SHEET CONTAINING CONTOUR-DEPENDENT DIRECTIONAL IMAGE AND METHOD FOR FORMING THE SAME

Abstract: A sheet having a set of axial markings viewable as a contour-dependent, directional image. The image is intact only when the sheet is in substantially a predetermined contour and viewed within a predetermined conical field of view. The figure shows a cross section of an embossed card (60) to the surface of which a microlens sheeting (62) comprising a monolayer of microspheres (64), a binder layer (66), and a layer of adhesive (68) was adhered before embossing. The card and adhered microlens sheeting have been embossed with desired characters (70, 72), such as with the account numbers on a credit card, and the sheeting has assumed a nonplanar contour. The embossing pattern has been altered by removing some characters (70, 72) and adding another (74). Due to the change of the contour of the card, the spatial arrangement between the individual axial markings has been altered, thereby disrupting the image, as represented by the nonparallel arrangement of the lines. Also a method for forming such sheets is presented.

Description: Field Of Invention The present invention concerns sheets containing directional images. A directional image is viewable only at or within a range centered on a predetermined angle, a region described herein as a conical field of view. The present invention also concerns a method for forming such images. Background Art In view of several recent incidents, manufacturers of over-the-counter drugs have become concerned that their products be sold in tamper-evident or tamper-resistant packages. The purpose of such security measures is to verify that the container has not been opened and thus that the contents have not been tampered with in some way. The measures must be inexpensive enough to be produced in bulk and should dependably provide a readily apparent indication of tampering. A low cost seal for a container is provided by paper glued across its opening. However, it usually is possible to peel off the paper and then reseal it, leaving an appearance quite similar to that of an unopened article. More sophisticated seals are caps that must be permanently structurally altered in order to be opened, as taught in U.S. Pat. No. 3,9l3,772 (Ochs); U.S. Pat. No. 4,446,979 (Goch); U.S. Pat. No. 4,457,437 (Heath); and U.S. Pat. No. 4,474,304 (Jacobs). Also known are seals which provide visual or audible indication whether a desired vacuum or pressure is intact, as taught in U.S. Pat. No. 3,l52,7ll (Mumford et al.); U.S. Pat. No. 3,l60,302 (Chaplin); and U.S. Pat. No. 3,443,7ll (Olson). U.S. Pat. No. 4,480,749 (Lauris et al.), shows thermoplastic seals made secure with temperature indicators. Embossed cards, such as credit cards, also present an area where tamper-resistance and counterfeit-resistance are desired. One frequently encountered form of fraud is to alter the embossed characters on a card, thereby changing the account number or expiration date indicated thereon. One security technique commonly used to combat such fraud involves "tipping", i.e. color coating the raised portions of the embossed characters. However, this coating is often easily removed, following which the original characters may be flattened and the card reembossed with new characters which may then be colored to give the card a genuine appearance. Another common security measure is to use a material that stress-whitens, such as is seen in labelling strips and some bank and credit cards. Flattening of the raised area may cause the card to revert to its original color, however, and the card may then be stamped with fraudulent information. There is a need in both the over-the-counter drug industry and in the credit card industry for better counterfeit-resistant security measures to indicate attempted tampering. Such measures should provide a clear, easily read indication of tampering and should be resistant to imitation or duplication designed to circumvent the intended function. Other Background Art Sheeting in which an image of a pattern or legend is built into the sheeting has found a number of important uses, particularly as distinctive labels useful to authenticate an article or document. Such sheetings as taught in U.S. Pat. No. 3,l54,872 (Nordgren); U.S. Pat. No. 3,80l,l83 (Sevelin et al.); U.S. Pat. No. 4,082,426 (Brown); and U.S. Pat. No. 4,099,838 (Cook et al.), have found use as validation stickers for vehicle license plates and as security films for driver's licenses, government documents, phonograph records, tape cassettes, and the like. These references teach that the legends must be incorporated into the sheeting when the sheeting is being made. U.S. Pat. No. 4,200,875 (Galanos) discloses a method of forming directional images in retroreflective sheeting which comprises a monolayer of glass microspheres and a reflective masking layer. In that method, laser irradiation of the retroreflective sheeting in an imagewise fashion causes structural alterations or modifications in the sheet which are viewable as a directional image. Hockert et al., Jap. Application No. l9824/84, filed February 6, l984, Kokai No. l48004/84, laid open August 24, l984, discloses a sheet which comprises a monolayer of closely spaced transparent microlenses and a set of axial markings, each marking disposed at the rear of a microlens such that the set of markings is visible as an image only when viewed within a predetermined conical field of view. Both of the latter two references teach that directional images may be formed in the sheetings after the sheetings are constructed. The latter reference discloses a sheeting wherein certain useful optical effects may be achieved, such as images which are colored, rather than just black and white. Also, sheets of the latter reference are useful as transparent overlays to authenticate documents or articles to which they