NZ200303A - Bank note authentication by diffraction grating structure bonded to note - Google Patents

Description

7 ^ 0 3 ® 3 Priority Date(s): ...................

Complete Specification FsSed: 14''frt&i Class: . l?P................

Publication Date: ... £)3'hjfi\/' .Al. . P.O. Journal, No: Patents Form No.5 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION "SHEET-MATERIAL AUTHENTICATED ITEM WITH REFLECTIVE-DIFFRACTIVE AUTHENTICATING DEVICE" ~r,WE rCA CORPORATION, a corporation organized under the laws of the State of Delaware, United States of America, and having its principal place of business at 30 Rockefeller Plaza, New York City, New York 10020, U.S.A. hereby declare the invention, for which I/we pray that a patent may be granted to rae/us,,and the method by which it is to be performed, to be particularly described in and by the following -statement:- (felb^ed by P^e ^ -I* RCA 75,742 SHEET-MATERIAL AUTHENTICATED ITEM WITH REFLECTIVE-DIFFRACTIVE AUTHENTICATING DEVICE This invention relates to devices for authenticating various items of sheet materials which are subject to counterfeiting, such as bank notes and other valuable documents, credit cards, passports, security passes, and phonograph records or their covers, for example.

Recently, there have been great advances in photocopying machines for the color copying of documents. It is believed that these advances will continue. Therefore, in the near future, it is likely that it will be possible to make a color photocopy of a bank note that the man-in-the-street is incapable of (or, at least, will have great difficulty in) distinguishing from the original. More particularly, while it is true that a genuine bank note makes use of authenticating means (such as inks of many colors and intricate engraved designs on special paper which sometimes contain water-marks or embedded colored platelets or metal threads) which currently permit an expert to distinguish a counterfeit from a genuine bank note, ~~ the unsophisticated layman is in no position to make use of such authenticating means. Therefore, as color photocopying improves, a real danger exists that the man-in-the-street will lose faith in the genuineness of paper currency. Such a situation could create havoc.

In order to be practical, any authenticating device for solving this problem must, inter alia, meet all four of the following criteria: 1. The authenticating device must produce an effect which is not capable of being reproduced by any type of color copier. . 7 ^ * •>, I 2 RCA 75,742 2. The effect produced by the authenticating device must be quickly and readily discernable, under ordinary lighting conditions, without requiring any significant degree of skill on the part of an unsophisticated layman. 3. The sophistication and capital cost of the equipment required to fabricate an authenticating device and to securely attach the authenticating device to an authenticated item (e.g., bank note, etc.) must be high enough to be beyond the reach of would-be counterfeiters. 4. Under high volume conditions, the additional per-unit cost of fabricating an authenticating device and securely attaching it to the authenticated item 15 (including both the amortization of the high capital cost of the equipment, as well as the per-unit variable cost) must be low enough not to constitute an impediment to its use.

Reference is made to U.S. Patent 4,186,943, 20 which issued February 5, 1980, to Lee, and to British Patent 1,394,021, which issued May 14, 1975. Each of these patents is concerned with an authenticating device for items such as bank notes. The authenticating devices disclosed in these patents fulfill the first 25 three of the above criteria, but not the fourth criterion. More specifically, the authenticating devices disclosed in these patents are comprised of a plastic strip or sheet substrate coated with a sufficient number of separate overlaying layers of quarter-wave (at a specified 30 wavelength within the visible spectrum) films of dielectric materials to operate effectly as a transmissive-reflective color filter. Such a color filter, when illuminated by polychromatic (e.g., white) light, selectively reflects most of the light in a certain portion of the visible 35 spectrum and selectively transmits most of the light in the remaining portion of the visible spectrum. Thus, the color of the reflective light and the color of the transmitted light are different from each other. By properly specifying the value of the wavelength of the 40 quarter-wave dielectric films, the respective colors of ■' ? 093 0 & 1 3 RCA 75,742 the reflected and transmitted light may be made substantially complementary (and, therefore, very quickly and readily distinguishable from each other by an 5 unskilled person). Further, the spectral portion (color) of reflected light from such a device which is observed depends on the angle at which it is illuminated and viewed. Therefore, the observed color of thiis portion changes as the device is tilted in relation to the 10 direction of the illuminating light. This change in observed color with angle also may be used to provide a change between two complementary reflected colors with a change in viewing angle.

While such an authenticating device works well IB for a bank note or the like, its per-unit cost of fabrication is inherently quite high. The reason for this is that each of the multi-layers of dielectric film is comprised of a different dielectric material having a different index-of-refraction. Each of these layers 20 must be successively and separately laid down (by such means as evaporation or sputtering deposition in a vacuum) with great accuracy to insure that the thickness of each particular dielectric material is equal to one-quarter of the specified wavelength of visible light 25 traveling through that particular dielectric material (i.e., having the index-of-refraction of that particular dielectric material). The cost of such successive and separate laying down is not a one-time capital cost, • which can be amortized over a high volume of bank notes, 30 but is part of the per-unit variable cost that applies to each and every authenticating device.

