The present invention relates to storage of data in holographic media. In particular, the present invention relates to storage of holographic data in a card format.
Holography is a familiar technology for displaying three dimensional images. Basically, two coherent light beams are caused to intersect in a holographic medium. An interference pattern or grating pattern results that is unique to the two beams and which is written into the medium. This grating pattern is referred to as the hologram and has the property that if it is illuminated by either of the beams used for recording, the illuminating beam diffracts in the direction of the second writing beam. To an observer, it appears as if the source of the second beam is still present at an observation plane.
However, holograms are becoming more common for use in other types of applications such as security and data storage. In data storage applications, as is well understood by those skilled in the art, a page of data is used as a source image and a detector array is placed at the observation plane. Additionally, due to Bragg effects, many holograms may be multiplexed within the same volume of holographic material by slightly changing the angle of the reference beam with each different data page. Large numbers of holograms can be multiplexed this way in a small volume of recording material, providing high data storage potential. A complete discussion of storage holograms can be found, for example, in John R. Vacca, Holograms and Holography Design, Techniques, and Commercial Applications, Charles River Media, Inc., 2001 (xe2x80x9cVaccaxe2x80x9d). Generally, data stored in holographic media is only machine readable.
With respect to security applications, it is well known to include holograms on credit cards to prevent duplication of these items. A hologram is useful in this context because of the relative difficulty involved in counterfeiting a hologram as compared to printed designs, embossed features and even photographs. However, security holograms used on credit cards are generally embossed only on the surface of the card. As such, while holograms in general are relatively difficult to duplicate, a hologram on the surface of a card can be somewhat easier to duplicate or alter.
One potential solution to the problems associated with relative ease of duplication of surface holograms is offered in U.S. Pat. No. 6,005,691 for xe2x80x9cHigh-Security Machine-Readable Holographic Cardxe2x80x9d to Grot et al. Grot et al. discloses a hologram card which includes a first plastic material formed to include localized topological features constituting a diffractive optical element. The diffractive optical element is structured to generate a hologram image. The hologram card also includes a protective layer which is chemically bonded to and directly contacts the topological features constituting the diffractive optical element. While the hologram card of Grot et al. includes a protective layer to make any hologram included in the diffractive element more difficult to duplicate, the card includes only a surface hologram, which holds a relatively small amount of information. That is, the hologram card disclosed in Grot et al. is relatively inefficient.
Additionally, while credit cards, and drivers licenses and identification cards, can typically store some information in a magnetic stripe often included with such cards, the amount of information such magnetic stripes can store can be relatively low.
An apparatus in accordance with the present invention includes a multi-layer holographic card having a section containing holographic machine readable data as well as security and/or presentation information which may be either machine or human readable and may also be holographic. Such a holographic card can advantageously store a relatively large amount of data in holographic format. Further by providing a protective layer over a holographic layer, a holographic card in accordance with the present invention can advantageously provide increased security for data.
In particular, a holographic card for storing data in holographic format in accordance with the present invention includes a holographic layer for containing holographic information and a transparent protective layer overlaying the holographic layer. A typical card is formed by sandwiching the holographic layer between two protective layers or substrates. The holographic layer includes at least a first data section for containing machine readable holographic data. The card also includes a second data section containing either or both presentation or security data. The second data section is preferably included in the holographic layer, but can also be separate therefrom, particularly when non-holographic data is contained therein.