might be adhered. Summary Of Invention This invention provides a convenient, economical, counterfeit-resistant means of reliably indicating attempted tampering with articles such as containers of embossed cards by virtue of a new sheet that is applied to the articles. The invention is a sheet bearing a directional image which is viewable when looking within a predetermined conical field of view directed towards one face of said sheet hereinafter referred to as the front face of said sheet, said sheet comprising: (A) a monolayer of closely spaced transparent microlenses; (B) means for securing the microlenses to said front face of the sheet; (C) a set of axial markings, namely markings whose center points are respectively located on axes that extend through the optical center of individual microlenses and intersect at a common viewing point or line or are parallel to one another; (D) each axial marking being located at the rear of a microlens, that is to say the side of said microlens opposite said front fact of said sheet; and (E) each axial marking being located at the rear of a microlens at a position where the set of said axial markings is visible as an intact image only when said sheet is in substantially a predetermined nonplaner contour and is viewed within said predetermined conical field of view. The invention also provides a method of making said sheet, said method being defined in the accompanying Claim 15. The new sheet of the invention may also be used as a means for verifying the authenticity of an article to which it is attached. In addition, a sheet containing one or more contour-dependent, directional images may have many other applications, such as a sign with changeable messages for instance. "Axial markings" are defined as visibly distinctive structures of limited area, such as coated spots, openings in otherwise continuous layers, deformities within a layer, or deformities within individual microlenses, whose center points are respectively located on axes that extend through the optical center of individual microlenses and intersect at a common viewing point or line, or are parallel to one another. Detailed Description of Invention To form a sheet containing a contour-dependent, directional image, a sheeting comprising a nomolayer of microlenses, sometimes referred to herein as microlens sheeting, is arranged in a selected contour. This contour may be defined by a surface, such as the area of embossing on an embossed card, to which the microlens sheeting is adhered as a surface-conforming overlay, or the sheeting itself may assume a contour. For instance, microlens sheeting may be used to form a vacuum seal for a container; such seals typically have a concave contour. In this regard, it is useful for the microlens sheeting to be sufficiently flexible to conform to an underlying surface or in response to a contouring force while retaining its structural integrity and optical properties. When the sheeting is arranged in the desired contour, a beam of highly collimated light, such as a laser beam, is directed at the sheeting in an imagewise fashion at a selected angle of incidence, thereby forming an axial marking at the rear of each microlens which the beam strikes. Q-switched, pulsed laser beams are typically the most efficient type of collimated light for forming axial markings in microlens sheeting. The set of axial markings is visible as an intact image only when the sheet is in substantially the contour in which it was irradiated and is viewed within a conical field of view which is centered on the angle of incidence at which the sheeting was irradiated. Additional contour-dependent, directional images, each viewable within a different conical field of view or when the sheet is arranged in a different contour, may be formed in the sheet according to the same method. The collimated light may scan the entire face of the microlens sheeting, or only selected portions thereof, such as through a mask, to form a set of axial markings, or the set of axial markings can be formed by irradiating the sheeting in a specially shaped scanning pattern. Whether the image extends uniformly across the whole sheet or merely a portion thereof, it has the visibly distinctive contour-dependent, directional quality which is an object of this invention. The precise location or level within the sheeting at which the axial markings are formed is determined by the construction of the microlens sheeting and the method used for forming directional images. For instance, each axial marking may be a deformity formed in a microlens itself, or an opening formed in a layer which is moderately spaced therefrom. As used herein, "at the rear of" is meant to refer to all such locations. The angle of incidence at which the collimated light is directed at the microlens sheeting determines the central axis of the conical field of view within which the resultant image is visible. The angular width of that conical field of view is related to the size of the axial markings, which is determined in part by the energy density of the irradiation applied to the sheeting to create the markings. If a beam with higher energy density is used to form the axial markings, the image will have a wider conical field of view. Similarly, if a beam with a lower energy density is used, the image will have a narrower conical field of view. When sheeting comprising a monolayer of microspheres substantially all having an average diameter of approximately 50 to 60 micrometers is irradiated to form axial markings from 3 to l0 micrometers in diameter, the contour-dependent, directional image is viewable only within a conical field approximately l0 to 20 degrees across. The set of axial markings is visible as an intact image only when the imaged sheet is in substantially the same contour in which it was imaged and is viewed from within the predetermined conical field of view. When