Reference is made to U.S. Patent 4,033,059, which issued July 5, 1977, to Hutton et al., and to U.S. Patent 4,124,947, which issued November 14, 1978, to Kuhl et al. Each of these patents discloses an authenticating device comprised of an imprint of intaglio pattern elements on a substrate. The intaglio pattern elements are in the form of one or more fields of closely juxtaposed, non-intersecting and non-contacting, lengthwise-extending mounds or peaks. The reflected 1 4 RCA 75,742 color of the mounds contrasts in luminance with that of the substrate (i.e., one has a relatively light reflective color and the other has a relatively dark reflective 5 color). However, as long as the authenticating device is viewed from an angle such that both the mounds, themselves, and the intervening substrate spaces between adjacent mounds are within the line of sight of a viewer, the luminance of the observed reflected color is an 10 integration of that of the mound reflected color and the substrate reflected color. The color hue of the integrated observed reflected color turns out to be fairly close to that of the mounds color, although the saturation of the integrated observed reflected color 15 is substantially different from that of the mounds color. On the other hand, when the mounds are viewed within a band of angular orientations, with respect to the substrate, such that the presence of the mounds occlude the intervening spaces between adjacent mounds, the 20 observed color is solely that of the mounds, themselves, (which contrasts in luminance with the aforesaid integrated color). In the case of Kuhl et al., when the mounds are viewed within such a band, one or more very thin lines, which extend in a direction substantially 25 perpendicular to the lengthwise extending mounds and have the color of the substrate, become observable in contrast to the mound color background. These thin lines are achieved by breaking each lengthwise extending mound into the same given set of two or more slightly spaced 30 longitudinal segments, the corresponding spaces between longitudinal segments of adjacent mounds being aligned with one another. Thereby, the above-mentioned observable thin lines appear when viewed within such a shallow (in a plane perpendicular to the surface of the substrate), 35 extremely narrow (in a plane parallel to the surface of the substrate) band of angular orientations, because the mound segments then occlude the relatively wider spaces of substrate color between adjacent mound segments.

In any case, the band of angular orientations 40 is defined by such factors as the height of the mounds, 7 !*>>. a ' k n •% 1 5 RCA 75,742 the size of the intervening spaces between adjacent mounds, the angular orientation of the lengthwise extending mounds with respect to the line of sight of the viewer, 5 the shape of the mounds and the relative location of different angularly-oriented fields of mounds with respect to one another. A "contrast" image (formed by appropriately changing the values of one or more of these parameters for the image with respect to those of the 10 background), which image is incapable of being discerned from the background when viewed at most angular orientations, becomes discernable (by contrast with the background) when viewed from angular orientations within this band.

On a relative basis, the per-unit cost of an intaglio-imprinted indicating device is substantially lower than that of a transmissive-reflective color filter authenticating device, discussed above. However, on an absolute basis, the per-unit cost of an intaglio-imprinted 20 authenticating device is still somewhat too high to satisfy the fourth criterion, set forth above. Furthermore, the intaglio-imprinted authenticating device is much less effective than is the transmissive-reflective color filter authenticating device in meeting the 25 requirements of the second criterion, discussed above.

It takes very little skill on the part of an unsophisticated layman to readily and quickly observe a change in complementary colors (each of which is observable over a relatively large but different angular orientation) 30 when a transmissive-reflective color filter authenticating device is observed at different angles or, alternatively, observed, respectively, in reflected or transmitted light. However, in the case of an intaglio-imprinted authenticating device, an image is observed by either 36 a change of contrast in luminance, with respect to that of the background, when the authenticating device is viewed from within a certain prescribed band of angular orientations or, in the case of Kuhl et al., by the appearance of a very thin line of contrasting color 40 when viewed within a certain extremely narrow and also 00303 6 RCA 75,742 shallow band of angular orientations. In either case, it inherently takes a significant amount of time, as well as some skill, for a viewer to orient properly the intaglio-imprinted authenticating device to reveal the "contrast" image thereon. While this significant amount of time may not be an impediment to the use of an intaglio-imprint authenticating device on such an authenticated item as a stock certificate, which can be observed relatively slowly, it is an impediment to its use on a bank note or the like, which must be observed quickly by an unskilled layman (such as a cashier in a store or in a box office, by way of example).

