. The printed color image may be comprised of any color images that can be printed by printers, such as cyan, magenta and yellow, for example The human eye has difficulty perceiving color edges without contrast. In accordance with the present invention, contrast is provided by a high resolution black- and-white embossed image. When the high resolution black-and-white embossed image is overprinted with transparent ink, the effect of a high resolution color image is created.

As discussed above, an embossed black-and-white image can be produced by placing a master which is comprised of a substrate and a cured polymer layer in contact with a layer of liquid photopolymer and exposing the liquid photopolymer to ultraviolet light. Alternatively, a metal embossing master may be created from any of the photopolymer masters by electroforming. For example, the master shown in Fig. 7 may be coated with a thin layer of metal, such as silver, by vapor deposition in order to render it electrically conductive. Electrical contact is then made to the metal surface and a layer of nickel is deposited onto the surface by conventional electroforming processes. After deposition of a sufficient thickness of nickel, the nickel plated photopolymer layer is removed from the electroplating bath and the nickel master is separated form the photopolymer layer. Embossments may then be generated from the nickel master by the soft embossing procedure discussed above with respect to Fig. Id.

Figs. 13a-13c illustrate an alternative embodiment for creating the light control material of the present invention wherein the light control material is intended to operate in the transmissive mode. As shown in Fig. 13a, a substrate such as glass or quartz 216, has a field of tapered structures 218 formed on its surface by any of the methods discussed above. The tapered structures 218 are covered with a layer of metal 214 which is covered with a layer of photoresist 212. The layer of photoresist 212 is exposed to ultraviolet light through mask 210 and the exposed areas are developed away, as shown in Fig. 13b. The resulting photoresist pads 220 shield metal layer 214 during a wet etching step such that only the uncovered portions of metal layer 214 are etched. During the wet etching step an acid is used to etch through the metal layer 214 and through the tapered structures located below the etched portions of the metal layer. The resulting structure is shown in Fig. 13c. The structure in Fig. 13c functions in the same manner as that shown in Fig. 8b when it is covered with a reflective material. The structure of Fig. 13c can also function in the transmissive mode wherein light passes through the structure to create a black-and- white image. The tapered structures 222 function as light traps by absorbing light which impinges on them. In this embodiment, the surface shown in Fig. 13c is not coated with a reflective material. However, the tapered structures 222 may be coated with a reflective material to enhance their light absorbing characteristics.

As shown in Fig. 14, the structure of Fig. 13c is capable of operating in the transmissive mode to produce a color image by placing additive or substractive color transparencies 225 and 226 in contact therewith or by overprinting the black-and- white image with a transparent color ink image. In essence, the structure of Fig. 13c can be used to produce color images in the same manner that the structures of Figs. 11 and 12 are used to produce color images, with the exception that the structure of Fig. 13c may be operated in the transmissive mode. Photocopy machines generally view documents from a direction which is normal to the surface of the document. Under certain circumstances, it is desirable to have a document which is capable of being read by an individual, but which is incapable of being reproduced by a photocopy machine. The invention accomplishes these and other objectives. Fig. 15 illustrates one embodiment of the present invention wherein the light control material can be used to create a copy-resistant document. Light control structure 252 comprises a substrate 258 which is bonded to a pattern of light reflectors 253 and light absorbing surfaces 254. A copied or printed image 255 can be produced on substrate 258. Ray 250 represents the direction from which a photocopy machine generally views a document. As shown in the drawing, light impinging on structure 252 from direction 250 is incident upon one of the reflective surfaces 253. The light is reflected from reflective surface 253 into the light absorbing surface of light trap

254. The light trap 254, as further described below, absorbs the light that enters it and the document appears black. When viewed from the direction 251 directly above the light trap, the document appears black because the light goes directly into the light trap. Therefore, to a photocopy machine which is viewing the document from a direction normal to the surface, the document will appear black and will be uncopyable. However, when the image is viewed from a position 257, light impinging on the reflective surface from direction 256 will be reflected and the image will be bright. This allows an individual to hold the document in such a manner that the document can be read.

Fig. 16 illustrates a second embodiment of the invention wherein the reflective surfaces 262 are curved. Light coming from direction 260 normal to the surface of the document is reflected by curved reflector 262 into the light trap 263. Therefore, the image is rendered incomprehensible to a copy machine. However, when the image is viewed over a range of angles 261, the image will appear bright. This allows an individual to read the document while viewing it over a relatively wide range of angles.