the contour of the sheet is changed, the spatial relation between the various axial markings which compose the image is changed, thereby distorting the image's appearance, or if the change of contour is of sufficient magnitude, preventing any more than portions of the image from being simultaneously visible at any angle. The sensitivity of the image to changes in the contour of the sheet is inversely related to the angular width of the conical field of view within which the image is visible. Images which are visible only across very narrow conical fields of view are sensitive to minor changes in contour. Such images, however, typically have less contrast as against the surrounding background and are somewhat more difficult to locate than images which are visible across very wide conical fields of view. Images which are visible across very wide conical fields of view may remain intact even after substantial changes in contour. The optimum width of the conical field of view and related sensitivity of the image to changes in the contour of the sheet are dependent upon the particular application and may be readily determined by experiment. Sheets used to authenticate embossed cards, for instance, would be most useful if sensitive to minor changes in contour, whereas sheets used as changeable signs where high visibility over a wide field may be desired should be less sensitive to changes of contour. The microlens sheeting used in this invention should be flexible and remain structurally sound, i.e., the microlenses should remain securely anchored to the sheeting and not separate from the sheeting when the sheeting is shaped into various contours. The degree of flexibility required is determined by the intended application. The means for securing the microlenses to the sheet may be a layer of binder material, such as a polymeric material, or some other construction by which the microlenses are held in a monolayer arrangement, as, for instance, in U.S. Pat. No. 3,503,3l5 (de Montebello). Among the types of microlens sheeting which may be useful in this invention are "exposed-lens" sheeting, wherein the microlenses protrude from the front of the sheeting; "encapsulated-lens" sheeting, disclosed in U.S. Pat. No. 3,l90,l78 (McKenzie) which is similar to exposed-lens sheeting in that the microlenses protrude from a binder layer, but further comprises a transparent film covering the microlenses; and "embedded-lens" sheeting, wherein the microlenses are embedded in a transparent protective layer. The microlenses in encapsulated-lens and embedded-lens sheeting may be less likely to separate from the sheeting than those in exposed-lens sheeting when the sheeting is shaped into severely convoluted contours; however, such sheetings tend to be more difficult to bend or conform to underlying contours than exposed-lens sheeting. Retroreflective embedded-lens and encapsulated-lens sheetings remain retroreflective even when wet, whereas exposed-lens sheeting does not, making encapsulated-lens and embedded-lens sheetings preferred for many outdoor applications, such as for traffic signs. In most applications of this invention, the microlens sheeting is preferably retroreflective, because the images formed therein are typically more easily verified than those formed in non-retroreflective sheeting. The microlenses are preferably spherical because they may be imaged from a wide range of angles of incidence, and if the sheeting is retroreflective, it will be retroreflective over a wide range of angles. An example of sheeting comprising microlenses which are not spheres is disclosed in de Montebello. Glass microlenses are preferred because they tend to be more durable than microlenses formed from other materials. Microspheres with a refractive index within a range of l.5 to 2.7 are useful; however, presently available microspheres with refractive indices above 2.4 may introduce coloring. Colorless clarity may be preferred in some applications, such as where a sheet containing the contour-dependent, directional image is adhered as a transparent overlay over information on a document. The microspheres preferably have an average diameter of l0 to l00 micrometers, and are ideally of a substantially uniform diameter of approximately 50 to 60 micrometers. If the microspheres are much larger, the sheeting tends to become thicker and more difficult to bend, and images formed therein will have less resolution. If the microspheres are much smaller, images formed on the sheeting will have greater resolution, but it is more difficult to distribute the microlenses uniformly when constructing the sheeting. As the microspheres become smaller than approximately l0 to 20 micrometers, diffraction losses may cause the sheeting to be less brightly reflective, thus reducing the contrast between the image and the surrounding background, thereby rendering the image more difficult to locate. This invention may be used to provide a secure, tamper-evident means of providing a readily apparent indication that a container, such as a bottle or jar, has been opened or that an attempt to open it has been made. Many such containers are presently sealed under a vacuum which typically causes the seal of the container to assume a familiar concave shape. When the container is opened, the vacuum is lost and the shape of the seal changes. If the seal comprises a sheet containing a contour-dependent, directional image, the image is disrupted providing a readily apparent indication that the container has been opened. Conveniently, microlens sheetings may be made in physically unsubstantial or flimsy form so that an attempt to tamper with the seal is likely to cause a tear or other obvious physical damage to the sheet in addition to disrupting the contour-dependent, directional image contained therein. The image may be a special safety message explaining the importance of it being intact