The present invention is directed to an authenticating device which meets all of the above set forth four criteria. In the present invention an authenticating device comprises a diffractive phase structure embossed or cast on the surface of a substrate, which forms a special type of color filter. In principle, such a filter may be either a transmissive filter or a reflective filter. However, for reasons described in more detail below, a reflective filter is much more practical for use in an authenticating device. An \ authenticating device incorporating the present invention may make use of the teachings of one or more of U.S. __ Patent 3,957,354, which issued to Knop on May 18, 1976; U.S. Patent 3,961,836, which issued to Knop on June 8, 1976, and U.S. Patent 4,062,628, which issued to Gale on December 13, 1977. r More specifically, an authenticating device incorporating the present invention includes a substrate bonded to the sheet material of which an authenticated item subject to counterfeiting is comprised. The substrate has a predetermined reflective diffraction grating structure formed as a relief pattern that is situated on at least one region of a viewable surface of the substrate. The reflective diffraction grating structure is filled and covered with a transparent i 7 RCA 15,142 material which exhibits a given index-of-refraction. The relief pattern forming the structure has specified grating profile, physical amplitude, and line frequency 5 parameters, such that the structure operates to separate polychromatic illuminating light incident thereon into at least one pair of adjacent, separate and distinct, reflective beams of contrasting colors, in which the narrowest angular dimension of the beam-width of each 10 of the beams at a distance of thirty centimeters is at least two milliradians. The transparent material is bonded to the viewable surface of the substrate in a manner sufficiently secure to prevent the transparent material from being removed from the structure without 15 effectively destroying the structure. As used herein, the expression "adjacent, separate and distinct, reflective beams of contrasting colors" is meant to exclude the case of adjacent and contiguous color portions of the continuous spectrum of visible polychromatic (e.g., 20 so-called "white") light, since such adjacent portions are neither "separate and distinct" from each other nor do they have colors which are "contrasting".

In the drawings: FIG. 1 is a schematic diagram of a first 25 embodiment of an authenticated item having an authenticating device, comprised of single integrated structure, bonded thereto; FIG. la is a schematic diagram of a second ■ embodiment of an authenticated item having an 30 authenticating device, comprised of a plurality of spaced integrated structures, bonded thereto; FIGS. 2a and 2b schematically illustrate first and second embodiments of an integrated structure of the type employed by the authenticating devices of 35 FIGS. 1 and la; FIGS. 3 and 3a schematically illustrate one species of an authenticating device integrated structure, in which an integrated structure is comprised of a single region diffraction grating structure, and 40 FIGS. 4, 4a, and 4b schematically illustrate 2 0 A -* n 1 8 RCA 75,742 other species of an authenticating device integrated structure in which an integrated structure is comprised of a plurality of contiguous different diffraction 5 grating regions.

Referring to FIG. 1, there is shown authenticated item 100 comprised of a sheet material, such as plastic or paper. In describing the present invention, for illustrative purposes, it often will be assumed that 10 authenticated item 100 is a bank note. However, it should be understood that authenticated item 100 may take other forms, such as other types of valuable documents having intrinsic value, credit cards, passports, security passes, or phonograph records or covers therefor, 15 by way of examples. In any case, authenticated item 100 has bonded thereto authenticating device 102. Authenticating device 102 consists of a single integrated structure of the type shown in either FIGS. 2a or 2b. In FIG. la, authenticating device 102a consists of a plurality of 20 two (or more) spaced integrated structures, each of the type shown in either FIGS. 2a or 2b.

As indicated in FIGS. 2a and 2b, integrated structure 201a or 201b, comprising authenticating device 102 or 102a, includes substrate 200 having a bottom 25 surface 202 that is bonded to authenticated item 100 and a top surface 204 having a predetermined reflective diffraction grating structure 206 formed thereon.

Transparent material 208 fills and covers reflective diffraction grating structure 206. Further, transparent 30 material 208 exhibits a given index-of-refraction'larger than unity. Transparent material 208 is preferably a plastic material, such as polyvinylchloride (PVC) or polycarbonate polyester, by way of example. (The index-of-refraction of such materials is nominally about 1.5). 35 While substrate 200 may be comprised of metal, it is preferably also comprised of a plastic or an adhesive material.

In the case of integrated-structure embodiment 201a, shown in FIG. 2a substrate 200 may be formed from 40 a thermoplastic sheet having diffractive structure 206 2 00^^3 • 1 9 RCA 75,742 embossed or cast on surface 204 thereof. Structure 206 may be made reflective by vacuum (e.g., evaporation or sputtering) deposition of a thin metal (e.g., aluminum) 5 film. Transparent material 208 may then be added by laminating a plastic layer to surface 204 or, alternatively, by applying a coating of a plastic monomer or plastic solution which is thereafter cured into a solid layer. In this case, transparent material 208 and substrate 200 10 may be made of either the same or different plastics.

