A tamper-evident label includes two parts that are separated from each other when an article to which they are attached is opened. The two parts have matching surface relief patterns secured to each other at an interface, and the indexes of refraction of the parts are substantially equal such that the patterns have no optical effect when attached to each other. When the two parts are separated, however, the patterns generate an image drawing attention to their separation. The patterns are preferably holographic.

TECHNICAL FIELD 
This invention relates to the art of tamper-evident labels employing 
elements having surface relief patterns that create visible images, such 
as holographic patterns. In the preferred embodiment, the invention 
relates to a label that is separable into two holographic elements to 
evidence opening of the container or tampering. 
BACKGROUND 
Numerous types of labels are known for indicating whether a container has 
been opened or subjected to tampering. These labels are known as 
tamper-indicating or tamper-evident labels. Each of these labels has a 
feature that is altered when the container is opened to indicate that the 
container has not remained sealed. One such label is simply a strip of 
paper, or other easily torn material, that bridges a joint between a 
container's top and the container's body. When the top is removed, the 
tape is severed. Another tamper-evident label, shown, in U.S. Pat. No. 
3,935,960 (Cornell) uses a layer of material that changes color when it is 
flexed. The layer is placed on the container such that it is necessarily 
flexed when the container is opened to indicate tampering. Also, U.S. Pat. 
Nos. 4,121,003 (Williams) and 4,184701 (Franklin) show labels that 
indicate tampering by providing the label with an adhesive that contains 
information, parts of the adhesive remaining on both the portion of the 
label staying on the article and the portion of the label that is 
detached. 
It is also known to use labels that are difficult to produce, such as those 
having holograms, to authenticate an article. For example, U.S. Pat. No. 
4,501,439 (Antes) discloses a hologram attached to an article for 
authenticating the article, the image generated by the hologram being read 
only by a specially-designed instrument. U.S. Pat. Nos. 4,921,319; 
5,044,707; and 5,085,514 (Mallik) also show authenticating holograms, 
which allow documents such as passports to be viewed through the hologram. 
SUMMARY OF THE INVENTION 
In accordance with the invention, a product particularly useful as a 
tamper-evident label includes elements with surface relief patterns that 
are capable of producing optical images when illuminated with light. The 
label is placed on the container initially such that the relationship 
between the elements precludes generation of the images. This 
relationship, however, is necessarily disturbed when the container is 
opened or the label removed, whereby the images are readily viewed to 
indicate that the container has been opened. 
In the preferred embodiment, the surface relief patterns are holographic, 
but non-holographic patterns are also contemplated. The patterns may be 
generated in a variety of ways, including photographic recordation of 
interference or other patterns and computer generation. A first one of the 
elements is preferably made of a curable resin that is cast and cured, for 
example, by actinic or other radiation, including electron beam 
irradiation and thermal radiation. Such techniques for production of a 
hologram are known in the art. Alternatively, the first element is hard 
embossed, etched or engraved with known photographic or other techniques. 
The second element is preferably formed by applying a material in liquid 
form to the surface relief of the first element, whereby it forms a 
replica of the surface relief pattern on the first element. The materials 
used for the two elements are preferably transparent and have 
substantially the same indexes of refraction, whereby no image is visible 
when the elements are in this initial configuration because there is no 
diffraction at the interface between the two matching patterns. 
The material used for the first element includes a release agent that 
allows the second element to be detached from the first element while 
retaining the surface relief pattern. Thus, when the second element is 
detached from the first element, each of the elements has the same surface 
relief pattern thereon, and each is capable of forming an image. For 
example, if the surface relief pattern is capable of creating an image of 
the word "void," detachment of the second element from the first results 
in separate elements each having "void" easily visible thereon. 
The label may have printing on other of its surfaces. For example, the 
second element may carry on its bottom surface a printed label indicating 
the contents of the container. Preferably, however, the bottom surface 
carries a second hologram. The image generated by the second hologram is 
visible when the first element is attached because the matched indexes of 
refraction of the two elements prevents formation of images by the 
holograms forming the interface between the elements.