or a pattern such as a company logo or trademark with accompanying directions on the container explaining how to verify that the seal is intact. The image is preferably of sufficient detail and size that it cannot be easily imitated and that disruptions will be discernible. However, the image should not be so complicated that changes induced by tampering will not be noticed by the intended beneficiary, the consumer. A photographic image is ideally suited to indicate tampering. The familiar format and subtle detail which may be achieved with such an image are useful to provide a very reliable means for indicating tampering wherein changes in the image are readily apparent. A photographic image can be formed by irradiating the sheeting through a half-tone mask which contains a photographic image. Sheets of the invention containing contour-dependent, directional images may be useful as a counterfeit-resistant means of authenticating an embossed card, such as a credit card, and providing a readily apparent indication that the embossing has not been tampered with. A sheet containing a contour-dependent, directional image may also provide a means to authenticate a security pass. An exemplary embodiment is a security pass in the form of a card bearing a picture of the card-holder and pertinent written information over which is adhered a substantially transparent, preferably retroreflective, microlens sheeting which does not interfere with the legibility of the underlying matter. Means of authenticating the card is provided by arranging the card, with adhered microlens sheeting, in a selected contour and forming a contour-dependent, directional image therein. The card may thereafter be authenticated by arranging it in the selected contour and verifying that the image is intact. Greater security may be achieved by using a contour-defining template and mechanical reading device to read the directional image. When such devices are used, contour-dependent, directional images which have very narrow conical fields of view and are therefore more sensitive to changes in contour may be selected. Sheets containing more than one set of axial markings to provide more than one contour-dependent, directional image may be formed. When two images are formed at the same contour, it will usually be preferable to select angles of incidence that are sufficiently different that the respective conical fields of view of the two images do not overlap or interfere with each other. Alternatively, an additional set of axial markings may compose a contour-dependent, directional image which is viewable only when the sheet is arranged in a contour different from the contour wherein the first image is visible. A sheet containing a contour-dependent, directional image may be used as a sign wherein the message is rendered visible or not as desired by changing the shape of the sign. For such an embodiment, it is typically convenient to adhere a microlens sheeting to a contour-defining means, such as a metal or plastic panel, which while being flexible, will assume a contour in which the contour-dependent image is visible and at least one other contour. For example, a metal panel which flexes from a concave to a convex contour is suitable. The sheeting may be adhesively bonded to the metal panel, and one or more sets of axial markings formed therein, each set composing a contour-dependent, directional image viewable in a contour which the panel assumes. Another suitable metal panel may be normally flat, at which contour one contour-dependent, directional image is formed; while one or more other contour-dependent, directional images are formed at one or more cylindrical contours. Several different embodiments of microlens sheeting may be used to impart different advantages to the practice of this invention. For instance, as disclosed in the aforementioned Hockert et al. application, certain axial markings may be impregnated with a colored coating composition to provide a colored image. Such sheeting could be used to provide a sign bearing two or more contour-dependent, directional images wherein the images have different colors thereby rendering the images more distinguishable in the same fashion that traffic lights are color coded. Sheets bearing contour-dependent, directional images may be used to determine if an article is in a particular contour, or may be arranged in predetermined contour to render a particular image viewable. Other applications wherein sheets bearing contour-dependent, directional images may be useful will be obvious to those skilled in the art. Description Of Drawings FIGURE l shows a cross section of a container having a microlens sheeting adhered across the mouth thereof as a vacuum seal in which a contour-dependent, directional image is formed to provide a tamper-evident means of verifying that the seal has not been opened. FIGURE 2 shows a cross section of the container and seal shown in FIGURE l after the seal has been breached causing disruption of its contour-dependent, directional image, thereby providing a readily apparent indication of tampering. FIGURE 3 shows a cross section of a container and its screw-on cap onto which a microlens sheeting has been adhered to overlap the cap and neck of the container, the sheeting having a contour-dependent, directional image formed therein to provide a tamper-evident means of verifying that the cap has not been tampered with. FIGURE 4 shows a cross section of a metal panel to which a microlens sheet has been adhered to provide a sign with a changeable message. FIGURE 5 shows a cross section of an embossed card having a microlens sheeting adhered to the surface thereof so as to conform to the surface of the card in an embossed area; a contour-dependent, directional image has been formed in the microlens sheeting to provide a tamper-evident means of authenticating the card and the embossing pattern