In the case of integrated-structure embodiment 201b, shown in FIG. 2b, transparent material 208 comprises the original plastic sheet in which the diffraction grating structure 206 is embossed or cast, and substrate IB 200 may comprise a laminated plastic layer or either a plastic monomer or plastic solution that is later cured into a thin plastic layer. Alternatively, substrate 200, in the case of integrated-structure 201b, may be comprised of the adhesive material that bonds its bottom 20 surface to authenticated item 100. In other respects, the embodiments of FIGS. 2a and 2b are substantially similar.

In both the embodiments of FIGS. 2a and 2b, the combined thickness of substrate 200 and transparent 26 material 208 is t. As indicated by the arrow, the viewable surface of the reflective diffraction grating ' structure 206 is illuminated from above, through transparent material 208, by polychromatic light illumination 210. The grating structure 206 reflects 30 the polychromatic illuminating light incident thereon, in manner determined by its physical amplitude A, its lines frequency d and the spatial waveform or shape of its periodic grating profile. In accordance with the principles of the present invention, structure 206 3B has specified grating profile, physical amplitude A and line frequency d parameters, such that structure 206 is operative to separate polychromatic illuminating light 210 incident thereon into at least one pair of adjacent, separate and distinct, reflected beam of 40 contrasting colors, wherein the size of the narrowest s f 1 10 RCA 75,742 angular dimension of the beam-width of each of the beams at a distance of 30 cm is at least 2 milliradians.

Examples of reflective diffraction gratings structures 5 having such specified grating profile, physical amplitude and line frequency parameters are discussed in more detail below in connection with FIGS. 3 and 4.

It is often important that the overall thickness t of the integrated-structure forming authenticating 10 device 102 or 102a be quite small. For instance, in ^tacking bank notes in a stack of a predetermined number of bills, an angular tilt occurs which has a value dependent upon the product of the thickness t and the number of bills in the stack. In principle, the sheet 15 material could include a declivity or recess having a depth equal to t for receiving the authenticating device. However, this adds to the cost. It is simpler and cheaper to maintain the maximum angular tilt low enough to prevent any bill from slipping off the stack, which 20 is the case when the thickness t is maintained less than 0.0005 mils (i.e., about 12.5 ym). However, to maintain sufficient strength to prevent tearing of an integrated structure having such a small overall thickness t, the physical amplitude A should be maintained 25 as small as possible. Further, the speed at which a diffractive structure having a relatively small ratio of physical amplitude A to line spacing d can be embossed (by compression molding of a thermoplastic sheet) is much faster (and, therefore, provides a 30 significant reduction in variable unit cost) than is the case when this ratio is large. This is another reason why the physical amplitude A should be made as small as possible.

A diffraction grating, whether reflective or transmissive, formed by a surface relief pattern is a phase grating. The optical effect produced by a phase grating depends on the value of its optical amplitude a, measured in free-space wavelengths X, rather than directly on the value of its physical amplitude A.

However, the optical amplitude a is proportional to the 40 1 40 2 0 03 0 11 RCA 75,742 physical amplitude A, although the constant of proportionality differs significantly between transmissive and reflective gratings. Specifically, for a transmissiye grating, the constant of proportionality is l/(n^ - n), where n is the index-of-refraction of transparent material 208 and n^ is the index-of-refraction of a transparent substrate corresponding to substrate 200. Dielectric materials of the type (such as plastic or adhesive) -that are practical for fabricating integrated structure 201a or 201b have indices-of-refraction which do not differ much from one to another. Therefore, although the index-of-refraction of such dielectric materials is high relative to unity (e.g., in the vicinity of 1.5), the difference in indices-of-refraction (n^ - n) of any two of such dielectric materials is small relative to unity, resulting in a large constant of proportionality l/(n^ - n) for a transmissive grating.

This renders the physical amplitude A of a transmissive grating quite large relative to its optical amplitude a. On the other hand, the constant of proportionality for a reflective grating is l/2n, which is much smaller than unity, since n is larger than unity. This renders the physical amplitude A of a reflective grating quite small relative to its optical amplitude a. As discussed above, there are significant benefits to be had by maintaining pysical amplitude A relatively small. It is for this reason that reflective diffraction gratings, rather than transmissive diffraction gratings, are employed in the authenticating devices of the * present invention.

Further, the fabrication of the integrated structures 201a and 201b, FIGS. 2a and 2b, lend themselves to continuous-flow processing techniques, which reduce substantially the variable unit cost of fabricating such a structure. For example, in the case of the embodiment of FIG. 2a, a plastic sheet from a first plastic roll (constituting substrate 200) is passed, in turn, through embossing rollers (which emboss grating structure 206), through a vacuum deposition chamber (which metalizes 1 12 RCA 75,742 surface of substrate 200 with a reflective coating), then through lamination rollers which simultaneously also receive a laminating plastic having a laminating 5 coating thereon from a second plastic roll (which constitutes transparent material 208). The laminate emerging from the lamination rollers passes through an adhesive application chamber, where the adhesive for bonding to the authenticating item is applied. In the 10 case of the embodiment shown in FIG. 2b, the entire lamination step may be omitted, since the bonding adhesive itself may constitute substrate 200, while the embossed plastic from the first-mentioned plastic roll constitutes transparent material 208.