Fig. 17 shows another embodiment wherein the structure, having light reflectors 267 and light absorbing surfaces 270, is black when viewed from above, and bright when viewed from other directions. Fig. 18 illustrates an embodiment of the light control device in which a document can only be viewed from a direction normal to the surface of the structure. In essence, the light control device of the present invention can be made such that when an image produced thereon is viewed from one direction, or over one particular range of angles, the image will appear dark and when it is viewed over another range of angles, the image will appear bright. The light control device can be tailored in accordance with the desired viewing angles.

Once the embossment 163 shown in Fig. 2g is made, the light absorbing surfaces can be selectively formed therein by the masking, exposure, development, and etching steps described above with regard to Figs, la-lc to obtain results such as those shown in Figs. 19a, 19b and 19c, which correspond to Figs. 5a, 5b, and 5c discussed above. Fig. 19b illustrates what an embossment would look like if the structure in Fig. 19a was used as the master utilizing the soft embossing step of Fig.

2f. Fig. 19c illustrates the effect of reactive ion etching an inverted domed structure. The inverted domed structure is comprised, for example, of layers 156 and 157 shown in Fig. 2f. Depending on the manner in which the light traps are created, different ranges of viewing angles can be obtained. This can be seen in Figs. 19d and 19e, which are enlargements of Figs. 19b and 19c, respectively. The light traps in Fig. 19d project outward as a result of the inward projection of the light traps of the master of Fig. 19a. In Fig. 19e, the result of reactive ion etching directly into the photopolymer is an inward projection of the light traps. Therefore, more of the light reflected from the structure in Fig. 19d will impinge on the light traps thereby resulting in a slightly narrower range of viewing angles than the range of viewing angles provided by the structure of Fig. 19e. Figs. 20a-21 illustrate an alternative embodiment for creating the light control material of the present invention. In accordance with this embodiment, the light control material has a holographic surface formed therein. The holographic surface is comprised of a cured layer of photopolymer. The holographic surface is reactively ion etched to form light traps uniformly throughout the holographic surface. By reactively ion etching the holographic surface, enough of the holographic surface is preserved such that the hologram provides bright reflection when viewed from one set of viewing angles but is dark when viewed from a second set of directions due to the light traps.

In accordance with this embodiment, the light control material of the present invention is generated from a master having a hologram formed therein. As shown in Fig. 20a, a suitable master may be comprised of substrate 287 and layer 288 having a holographic surface 290 formed therein. Since the master depicted in Fig. 20a is readily available on the market, it is unnecessary to fabricate a master. However, a master may be fabricated in accordance with the present invention as discussed below with respect to Figs. 22a and 22b. As shown in Fig. 20b, the master comprised of layers 287 and 288 is placed in contact with liquid photopolymer 292 which is in contact with substrate 291. The structure is exposed to ultraviolet light which causes photopolymer 292 to harden and bond to substrate 291. Fig. 20c depicts an embossment comprised of layers 291 and 292. The holographic surface 290 has been transferred into the hardened photopolymer 292.

The arrows in Fig. 20d depict the reactive ion etching of layer 292. Fig. 21 is an exaggerated cross-sectional illustration of the structure of Fig. 20d after photopolymer layer 292 has been subjected to reactive ion etching. Oxygen is preferably used as the reactive gas during reactive ion etching. When the photopolymer layer 292 is etched, portions of the holographic surface 300 are removed, thereby creating the tapered structures 305 shown in Fig. 21. The dashed line 310 depicts the previous surface contour. When the etched surface is coated with a reflective layer of metal (not shown), the tapered structures function as light traps, as discussed above with respect to Fig. 6. The tips of the tapered structures approximate the original surface contour 300. Through experimentation it was discovered that from one set of angles the tapered structures 305 approximate the holographic surface 300 (i.e., they follow the contour of the original holographic surface) and provide bright reflection. Light waves 311 reflected from the tips of structures 305 interfere in accordance with the holographic pattern 300 formed in layer 292 to recreate a holographic image, which, in this case, is simply a bright surface. The angles from which the surface will look bright is determined by which of a variety of holograms is selected as the master. From whatever angles the holographic surface is not intended to produce brightness, the surface of layer 292 will appear dark because of the light traps generated throughout the surface. Therefore, an image carried either on the upper surface of layer 292 or on the surface of a transparent substrate covering the upper surface of layer 292 (not shown) will be bright when viewed from one particular set of directions and dark when viewed from other directions. A person interested in reading the document simply adjusts the angle from which the document is being viewed until the image printed thereon becomes readable. The hologram is generally selected to provide only off-axis viewing angles since copy machines generally read a document from a direction normal to the surface of the document. Therefore, an image carried on the surface of the material will appear dark when viewed from a direction normal to the surface, which is generally the direction from which a copy machine reads the document.