it bears. FIGURE 6 shows the cross section of the embossed card shown in FIGURE 5 wherein the pattern of embossing has been altered, causing a disruption of the contour-dependent, directional image thereby providing a readily apparent indication that the embossing has been tampered with. FIGURE l shows a cross section of a container l0 with a vacuum seal l2 which comprises a microlens sheeting l4 and underlying adhesive layer l6. The vacuum in the container has caused the seal to assume a concave contour. A contour-dependent, directional image may be formed in the microlens sheeting by irradiating the sheeting with a beam l8 of highly collimated light. The angle of incidence shown here is essentially normal to the plane of the top of the container. The beam is directed through a mask 20 so the sheeting is irradiated in an imagewise fashion to form an imagewise set of axial markings. Lines A, B, C, D, and E represent portions of the irradiation which is transmitted through the mask and can also represent the central axes of the conical fields of view of the axial markings formed by those portions of irradiation. When the mask is removed, the set of axial markings formed is viewable as an intact image from within a conical field of view centered on the angle of incidence of the irradiation if the sheet is in substantially the contour it was in when the axial markings were formed. When the image is intact, all portions of the image are simultaneously viewable within said conical field of view. A cap may be screwed onto the container, secured by the threads 22 shown on the side of the neck of the container. A cap is useful to protect the seal from accidental damage. As shown in FIGURE 2, if the seal is tampered with or opened, the vacuum will be lost so that the seal will assume a contour 30 different from its original one 32. The different contour alters the spatial orientation between different parts of the sheet, thereby disrupting the image contained therein. The image is no longer intact, as shown by the nonparallel arrangement of lines A', B', C', D', and E', each of which indicates the axis of the conical field of view at which a portion of the image can be viewed. The conical fields of view of the different portions of the image no longer overlap, therefore all parts of the image are not simultaneously visible from any perspective, providing a readily apparent indication that the seal has been tampered with. FIGURE 3 shows a microlens sheeting 40 adhered with an adhesive 42 to overlap the cap 44 and neck 46 of a container to provide a tamper-evident means of sealing the container according to the invention. The microlens sheeting has been irradiated to form a contour-dependent, directional image. Lines F', G', H', I', J', and K' represent the central axes of the conical fields of view of portions of the image. If the sheet is arranged in substantially the same contour it was in when the set of axial markings was formed, the image will be intact, represented by the parallel arrangement of lines F', G', H', I', J', and K'. FIGURE 4 shows a microlens sheeting 50 having an adhesive layer 52 on the back side adhered to a metal panel 54 which is capable of assuming a concave contour 56 and a convex contour 58. A set of axial markings which is viewable as an intact image when the metal panel is in its concave contour is disrupted when the panel assumes the convex contour. FIGURE 5 shows a cross section of an embossed card 60 to the surface of which a microlens sheeting 62 comprising a monolayer of microspheres 64, a binder layer 66, and a layer of adhesive 68 was adhered before embossing. The card and adhered microlens sheeting have been embossed with desired characters 70, 72, such as with the account numbers on a credit card, and the sheeting has assumed a nonplanar contour. The microlens sheeting has been stretched in the embossed areas to conform to the new contour of the surface of the card. A tamper-evident means of authenticating the card and the embossing pattern it bears is provided by irradiating the sheeting to form a set of axial markings therein which is viewable as a contour-dependent, directional image. The angle of incidence, which defines the central axis of the conical field of view, is shown here as essentially normal to the card. Any other useful angle at which the sheeting could have been imaged might have been chosen. Each of the lines represents the central axis of the conical field of view of an axial marking. When the sheet is arranged in the contour it was in when the contour-dependent, directional image was formed, the image will be intact, represented by the parallel arrangement of the lines. In FIGURE 6, the embossing pattern shown in FIGURE 5 has been altered by removing some characters 70, 72 and adding another 74. Due to the change of the contour of the card, the spatial arrangement between the individual axial markings has been altered, thereby disrupting the image, as represented by the nonparallel arrangement of the lines. The image is no longer intact due to disruptions caused by both the removal of original, authentic embossing, and the addition of new, counterfeit characters, thereby providing a readily apparent indication of tampering, an object of this invention. These FIGURES, which are not to scale, are representations of only some of the embodiments in which sheets containing contour-dependent, directional images may be useful. They are intended to be only nonlimiting aids to the written description of this invention. The utility of this invention will be further explained by the follwoing nonlimiting examples. Example l This example discloses one embodiment of the invention which provides a secure, tamper evident means of indicating that a container has been opened or that an attempt to open it has been made. A piece of "SCOTCHLITE" Brand High Gain Reflective Sheeting No. 76l0, available