Reference is made to FIGS. 3 and 3a, which schematically illustrate a first species of a diffraction grating structure having specified grating profile, physical amplitude and line frequency parameters, such that the structure is operative to separate polychromatic 20 illuminating light incident thereon into at least one pair of adjacent, separate and distinct, reflected beams of contrasting colors, wherein the size of the narrowest angular dimension of the beam-width of each of the beams at a distance of 30 cm is at least 2 milliradians. 25 ilore particularly, diffraction grating structure 300 comprises a single region having a narrowest dimension W. As shown in FIG. 3a, diffraction grating structure 300 has a rectangular grating profile, a physical amplitude A and a line spacing d (i.e., a line frequency d . 30 Further, as indicated in FIG. 3a, grating structure 300 has a duty cycle of b/d. Only the top and bottom surfaces of structure 300 are covered by metalized reflective elements 302 and 304, which metalized reflective elements are applied by vacuum deposition. 35 Such vacuum deposition leaves the substantially vertical sides 306 of the rectangular profile of diffraction grating structure 300 substantially free of metal.

It is assumed that diffraction grating structure 300 is embossed in a plastic substrate 200 (as shown in 40 FIG. 2a) and transparent material 208 is also made of 7? ; i 13 RCA 75,742 plastic (preferably the same plastic as substrate 200), :then transparent material 208 can be bonded to the unmetalized portion of viewable surface 204 of substrate 5 200 in a manner sufficiently secure to prevent removal thereof from diffraction grating structure 300 without effectively destroying structure 300. The same results hold in the case where diffraction grating structure 300 is embossed in a plastic transparent material 208 (as 10 shown in FIG. 2b) and substrate 200 is in the form of either a plastic or an adhesive material. metalization of only the top and bottom surfaces of a rectangular grating is not the only way to prevent removal 15 of the transparent material without effectively destroying the grating structure. A grating structure having any type of profile may be so thinly metalized such that there are minute voids of metal distributed over the profile. Alternatively, a sufficiently strong metal-plastic bond 20 between transparent material and a totally metalized diffraction grating structure may be used. disclosed in detail in the aforesaid U.S. Patent 3,957,354, which, inter alia, may be comprised of a rectangular 25 profile diffraction grating such as diffraction grating structure 300. More particularly, diffraction grating structure 300 operates to separate polychromatic illuminating light incident thereon into a zero-order reflective beam and one or more higher diffraction order 30 reflective beams. In accordance with the teachings of U.S. Patent 3,957,354, the respective colors of the zero-diffraction ord^r and each of the higher diffraction orders of a rectangular profile diffraction grating depends on the wavelength spectrum characteristics of 36 the polychromatic illuminating light and the optical amplitude a (which, as has been discussed above, is proportional to the physical amplitude A) of the rectangular profile diffraction grating. Furthermore, the resultant color of the sum of all the higher 40 diffraction order beams is the complement of the color It should be understood, however, that Diffractive-subtractive color filters are .■ . 20030 1 14 RCA 75,742 of the zero diffraction order.

As is known in the art, the angular separation between any pair of adjacent diffraction orders is a 5 direct function of line frequency (i.e., d . By making the line frequency sufficiently high, the diffraction angle becomes big enough to separate each pair of adjacent diffraction orders into separate and distinct beams. For fine-line gratings, (i.e., where the line space d 10 has a value less than twice the wavelength of the light), the diffraction angle becomes large enough that only the zero and the first diffraction orders may occur. In this case, the color of the first diffraction order is xthe complement of the color of the zero diffraction IB order, so that the respective colors of the zero and first diffraction orders contrast with one another to a great extent. However, even at lower line frequencies, the physical amplitude A may be specified at such a value (in accordance with the teachings of aforesaid U.S. Patent 20 3,957,354) that the zero and first diffraction order have contrasting colors.