Once the structure shown in Fig. 21 has been created, embossments can be produced from it by using the soft embossing procedure discussed above with respect to Fig. 20b.

Figs. 22a-22d illustrate an alternative embodiment of the present invention wherein a holographic surface is formed in a layer of material and the surface is reactively ion etched at selected locations to remove portions of the holographic surface.

Fig. 22a illustrates a light control material comprised of a substrate 315 having a layer of photoresist 316 thereon. The photoresist is exposed to ultraviolet light through a mask 313. Preferably, the mask is a gray scale pattern which creates variable exposure in the photoresist as indicated by the dashed line. After exposure, the exposed portions of the photoresist 314 are developed away thereby creating a hologram in photoresist layer 316. Fig. 22b illustrates how the hologram is transferred into a layer of photopolymer. Substrate 315 having the photoresist layer 316 thereon is placed in contact with a layer of liquid photopolymer 321 which is in contact with a substrate 320. The liquid photopolymer 321 is exposed to ultraviolet light which causes the photopolymer to harden and the hologram to be formed therein. As shown in Fig. 22c, the photopolymer layer 321 having the hologram formed therein is covered with a thin layer of metal 340 such as chrome. The layer of chrome is then covered with a layer of photoresist 341. Selected areas in photoresist layer 341 are exposed to ultraviolet light through mask 342. The exposed photoresist is then developed away leaving selected areas in the chrome layer 340 uncovered. The uncovered areas of the chrome layer are etched away with an acid bath leaving selected areas in the photopolymer layer 321 uncovered. The structure is then reactively ion etched, as indicated by the arrows in Fig. 22d which causes fields of tapered structures 343 to be formed in the photopolymer 321 at selected locations. The surface of photopolymer layer 321 having the hologram and the tapered structures formed therein is coated with a thin layer of reflective metal (not shown). The photopolymer layer 321 is then overcoated with a strong adhesive 345 such as a solvent-based lacquer coating, an actinicly cured polymer, or a melt process polyethylene. Preferably, the overcoat has a low viscosity which allows the overcoat material to penetrate deeply into the fields of tapered structures.

Once the overcoat 345 hardens (Fig. 22e), a strong physical bond is created between the overcoat and the tapered structures due to the shapes of the tapered structures. If force is used to separate overcoat 345 from photopolymer layer 321, significant portions of the overcoat will remain bonded to the photopolymer while other portions of the overcoat may be removed. Since neither the portions of overcoat 345 which have been removed nor the surface of photopolymer layer 321 having portions of the overcoat bonded to it can be used to replicate the hologram, the hologram cannot be counterfeited. Furthermore, the areas in the hologram which are reactively ion etched are so small that the hologram is preserved. Also, if the hologram and the tapered structures are coated with a reflective layer of metal (not shown), the fields of tapered structures will function as light traps. By forming light traps at selected locations in the hologram, the contrast of a holographic image generated by the hologram can be enhanced. Therefore, in accordance with this embodiment, the contrast of a holographic image produced by a hologram can be enhanced while making the hologram counterfeit proof.

Once the hologram has been reactively ion etched, as shown in Fig. 22d, the structure can be used as a master from which embossments can be generated by using the soft embossing method discussed above with respect to Fig. 20b. Once the embossment has been generated (not shown), it is overcoated with a coating which strongly adheres to the metallized embossed surface as discussed above to prevent the hologram from being reproduced. Prior to applying the overcoat the surface of the etched hologram is covered with a reflective layer of metal such as aluminum which gives the fields of tapered structures their light absorbing characteristics causing them to operate as light traps.

Preferably, NO A 61 photopolymer is used to create the light control material of the present invention. Although this type of photopolymer was found through experimentation to be suitable for use with the present invention, it is possible that other types of photopolymers may also be suitable for such use. Shipley 1800 Series photoresist is preferably used to create the light control material of the present invention but other types of photoresist may be suitable for use with the present invention. While the invention has been disclosed in preferred forms, it will be apparent to those skilled in the art that many modifications can be made to the invention without departing from the spirit and scope of the invention. For example, the present invention is not limited with respect to the types of materials used to create the masters or embossments. Any materials which have the desired physical and optical properties may be used with the present invention. The embossments may also be generated by extrusion embossing wherein a molten extrudate is placed in contact with the master. When the extrudate cools it hardens and the resulting embossment is separated from the master.