from the Minnesota Mining and Manufacturing Company of St. Paul, Minnesota, was placed microlens side down on a flat surface. The sheeting comprises a monolayer of closely spaced transparent glass microspheres, substantially all of which have diameters of approximately 50-70 microns and a refractive index of approximately l.9. The microspheres have a chemically deposited reflective layer on the rear surface and are partially embedded in a polymeric binder containing a black pigment. The back side of the sheeting is coated with a pressure sensitive adhesive which is protected by a release liner. The release liner was removed to expose the pressure sensitive adhesive. A glass l00 milliliter bottle having a mouth approximately 4.5 centimeters in diameter was heated briefly to warm the bottle and the air contained therein. The mouth of the bottle was then pressed against the exposed adhesive of the 76l0 sheeting, thereby causing the sheeting to adhere to the bottle and form a seal. The sealed bottle was then allowed to cool to room temperature. As the air in the bottle cooled and contracted, the sheeting was pushed into the bottle mouth, assuming the familiar concave shape of a vacuum seal. The sheet of 76l0 sheeting was trimmed to remove the excess portions which extended beyond the mouth of the bottle. A flat mask bearing a repeating legend as open areas in a reflective aluminum vapor coat on a transparent film was placed across the mouth of the bottle and seal. The mask was prepared according to U.S. Pat. No. 4,3l4,022 (Fisch). The sheeting was then irradiated through the mask with the beam from a Model 5l2 Q Neodymium:Yttrium Aluminum Garnet (Nd:YAG) Q-switched laser available from the Control Laser Corporation of Orlando, Florida. The wavelength of the beam emitted by the Model 5l2 Q laser was approximately l.064 micrometers. The laser was adjusted to emit a beam with an average power of approximately l0 watts, a pulse rate of approximately 3.4 kilohertz, a pulse duration of approximately 200 nanoseconds, and a diameter at the surface of the mask of approximately two millimeters. The beam was scanned across the mask and bottle seal at an angle of incidence to the mask of approximately 0°, i.e., essentially normal, at a speed of approximately three meters per second, by scanning the beam across the mask in one direction then moving it approximately one millimeter in a transverse direction before again scanning across the mask. When the entire mask had been irradiated in this fashion, the mask was removed. The repeating legend was visible as a dark image within a conical field of view which was approximately 20° wide and centered on the angle of incidence of the laser irradiation. The repeating legend was visible over the entire surface of the seal from any angle within this conical field of view. The image was visible in both ordinary diffused light and under retroreflective light conditions. In accordance with this invention, the image is intact only if the seal is in substantially this original contour and is viewed within the conical field of view. If the contour of the seal is changed, such as if the seal is removed then reattached, the image will no longer be intact. A second bottle seal was prepared and an image formed therein according to the method described above. Similar results were achieved. To simulate tampering, a small section of the seal on the second bottle was lifted from the rim of the bottle, thereby breaking the vacuum seal and allowing air to enter the bottle. Because of the loss of the vacuum the seal was no longer in the same concave contour, but assumed a less concave, nearly flat contour with some undulations. The repeating legend was no longer visible over the entire surface of the seal from any one viewing angle. From within the original conical field of view, the legend was visible only in a region approximately two centimeters in diameter located in the center of the seal. When viewed at other angles slightly off normal, the image on different portions of seal was visible. Example 2 This example discloses another embodiment of this invention used to provide a secure, tamper-evident means of indicating that a container has been opened or tampered with. A piece of 76l0 sheeting like that used in Example l was cut into a strip approximately one centimeter wide and 20 centimeters long. The protective liner was removed from the back of the strip to expose the underlying adhesive. The strip was wrapped tightly around a screw-top bottle, to which the cap had been previously screwed on, overlapping the edge of the cap and adjacent bottle neck. The hand pressure and tension applied caused the strip of sheeting to stretch and conform to the contour of the underlying cap and bottle neck. A flat mask, bearing the legend "3M", was made similarly as that used in Example l. The mask was placed tangent to the strip of microlens sheeting wrapped around the bottle top. The mask and strip of sheeting were irradiated with a laser beam as described in Example l to form a set of axial markings which was visible as an image "3M". The image was visible within a conical field approximately 20° wide, the central axis of which was the angle of incidence of the irradiation which formed the axial markings. The bottle was rotated about its axis approximately 45° and the mask moved approximately 45° in the opposite direction and placed similarly as for the first scanning sequence. This region was irradiated in the same fashion as the first region. This procedure was then repeated until a series of images had been formed around the seal. Each image had an appearance similar to the first one formed. A second bottle was prepared in the same manner as the first. Similar results were achieved. The strip of microlens sheeting on the second bottle was removed to simulate tampering. The strip was very difficult to remove and tore