DETAILED DESCRIPTION OF THE INVENTION 
With reference to FIG. 1 of the drawings, a tamper-evident label 2 in 
accordance with the invention includes a first element 8 having a 
polyester layer 14 as a carrier substrate. The lower surface 10 of this 
first element is provided with a holographic surface relief pattern 
capable of generating a holographic image, for example, of the word 
"void." Holographic element 8 is preferably formed in a known manner by 
casting a resin with the desired holographic pattern, and curing that 
resin by illumination with actinic radiation. Such a procedure is 
disclosed, for example, in U.S. Pat. No. 5,085,514. The resin used in the 
present invention, however, differs from that generally known by the 
addition of acrylic functional polyester modified dimethyl polysiloxane, 
which acts as a release agent, 
A second element 4 is formed on the surface 10 of element 8 after curing 
such that the upper surface 6 of the element 4 flows into the surface 
relief on surface 10 to form a replica of the pattern on surface 10. The 
material used for the second element is preferably one that pours like a 
liquid but is "100% solids." That is, this material changes from a liquid 
to a solid only by polymerization because it does not contain any volatile 
components. The material used for the element 4 is essentially the same as 
that used for element 8, but does not necessarily contain the acrylic 
functional polyester modified dimethyl polysiloxane. The material forming 
the second element is then cured by illumination with actinic radiation. 
The label 2 is completed by the addition of an adhesive layer 12, for 
attaching the label to a product. A polyester layer 14 is used as the 
carrier substrate and to provide protection of the label. 
Preferred compositions for the two materials are set forth in the following 
table. 
TABLE 1 
______________________________________ 
COMPOSITION OF 
COMPOSITION OF 
ELEMENT 8 ELEMENT 4 
(wt. %) (wt. %) 
______________________________________ 
Multifunctional Acrylate 
Monomer and Oligomer 80 85 
Photoinitiator Blend 12 13 
Surfactant 2 2 
Acrylic Functional Polyester 
Modified Dimethyl 
Polysiloxane 6 
______________________________________ 
In the compositions set forth above, the amount of the acrylic functional 
polyester modified dimethyl polysiloxane may be from 1% to 10% in one or 
both of the elements. The preferred compound is sold under the designation 
"BYK-371" and may be obtained from BYK-Chemie, USA. The first three 
components may be the known commercial product "Radkote 801." 
It is within the contemplation of the invention that other types of curable 
resins could be used, including acrylates, polyesters, epoxies, vinyls, 
and silicones. 
The addition of the acrylic functional polyester modified dimethyl 
polysiloxane allows the second element to be detached from the first 
element while preserving the surface relief patterns on the surfaces 6 and 
10. As illustrated in FIG. 2, the first element 8 and its protective layer 
14 have been pulled from the second element 4. The surface relief patterns 
6 and 10 have been separated but have retained their original forms, 
pattern 6 being a replica of the pattern 10. 
The materials from which elements 4 and 8 are formed have substantially 
equal optical indexes of refraction. Thus, when the elements 4 and 8 are 
in contact as shown in FIG. 1, light passes through the interface without 
deviation, and the patterns, thus, do not generate an image. When the two 
elements are separated, however, as shown in FIG. 2, the surface relief 
patterns are exposed to air, permitting diffraction and the consequent 
formation of images. 
FIG. 3 illustrates a second embodiment of the invention. In accordance with 
this embodiment, the lower surface of the element 4 is provided with a 
second surface relief pattern 16. The pattern 16 is preferably a 
holographic pattern. In addition, a metal layer 18 is applied to the 
pattern 16 whereby the image generated by the pattern is easily visible. 
For example, the image generated by the pattern 16 may contain information 
about the product to which the label is attached and be designed 
artistically to attract attention to the product. 
The second surface relief pattern 16 is applied to the element 4 in 
substantially the same manner as is the pattern 10. 
FIG. 4 illustrates yet another embodiment wherein the lower surface of 
element 4 includes a printed pattern 20, which has been applied with any 
of several known techniques. The upper surface of element 8 contains a 
printed pattern 22, as well. When the first and second elements are 
attached as shown in FIG. 4, the printed patterns 20 and 22 are easily 
viewed. It will be appreciated that the embodiment of FIG. 4 may instead 
have only one of the printed patterns. 
FIG. 5 illustrates the application of a label in accordance with the 
invention to a container. In the illustrated application, the container 24 
is a CD-ROM container commonly known as a "jewel box." The label 2 is 
attached to the container such that it spans the joint 26 between the 
separable parts of the container. In this example, the polyester layer 14 
is rather thick and strong to prevent opening the container without first 
removing that layer. The second layer 4 and the metal layer 18, however, 
are quite thin, e.g., 2 microns, whereby once the layer 14 and first 
element 8 are removed, the parts of the container are easily separated. 
Thus, the container is effectively sealed until the polyester layer is 
removed, which exposes the word "void" on the two elements. 
It will be appreciated that a unique label and method for its manufacture 
have been described. Modifications within the scope of the appended claims 
will be apparent to those of skill in the art.