Claims

Claims:

1. A light control material comprising: a polymer layer having at least one light trap formed therein wherein said at least one light trap is comprised of a plurality of tapered structures which have relatively high aspect ratios and wherein said tapered structures are covered with a reflective material such that light entering said at least one light trap is reflected within said at least one light trap among the tapered structures until substantially all of the light which has entered said at least one light trap has been absorbed.

2. A light control material according to Claim 1 wherein said at least one light trap is created by reactively ion etching said polymer layer to thereby form the tapered structures and by coating said tapered structures with metal to form said reflective layer.

3. A light control material according to Claim 1 wherein said light control material is an embossment generated from a master by an embossing procedure.

4. A light control material according to Claim 1 wherein the polymer layer has a plurality of reflective areas and light traps formed therein, each of said light traps comprising a plurality of said tapered structures, wherein said tapered structures have relatively high aspect ratios and wherein said reflective areas and said light traps are covered with a reflective material whereby light impinging on the reflective areas is reflected and light entering the light traps is reflected within the light traps among the tapered structures until substantially all of the light which has entered the light traps has been absorbed.

5. A light control material according to Claim 4 wherein said light control material is an embossment generated from a master by an embossing procedure and wherein said light traps formed in said embossment correspond to light traps formed in said master.

6. A light control material according to Claim 4 wherein said light traps are created by reactively ion etching said polymer layer to thereby form the tapered structures and by coating said tapered structures with metal to form said reflective layer.

7. A light control material according to Claim 5 wherein the light traps formed in the master are formed in selected locations in the master, and wherein said locations are selected in accordance with a photographic negative.

8. A light control material according to Claim 4 wherein said polymer layer displays a black-and-white image, wherein the light traps provide the black in the black-and-white image and the reflective areas provide the white in the black-and- white image and wherein at least one color transparency overlays said polymer layer whereby the black-and-white image is converted into a color image.

9. A light control material according to Claim 8 wherein said light control material is an embossment generated from a master by an embossing procedure and wherein said light traps formed in said embossment correspond to light traps formed in said master.

10. A light control material according to Claim 9 wherein the light traps formed in the master are formed in selected locations in said master, wherein said locations are selected in accordance with a photographic negative.

11. A light control material according to Claim 9 wherein the light traps formed in the master are formed in selected locations in said master, wherein said locations are selected in accordance with a computer-generated mask.

12. A light control material according to Claim 4 wherein said polymer layer displays a black-and-white image, wherein the light traps provide the black in the black-and-white image and wherein the reflective areas provide the white in the black-and-white image and wherein said polymer layer is overprinted with transparent color ink, and wherein said color transparent ink converts the black-and-white image into a color image.

13. A light control material according to Claim 12 wherein said light control material is an embossment generated from a master by an embossing procedure and wherein said light traps formed in said embossment correspond to light traps formed in said master.

14. A light control material according to Claim 13 wherein the light traps formed in the master are formed in selected locations in said master, wherein said locations are selected in accordance with a photographic negative.

15. A light control material according to Claim 13 wherein the light traps formed in the master are formed in selected locations in said master, wherein said locations are selected in accordance with a computer-generated mask.

16. A light control material according to Claim 4 wherein said polymer layer is a substantially clear substrate having a surface on which the light traps are formed in selected locations.

17. A method for creating a light control material comprising the steps of: creating a master having fields of tapered structures formed therein at selected locations; utilizing an embossing procedure to generate an embossment from the master whereby the fields of tapered structures formed in said master create corresponding fields of tapered structures in a first surface of said embossment; and coating said first surface with a reflective material.

18. A method for creating a light control according to Claim 17 wherein said step of creating a master comprises the steps of: coating a cured layer of photopolymer with a thin layer of metal; coating the layer of metal with a layer of photoresist; exposing selected portions of the layer of photoresist to exposure radiation through a mask; developing away the exposed portions of photoresist, thereby leaving the layer of metal uncovered at locations at which the photoresist has been developed away; utilizing an acid bath to etch away the metal at the locations at which the photoresist has been developed away, thereby leaving selected locations of said layer of photopolymer uncovered; reactively ion etching said layer of photopolymer whereby said fields of tapered structures are formed in said layer of photopolymer at each of said selected locations.

19. A method for creating a light control material according to Claim 18 wherein said step of creating a master further includes the step of electroplating the layer of photopolymer with metal after the reactive ion etching step to create a metal master.