easily despite the attempt to remove it without damaging it. One portion of the strip was successfully removed and reattached to the bottle. The legend "3M" was disrupted because the contour which it assumed was substantially different from the original contour of that portion when the image was formed, providing a readily apparent indication that the bottle had been tampered with. Example 3 This example discloses an embodiment of this invention used to provide a secure, tamper-evident means of indicating that the embossing on an embossed card has been tampered with. A piece of No. 580-10 WAFT reflective sheeting, available from the Minnesota Mining and Manufacturing Company of St. Paul, Minnesota was adhered to the surface of a rigid sheet of polyvinyl chloride (PVC). The No. 580-10 WAFT sheeting is a wide angle flat top sheeting which comprises a monolayer of closely spaced transparent glass microspheres, substantially all of which have diameters of approximately 50 to 75 micrometers and a refractive index of approximately 2.26. The microspheres are partially embedded in a transparent polymeric space coat. The front of the microspheres is embedded in a polymeric top coat which has a flat surface and is approximately 0.0635 mm (2.5 mils) thick. The space coat conforms to the surface of the microspheres and there is a reflective aluminum vapor coat on the surface thereof which is disposed at approximately the focal length from the microspheres. On the opposite side of the aluminum vapor coat there is a layer of a carboxylic-based adhesive upon which is disposed a layer of a rubber-based adhesive which is in turn covered with a paper release liner. The sheet of PVC, which was approximately 0.613 mm (24.5 mils) thick, was obtained from Tenneco Polymers, Incorporated of Houston, Texas, and is similar to the material commonly used for credit cards. The WAFT sheeting was laminated to the sheet of PVC by laying the sheeting on the PVC after removing the paper release liner and compressing them on a platen press for approximately 10 minutes at a temperature of approximately 132°C (270°F) under approximately 0,7 to 1,7 MPa (100 to 250 pounds per square inch) of pressure. Individual blanks measuring approximately 54 by 86 mm (2 l/8" by 3 3/8") were die cut from the laminate and then embossed using a standard manual card embosser. The embossed blank cards were then irradiated with the beam from the same kind of laser as used in Examples l and 2. The entire surface of the card was raster scanned to form a contour-dependent, directional image therein. The resultant image was visible as a uniform pattern when viewed within a conical field of view centered upon the angle of incidence of the irradiation, under either ordinary diffuse light or retroreflective conditions. The imaged cards were immersed in boiling water for a few seconds to soften the PVC. A polyethylene terephthalate film was placed on each side of the cards which were then pressed by hand between two metal plates to flatten the embossing. After being allowed to cool, the flattened cards were re-embossed in the regions where previously embossed with different alphanumeric data. When viewed under either ordinary diffuse light or retroreflective conditions, the previously uniform directional image was disrupted in both areas where the original embossing had been flattened and where the new embossing had been added. The contour-dependent, directional image thus provided a readily apparent indication that the embossing on the card had been altered.

Claims: A sheet (14) bearing a directional image which is viewable when looking within a predetermined conical field of view directed towards one face of said sheet (14) hereinafter referred to as the front face of said sheet, said sheet comprising: (A) a monolayer of closely spaced transparent microlenses (64); (B) means (66) for securing the microlenses (64) to said front face of the sheet (14); (C) a set of axial markings (A-F), namely markings whose center points are respectively located on axes that extend through the optical center of individual microlenses and intersect at a common viewing point or line or are parallel to one another; (D) each axial marking being located at the rear of a microlens, that is to say the side of said microlens opposite said front face of said sheet characterized in that (E) each axial marking is located at the rear of a microlens at a position where the set of said axial markings (A-E) is visible as an intact image only when said sheet (14) is in substantially a predetermined nonplanar contour and is viewed within said predetermined conical field of view. A sheet as defined in Claim 1 further characterized in that said microlenses (64) are microspheres and have a refractive index between about 1.5 and 2.7. A sheet as defined in Claim 1 or 2 further characterized in that said microlenses (64) are glass. A sheet as defined in any one of Claims 1 - 3 further characterized in that said sheet (14) is retroreflective. A sheet as defined in any one of claims 1-4 further characterized in that said sheet (14) is substantially transparent. A sheet as defined in any one of Claims 1-5 further characterized in that said sheet (14) comprises a second set of axial markings, each disposed at the rear of a microlens such that said second set of axial markings is visible as an intact image only when said sheet is viewed within a second predetermined conical field of view. A sheet as defined in any one of claims l-6 further characterized in that said sheet (14) comprises an additional set of axial markings, each disposed at the rear of a microlens such that said additional set of axial markings is visible as an intact image oniy when said sheet is in substantially a predetermined contour different from said predetermined nonplanar contour. A sheet as defined in claim 7 further characterized in that said additional set