Contrast is enhanced if the respective colors of an adjacent pair of diffraction orders (e.g., the zero and first diffraction orders) are near maximum 2B saturation. In the case of diffraction gratings having line spacing d of at least 5 micrometers (i.e., in those cases where the Huygens-Kirchoff approximation is valid), maximum saturation occurs when the duty cycle b/d is at 50 percent. However, for any fine-line diffractive-30 subtractive color filter grating, maximum saturation occurs at a ratio of b/d determined by the particular solution of Maxwell's equations (taking into consideration all boundary condition parameters of the grating and the polarization parameters of the incident light). In 3B general, for fine-line rectangular profile diffractive-subtractive color filters, maximum saturation occurs at a value of the ratio b/d which is other than 50 percent and which is different for reflective and transmissive grating structures. By specifying many different sets of boundary conditions, a computer may be used to 1 15 RCA 75,742 solve Maxwell's equation by numerical analysis to thereby predict the fine-line reflective rectangular profile grating parameters d, b and A at which maximum saturation & of desired complementary color zero and first diffraction order reflected beams will result. Alternatively, if the reflective grating parameters d and A are specified, the value of the ratio b/d that yields a degree of saturation of the color produced thereby which is close 10 to the maximum can be easily determined experimentally by a cut and try method.

In order to meet criterion 2, discussed above, the visual effect on a viewer produced by at least one pair of adjacent (e.g., zero diffraction order on the first diffrac-15 tion order) reflective beams from diffraction grating structure 300 has to be quickly and readily discernable at a normal viewing distance from authenticated item 100, under ordinary lighting conditions, without requiring any significant degree of skill on part of the viewer. 20 If a normal viewing distance is assumed to be 30 cm (i.e., about 1 foot), the narrowest dimension V? of diffraction grating structure 300 should be, at the very least, sufficiently large tp subtend an angle of two milliradians (at this normal viewing distance of 30 cm), 25 in order to just meet criterion 2 (discussed above).

In other words, disregarding any divergence of either the reflected zero.order or higher order beams, the narrowest dimension of the beam-width of each of these beams is proportional to W, and at 30 cm must correspond 30 with an angular dimension, at the very least, of two milliradians. For optimum discernability, the narrowest angular dimension of the beam-width of each of the beams, at a distance of 30 cm, should be at least an order of magnitude larger than this (i.e., 20 milliradians or 35 more).

The expression "under ordinary lighting condition" needs some further explanation. When a diffraction grating is illuminated from a single collimated light source, it produces its most separate and distinct zero and higher diffraction order output - ';v', --A ' ,'Jju « 1 16 RCA 75#742 beams. However, when it is illuminated by substantially solely diffuse light, it produces its least separate and distinct zero and higher diffraction order output 6 beams. Ordinary lighting conditions, under which an authenticated item is normally viewed, consists of various separate, more or less collimated light sources together with a diffuse light background (with the exact relationship varying from one environment to another). 10 When the narrowest angular dimension of the beam-width of each of the pair of adjacent reflective beams of contrasting colors, at a distance of 30 cm, is at least 2 milliradians the beams are sufficiently separate and distinct, to be discerned, under "ordinary lighting 15 conditions", i.e., an unskilled viewer can readily locate and discern the reflective beams by merely tilting, that is, changing the angular orientation of, an authenticated item with respect to his line of sight.

Referring to FIG. 4, there is shown a species 20 of the present invention in which an integrated structure of an authenticating device is comprised of a plurality of contiguous regions, each region consisting of a different diffraction grating structure. More specifically, as shown in FIG. 4, region 1 diffraction 25- grating structure 400 is in the form of a circle of - diameter W and region 2 diffraction grating structure 402 is in the form of a rectangle that surrounds region 1 structure 400 and has a narrowest dimension that is greater than W. A first sub-species of grating structures 30 400 and 402 is shown in FIG. 4a and a second sub-species of grating structures 400 and 402 is shown in FIG. 4b. In accordance with the sub-species shown in FIG. 4a, both grating structures 400 and 402 have rectangular profiles and form diffractive-subtractive color filters 35 similar to grating structure 300. Grating structure 400 of FIG. 4a has a specified line spacing d and a specified physical amplitude A^, while grating structure 402 of FIG. 4a has the same specified line spacing d and a specified physical amplitude A£ different from A^. Since 40 grating structure 402 is contiguous with grating ■' •' 20030 i * 1 17 RCA 75,742 structure 400, a zero diffraction order of both grating structures 400 and 402 may be simultaneously viewable by a viewer. Further, since the line spacing d is the 6 same for grating structures 400 and 402, a first diffraction order of grating structures 400 and 402 in FIG. 4a also may be simultaneously viewable by a viewer. However, the respective angular orientations of the authenticated item with respect to the light of sight of 10 the viewer are different for simultaneously viewing the respective first diffraction orders from what they are for simultaneously viewing the respective zero diffraction orders.