20 A method for creating a light control according to Claim 18 wherein said mask is a photographic negative.

21. A method for creating a light control material according to Claim 18 wherein said mask is a computer-generated mask.

22. A method for creating a light control material according to Claim 17 further comprising the step of overlaying said embossment with at least one color transparency.

23. A method for creating a light control according to Claim 22 wherein said step of creating a master comprises the steps of: coating a cured layer of photopolymer with a thin layer of metal; coating the layer of metal with a layer of photoresist; exposing selected portions of the layer of photoresist to exposure radiation through a mask; developing away the exposed portions of photoresist, thereby leaving the layer of metal uncovered at locations at which the photoresist has been developed away; utilizing an acid bath to etch away the uncovered metal, thereby leaving selected locations of said layer of photopolymer uncovered; and reactively ion etching the layer of photopolymer whereby said fields of tapered structures are formed in said layer of photopolymer at each of said selected locations.

24. A method for creating a light control according to Claim 23 wherein said step of creating a master further includes the step of electroplating the layer of photopolymer with metal after the reactive ion etching step to create a metal master.

25. A method for creating a light control according to Claim 23 wherein said mask is a photographic negative.

26. A method for creating a light control material according to Claim 23 wherein said mask is a computer-generated mask.

27. A light control material for carrying an image and for rendering the image bright when viewed from a first set of angles and for rendering the image dark when viewed from a second set of angles, said light control material comprising: a substrate which is substantially transparent, said substrate having a first surface which is suitable for carrying an image thereon, said substrate having a second surface opposite and parallel to said first surface, said second surface having a plurality of reflective and light absorbing areas formed therein, said reflective and light absorbing areas arranged in a predetermined pattern to reflect light in a predetermined manner such that an image carried on said first surface is bright when viewed from a first set of angles with respect to said first surface but wherein the image is dark when viewed from a second set of angles with respect to said first surface.

28. A light control material according to Claim 27 wherein said substrate is a polymeric film and wherein said reflective and light absorbing areas are covered with a reflective layer of metal.

29. A light control material according to Claim 28 wherein the light absorbing areas are comprised of tapered structures which have large height-to-width ratios which cause light entering the light absorbing areas to be reflected among the tapered structures until substantially all of the light has been absorbed.

30. A light control material according to Claim 27 wherein said predetermined pattern determines the first and second sets of angles.

31. A light control material according to Claim 27 wherein said reflective areas are arcuate in shape.

32. A light control material for carrying an image and for rendering the image bright when viewed from a first set of angles and for rendering the image dark when viewed from a second set of angles, said light control material comprising: a first substrate having at least first and second surfaces, said first surface being suitable for carrying an image thereon, said first substrate being transparent, said second surface being opposite and parallel to said first surface; a second substrate bonded to said second surface of said first substrate, said second substrate having a plurality of reflective and light absorbing areas formed therein, said reflective and light absorbing areas arranged in a predetermined pattern to reflect light in a predetermined manner such that an image carried on said first surface is bright when viewed from a first set of angles with respect to said first surface but wherein the image is dark when viewed from a second set of angles with respect to said first surface.

33. A light control material according to Claim 32 wherein the first substrate is glass.

34. A light control material according to Claim 32 wherein the second substrate is cured photopolymer and wherein the reflective and light absorbing areas are covered with a reflective layer of metal.

35. A light control material according to Claim 34 wherein the light absorbing areas are comprised of tapered structures which have height-to-width ratios which cause light entering the light absorbing areas to be reflected among the tapered structure until substantially all of the light has been absorbed.

36. A light control material according to Claim 32 wherein said predetermined pattern determines the first and second sets of angles.

37. A light control material according to Claim 32 wherein said reflective areas are arcuate in shape.

38. A light control material for carrying an image and for rendering the image bright when viewed from a first set of angles and for rendering the image dark when viewed from a second set of angles, said light control material comprising: a substrate which is substantially transparent, said substrate having a hologram formed in at least one surface thereof, said hologram having tapered structures formed therein, wherein said hologram and said tapered structures are covered with a reflective material, and wherein said hologram having said tapered structures formed therein reconstructs a holographic image which illuminates an image carried by said light control material such that the image carried by said light control material is bright when viewed from a first set of angles, and wherein when the image carried by said light control material is viewed from a second set of angles said tapered structures covered with said reflective material function as a light trap and the image carried by said light control material is dark.

39. A light control material according to Claim 38 wherein said first and second sets of angles are pre-selected and wherein said hologram is selected from a variety of holograms in accordance with the pre-selected first and second sets of angles.