of axial markings is visible within said predetermined conical field of view. A sheet as defined in any one of claims l-8 further characterized in that said sheet (14) is attached across the mouth of a container (10) under a vacuum or positive pressure, said vacuum or positive pressure determining said predetermined nonplanar contour. A sheet as defined in any one of claims l-8 further characterized in that said sheet (62) is adhered as a surface conforming overlay across at least a portion of the embossed face of an embossed card (60), the embossing defining said predetermined nonplanar contour. A sheet as defined in any one of claims l-8 further characterized in that said sheet (40) is adhered to overlap a container and the cap (44) which seals such container so as to conform to the exterior contour of said container and cap, said exterior contour defining said predetermined nonplanar contour. A sheet as defined in any one of claims 1-8 further characterized in that said sheet is adhered to a document, is substantially transparent, and is adhered as an overlay to an information area of the document without interfering appreciably with the legibility of said information. A sheet as defined in any one of claims 1-8 further characterized in that said sheet (50) is attached to a contour-defining means, said contour-defining means being movable between at least two positions in which said sheet assumes differing predetermined contours (56,58), including said predetermined nonplanar contour. A sheet as defined in claim 13 further characterized in that said second contour is nonplanar. A method for making a sheet (14) with an image thereon as defined in any one of Claims 1 to 14 wherein said method comprises: (A) arranging into a nonplanar contour a sheet (14) comprising a monolayer of closely spaced transparent microlenses (64) secured to one face of the sheet (14) hereinafter referred to as the front face of said sheet; (B) directing highly collimated light in an imagewise fashion at the sheet at a selected angle of incidence to said front face to form an axial marking at the rear of each microlens which said light strikes, that is to say the side of said microlens opposite said front face of said sheet, which markings compose a set of axial markings (A-E), namely markings whose center points are respectively located on axes that extend through the optical center of individual microlenses and intersect at a common viewing point or line or are parallel to one another; and at a position where said image is visible as an intact image only when said sheet is in substantially said nonplanar contour and said front of said sheet is viewed within a conical field of view, the central axis of which being the angle of incidence of the irradiation. A method as defined in Claim 15 further characterized in that arranging said sheet into a nonplanar contour in step (A) comprises adhering said sheet across the opening of a container (10). A method as defined in Claim 15 further characterized in that arranging said sheet into a non-planar contour in step (A) comprises adhering said sheet to an article which seals the container. A method as defined in Claim 15 further characterized in that said sheet is adhered to the surface of a card (60) and arranging said sheet into a nonplanar contour in step (A) comprises enbossing said card thereby also embossing said sheet. A method as defined in Claim 15 further characterized in that said highly collimated light is a pulsed laser beam. A method as defined in Claim 19 further characterized in that said pulsed laser beam scans the entire face of said sheet. A method as defined in Claim 15 further characterized in that said method comprises after said step (B): (C) directing said highly collimated light in an imagewise fashion at a second selected angle of incidence to the face of said sheet (14) to form an axial marking at the rear of each microlens (64) which said light strikes, which markings compose a second set of axial markings which is visible as an intact image only when said sheet is in substantially said nonplanar contour and is viewed within a second conical field of view, the central axis of which being said second selected angle of incidence. A method as defined in Claim 15 further characterized in that said method comprises after said step (B): (C ) arranging said sheet (14) into a different contour; and (D ) directing said highly collimated light in an imagewise fashion at said selected angle of incidence to the face of the sheet to form an axial marking at the rear of each microlens (64) which said light strikes, which markings compose an additional set of axial markings which is visible as an intact image only when said sheet is in substantially said different contour. A method as defined in Claim 15 further characterized in that said method comprises, after said step (B): (C ) arranging said sheet (14) into a different contour; and (D ) directing said highly collimated light in an imagewise fashion at a different selected angle of incidence to the face of said sheet to form an axial marking at the rear of each microlens (64) which said light strikes, which markings compose another set of axial markings which is visible as an intact image only when said sheet is in substantially said different contour and is viewed within a different conical field of view, the central axis of which being said different selected angle of incidence.

Inventors: BRADSHAW, THOMAS I. C/O MINNESOTA MINING & MANUF., COOK, JACK E. C/O MINNESOTA MINING & MANUF., ORENSTEEN, BRUCE D. C/O MINNESOTA MINING & MANUF., Bradshaw, Thomas I. c/o Minnesota Mining &amp; Manuf., Cook, Jack E. c/o Minnesota Mining &amp; Manuf., Orensteen, Bruce D. c/o Minnesota Mining &amp; Manuf.
Applicants: MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1986
Main classification: G02B5
IPC codes: B32B7, G02B5, G02B3, B42D15, B32B3, B65D53