Referring now to the sub-species of FIG. 4 .15 shown in FIG. 4b, both grating structure 400 of region 1 and grating structure 402 of region 2 in FIG. 4b have sinusoidal grating profiles and have the same physical amplitude A. However, the line spacing d^ of grating structure 400 of FIG. 4b is different from the line 20 spacing d£ of grating structure 402 of FIG. 4b. As is known in the art, the arcsin of the diffraction angle of a diffraction grating is equal to the ratio of wavelength to line spacing. Therefore, each of grating structures 400 and 402 of FIG. 4b, in response to being 25 illuminated by polychromatic light, will produce higher diffraction orders in which the polychromatic light is -angularly dispersed into its component spectral colors. However, because the line spacings d^ and of grating structures 400 and 402 of FIG. 4b are different from one 30 another, different selected spectral colors may be derived at the same given angle of the first diffraction order for respective regions 1 and 2 (in the subspecies of FIG. 4b) by properly choosing the respective values of line spacings d^ and d£. By way of example, if line spacing d^^ of grating structure 400 of FIG. 4b is selected to be only 82% of line spacing d£ of grating structure 402 of FIG. 4b, then grating structure 400 will produce a reflected beam of green (530 nm spectral wavelength) light and grating structure 402 will produce 40 a reflected beam of complementary red (650 nm spectral y -f) 9 3 0 3 -1 18 RCA 75,742 wavelengths) light at the same given diffraction angle of the first diffraction order. Therefore, a viewer viewing regions 1 and 2 at this given diffraction angle 6 will see simultaneously a green region 1 and a red region 2. Furthermore, when viewing regions 1 and 2 of FIG. 4 at slightly different angles, the respective colors of regions 1 and 2 will no longer be green and red, respectively, but still will have essentially 10 contrasting colors within the entire spatial intersection of the respective dispersion spectrums of grating structures £00 and 402 of FIG. 4b, due to the difference in the respective values of d^ and d2 thereof.

It is desirable that the reflected beams from 15 regions 1 and 2 in the sub-species of FIG. 4b be as bright as possible. Therefore, the physical amplitude A of grating structures 400 and 402 in FIG. 4b preferably has a value which minimizes the amount of light which remains in its respective zero diffraction 20 order, which results in the light diffracted into the higher diffraction order reflected beams being maximized. Furthermore, to maximize the amount of light diffracted into each first diffraction order beam, it is desirable that both grating structures 400 and 402 of FIG. 4b be 25 fine-line diffraction gratings {discussed above), thereby suppressing the occurrence of higher diffraction, ~ order beams above the first diffraction order. In regard to maximizing the brightness of the first diffraction order of grating structures 400 and 40 2 of FIG. 4b, 30 reference should be made to the teachings of aforesaid U.S. Patents 3,961,836 and 4,062,628. 36 Returning to FIGS. 2a and 2b, transparent material 208 may be colorless or, alternatively, may include a dye operating as an absorbent subtractive color filter that is positioned in series with the reflective beams from the diffraction grating structure 40 206. Specifically, the dye should have spectral • ■ ? n o 3 0 « " 1 19 RCA 75,742 wavelength dependent transmission characteristics that enhance the color selectivity of the reflected beams from diffraction grating structure 206. In addition, 5 the presence of a dye within transparent material 208 enhances signal-to-noise ratio by reducing the glare produced by specular reflection from the metalized diffraction grating structure 206.

Further, transparent material 208 and/or other 10 components of the authenticating device are of a type which lend themselves to being doped with trace chemicals (e.g., magnetic, radioactive, etc.) that permit a more sophisticated analysis by an expert. 40 2 C 0 3 0 3

Claims (1)