40. A light control material according to Claim 38 wherein said light control material further comprises an overcoat bonded to said hologram and said tapered structures such that, if an attempt is made to separate said overcoat from said hologram and said tapered structures, part of said overcoat may be removed while at least a part of said overcoat will remain bonded to said hologram and to said tapered structures whereby said hologram is rendered counterfeit resistant.

41. A light control material according to Claim 40 wherein said tapered structures coated with said reflective layer of metal constitutes light traps wherein said light traps enhance the contrast of a holographic image created by said hologram.

42. A method for creating a copy-resistant document comprising the steps of: forming reflective and light absorbing surfaces in a polymer layer; bonding said polymer layer to a first surface of a clear substrate, said clear substrate having a second surface suitable for carrying an image thereon, said second surface being opposite of and parallel to said first surface.

43. A method for creating a copy-resistant document according to Claim 42 wherein the step of forming the reflective and light absorbing surfaces further includes the step of using photolithographic techniques to form a continuous pattern of uniformly-shaped surface areas in the polymer layer.

44. A method for creating a copy-resistant document according to Claim 43, wherein the step of forming the reflective and light absorbing surfaces includes the step of generating a first polymer mold which contains an imprint of the continuous pattern of uniformly-shaped surface areas formed in said polymer layer.

45. A method for creating a copy-resistant document according to Claim 44 wherein the step of forming the reflective and light absorbing surfaces includes the step of generating a second polymer mold from said first polymer mold, wherein the second polymer mold contains a replicate of the continuous pattern of uniformly- shaped surface areas formed in said polymer layer.

46. A method for creating a copy-resistant document according to Claim

44 wherein the step of forming the reflective and light absorbing surfaces includes the step of reactively ion etching selected areas in the imprint of the continuous pattern of uniformly-shaped surface areas generated in said first polymer mold to form fields of tapered structures in the selected areas, said tapered structures having large height-to- width ratios.

47. A method for creating a copy-resistant document according to Claim 46 wherein the step of forming the reflective and light absorbing surfaces includes the step of coating the surfaces with a reflective material, the reflective material giving the reflective surfaces their reflective characteristics and making the tapered structures reflect and absorb light such that light entering the fields is reflected among the tapered structures until substantially all of the light has been absorbed.

48. A method for creating a copy-resistant document according to Claim

45 wherein the step of forming the reflective and light absorbing surfaces includes the step of reactively ion etching selected areas in the replicated continuous pattern of uniformly-shaped surface areas to form fields of tapered structures in the selected areas, said tapered structures having large height-to-width ratios.

49. A method for creating a copy-resistant document according to Claim 48 wherein the step of forming the reflective and light absorbing surfaces includes the step of coating the surfaces with a reflective material, the reflective material giving the reflective surfaces their reflective characteristics and making the tapered structures reflect and absorb light such that light entering the fields is reflected among the tapered structures until substantially all of the light has been absorbed.

50. A method for creating a copy-resistant document according to Claim 46 wherein, once the fields of tapered structures have been formed in the first polymer mold, the first polymer mold becomes a master from which subsequent embossments can be generated.

51. A method for creating a copy-resistant document according to Claim 50 wherein, instead of generating embossments from the first polymer mold, a replica of the first polymer mold is created by electroforming and the replica is used as a master to generate embossments.

52. A method for creating a copy-resistant document according to Claim 48 wherein, once the fields of tapered structures have been formed in the second polymer mold, the second polymer mold becomes a master from which subsequent embossments can be generated.

53. A method for creating a copy-resistant document according to Claim 52 wherein, instead of generating embossments from the first polymer mold, a replica of the master is created by electroforming and the replica is used as a master to generate embossments.

54. A method for creating a copy-resistant document according to any one of Claim 50, 51, 52 or 53 wherein the embossments are generated by placing liquid photopolymer in contact with the master and with a substrate and exposing the liquid photopolymer to curing radiation which causes the photopolymer to harden and wherein the hardened photopolymer embossment is separated from the master and coated with a reflective material on at least one side.

55. A method for creating a copy-resistant document comprising the steps of: forming a hologram in a first surface of a polymer layer; reactively ion etching at least selected locations of said hologram to form tapered structures in said hologram; coating said hologram and said tapered structures with a reflective material.

56. A method for creating a copy-resistant document according to Claim 55 wherein the step of forming a hologram includes the step of selecting a hologram from a variety of different holograms and wherein said hologram is selected in accordance with predetermined optical characteristics.