1. WHAT WE CLAIM IS: 1. An authenticating device for use in authenticating an item of sheet material when said device is bonded to said item, wherein said device includes: a substrate having a predetermined reflective diffraction grating structure formed as a relief pattern that is situated on at least one region of a viewable surface of said substrate, a transparent material filling and covering said reflective diffraction grating structure, said transparent material exhibiting a given index-of-refraction, wherein said relief pattern forming said structure has specified grating profile, physical amplitude and line frequency parameters such that said structure is operative to separate polychromatic illuminating light incident thereon into at least one pair of adjacent, separate and distinct, reflected beams of contrasting colors, wherein the least beam width of each of said beams at a distance of thirty centimeters from the grating subtends an angle of at least two milliradians, and wherein said transparent material is bonded to said viewable surface of said substrate in a manner I sufficiently secure to prevent said transparent material from being removed from said structure without effectively ! destroying said structure. 0303 - 21 - 2. The device defined in claim 1, wherein said transparent material exhibits a given index-of-refraction greater than unity, and wherein said relief pattern forming said structure has such grating profile, physical amplitude and line frequency parameters that, in cooperation with said given index-of-refraction, said structure operates as a diffractive subtractive filter having predetermined transfer function characteristics. 3. The device defined in claim 1, wherein said transparent material exhibits a given index-of-refraction greater than unity, and wherein said relief pattern has a rectangular profile and such physical amplitude and line frequency parameters that, in cooperation with said given index-of-refraction, said structure diffracts through at least a certain angle, determined by said line frequency, a portion of said polychromatic illuminating light to discriminatingly transfer, as a predetermined function of wavelength of the wavelength spectrum of said polychromatic light, some certain fraction of each spectral component of said polychromatic illuminating light into diffraction orders higher than the zero diffraction order and transfer substantially the entire remainder of said polychromatic illuminating light into the zero diffraction order, said certain angle being sufficient to obtain zero and higher diffraction order beams that are separate and 200303 distinct from one another, whereby the colour of said zero diffraction order beam is the complement of the colour of the aggregate of all the diffraction order beams higher than said zero diffraction order. 4. The device defined in Claim 3, wherein said diffraction grating structure is comprised of a fine-line grating which diffracts said polychromatic illuminating light through a certain angle that is large enough to limit the number of higher diffraction orders to solely said first diffraction order, whereby the colour of said zero diffraction order beam is the complement of the colour of said first diffraction order beam. 5. The device defined in Claim 1, wherein said relief pattern is comprised of at least first and second contiguous regions, said first region being comprised of a first diffraction grating having first specified grating profile, physical amplitude and line frequency parameters, said second region being comprised of a second diffraction grating having second specified grating profile, physical amplitude and line frequency parameters, at least one of said second specified parameters of said second grating being different from the corresponding one of said first specified parameters of said first grating. 200303 6. The device defined in claim 5, wherein said first and second grating profiles are rectangular, wherein said first and second line frequencies are substantially identical, wherein said transparent material exhibits a given index-of-refraction, wherein said first physical amplitude has a first given value such that, in cooperation with said given index-of-refraction, said first grating operates as a diffractive-subtractive filter deriving a zero diffraction order beam of a first color and a first diffraction order beam of a second color, and second given value, different from said first given value, such that, in cooperation with said given index-of-refraction, said second grating operates as a diffractive-subtractive filter deriving a zero diffraction order beam of a third color which contrasts with said first color and a first diffraction order beam of a fourth color which contrasts with said second color. wherein said first and second gratings are fine-line diffraction gratings which diffract said polychromatic illuminating light through a certain angle that is large enough to limit the number of higher diffraction order beams to solely said first diffraction order reflected wherein said second physical amplitude has a -7 rpVia AjTr■? i w nl a i m £ iilC VAoV XUU J. J.1 _i_ A A J.41I V beam, and IBJUN1982 - 24 - :0C3Q3 wherein the respective first and second given values of said first and second physical amplitudes are such that said third color is the complement of said first color, whereby said fourth color is the complement of said second color. 8 . The device defined in claim 1, , wherein said transparent material includes a dye which forms an absorbent subtractive filter through which both said polychromatic illuminating light and said reflected beam pass, said dye filter having predetermined wavelength-dependent transmission characteristics which cooperate with the wavelength-dependent diffractive characteristics of said structure to enhance the color selectivity of the contrasting colors of said reflected beams. in which the respective top and bottom surfaces of said rectangular profile are substantially solely reflective and the respective side surfaces of said rectangular profile are substantially solely transmissive. 10. , An article comprising said item of sheet material and the device of any one of claims 1-9, wherein said substrate of said device is bonded to a viewable surface of said item of sheet material. .\2;\ / v . > ■' " " (<"7\ 9- The device defined in claim 1, wherein said structure has a rectangular profile 7 V <a>\ - 25 - 2 G C 3 Q 3 11. The article defined in claim 10/ wherein said authenticating device further includes, a separate other substrate spaced from said first-mentioned substrate and bonded to said sheet material1s viewable surface, said other substrate having another predetermined reflective diffraction grating structure formed as a relief pattern that is situated on at least one region of a viewable surface of said other substrate, and another transparent material filling and covering said other reflective diffraction grating structure, said other transparent material exhibiting a given index-of-refraction, wherein said relief pattern forming said other structure has specified grating profile, physical amplitude and line frequency parameters such that said other structure is operative to separate polychromatic illviminating light incident thereon into at least one pair of adjacent, separate and distinct, reflected beams of contrasting colors, wherein the least beam width of each of said beams from said other structure at a distance of thirty centimeters from said other structure subtends an angle of at least two milliradians, and wherein said other transparent material is bonded to said viewable surface of said other substrate in a manner sufficiently secure to prevent said other transparent material from being removed from said other structure without effectively destroying said other structure. vv , i/, x - 26 - ZOQ30 12. The device defined in claim 1, wherein said transparent material includes trace chemicals of a type which permit sophisticated analysis of the authenticity of said authenticating device. 13. An article comprising an authenticated item of sheet material which is subject to counterfeiting and an authenticating device bonded to a viewable surface of said item substantially as described herein with reference to any one of the embodiments shown in the accompanying drawings. BALDWIN, SON & CAREY iE APPLICANTS ATTORNEYS FOR Tht