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Diffractive Optically Variable Image Device (DOVID)

A diffractive optically variable image device (DOVID) is an optical security feature exhibiting visual effects based on the principle of diffraction. DOVIDs are incorporated in government-issued documents of value (banknotes, passports, visas, identification credentials and driver licenses) to increase their counterfeit resistance.[1], [2], [3] Brand protection is another application of DOVIDs. DOVIDs contain micro- or nanostructures in the form of diffractive gratings. Due to these structures, they exhibit optically variable effects such as dynamic chromatic, holographic, and kinematic effects, two- or three-dimensional images or color-changing effects, which ideally are easily recognized but are difficult to reproduce. Well known examples of DOVIDs are (i) holograms such as 2D or 3D or 2D/3D holograms based on mask illumination, dot matrix or e-beam origination technology and (ii) KINEGRAMs.[4], [5], [6]

A DOVID can also contain elements which are invisible to the unaided human eye such as microprint, kinetic microtext, or a variable laser-readable micro-image that is invisible when magnified under white light. DOVID structures can be incorporated in a foil and hot stamped on a paper document (e.g., banknote, visa), incorporated within the structure of a polycarbonate or composite card (e.g., passport, driver licenses) before personalization and integrated in a laminate that is heat sealed to the surface of a card after personalization (e.g., driver licenses, identity cards).[7]

1.      Composition of DOVIDs

In most cases DOVIDs are produced as a foil element in roll-to-roll mass production. Exceptions are DOVIDs which are directly embossed into a product. One example is the Kinebar, where a DOVID is embossed into the surface of a gold bar.[8]

Typically, DOVIDs are produced as a multi-layer foil system which is prepared on a polyester carrier either as a laminating foil or a transfer foil. In the transfer foil the polyester carrier is removed after transfer of the layers comprising the DOVID on the target substrate, whereas in the laminating foil the polyester carrier remains on the target substrate. The build-up of the foil typically comprises a separation layer for releasing the polyester carrier during transfer.

Other layers of the build-up are protective layer, replicate layer, reflection layer (metal or transparent high-refractive index material) and adhesive layer. Except for the reflection layer all other layers are typically made of polymeric material. The optically variable effect generating structures are replicated as a microrelief in a roll-to-roll step in the replicate layer, either by hot- or by UV-replication.

Built-up of a transfer foil. The reflection layer is needed due to the fact that the replicated structures are embedded in polymeric material with nearly identical optical properties. In most cases aluminum or a high-refractive index material (HRI) such as TiO2 or ZnS is used for this layer.

2.      Specific types of DOVIDs

DOVID types can be categorized as follows:

a.     Transparent

This DOVID feature consists only of dielectric material, typically including a high-refractive index material (HRI), and therefore is highly transparent. An underlying image or print may be seen clearly. Depending on the view angle the DOVID reveals an optically variable image. Transparent DOVIDs can be applied in patch form or as a laminate covering the entire surface of the substrate, e.g. the paper-based data page of a passport. This type of DOVID is not well suited for protecting banknotes due to the moisture sensitivity of the HRI material, but they are widely used for securing some government-issued identity documents.

b.     Metallized

This DOVID is opaque, exhibiting a fully metallized (mostly aluminum coated) diffractive image with optically variable effects. Fully metallized DOVIDs are typically applied in patch or stripe form. They are, for example, used for protecting paper-based documents, such as birth certificates or land titles. In the early days this kind of DOVID was also used on banknotes.

c.      Partially metallized

This DOVID contains a combination of opacity and transparency on the same surface. The originally fully metallized image is selectively de-metallized to emphasize specific texts or other sub-structures, and to create patterns or specific shapes by the metallization. For over thirty years, this type of DOVID has been used to protect banknotes and applied in patch or stripe form.

d.     Hybrid or Combi

This DOVID contains two or more interrelated visual areas, such as partially metallized and transparent, but revealing continuous visual effects for authentication. The combination of both (partially) metallized and transparent high-refractive index coatings in one feature is more complex and requires more sophisticated production techniques thus making it more difficult to counterfeit.

e.     Precision Metallized

A precision metallized DOVID is comprised of fine-line diffractive images which are in perfect register to the metallization. In contrast to conventional partial metallization, the tolerances approach zero. Typical designs include metallized Guilloche patterns with clearly defined movements and fully transparent intermediate areas (i.e., with no diffractive structures) revealing the underlying document or banknote security print. The precise, zero-tolerance registration between the diffractive images and the metallization enables design possibilities not achievable with normal partial metallization making it harder for counterfeiters to mimic the DOVID appearance.

3.      Incorporation of DOVIDs in documents

DOVIDs are applied either during the manufacturing process of the documents or at the document issuing office.

a.      For banknotes, DOVIDS are applied to the surface (as a patch or stripe) or incorporated as a windowed security thread. In most cases, some form of security print is applied over portions of the DOVID to integrate it into the document (layered security) and make counterfeiting more difficult.

b.      For paper-based identity documents such as visas, DOVIDS are incorporated in the same manner as for banknotes. For some documents, a security print is applied over portions of the DOVID to integrate it into the document (layered security).

c.       For paper-based identity documents, such as passports, DOVIDs are applied as a full-face laminate after personalization of the data page.

d.      For plastic-based (e.g., polycarbonate, Teslin, and composites) identity documents (identity cards, driver licenses and passport data pages), DOVIDS are often embedded within the card body during card layer collation and lamination process. This adds durability and increases security while enabling complex interaction with security features on other layers of the card (layered security). In rare cases DOVIDs are applied on the plastic surface like it is done for paper-based documents.

4.      References

1.      Wikipedia – Optically Variable Device: https://en.wikipedia.org/wiki/Optical_variable_device

2.      DSA: www.documentsecurityalliance.org

3.      Invariant characterization of DOVID security features using a photometric descriptor: https://ieeexplore.ieee.org/document/7351439

4.      Council of the European Union, Reference within the Public Register of Authentic Travel and Identity Documents Online: https://www.consilium.europa.eu/prado/en/LTU-HD-02001/index.html

5.      R. L. van Renesse, Optical Document Security, Third Edition, Artech House, Boston, London, 2005

6.      SPIE - Diffractive optical variable image devices generated by maskless interferometric lithography for optical security: https://www.spiedigitallibrary.org/conference-proceedings-of-spie/8001/800121/Diffractive-optical-variable-image-devices-generated-by-maskless-interferometric-lithography/10.1117/12.890750.short?SSO=1

7.      Council of the European Union, Glossary of the Public Register of Authentic Travel and Identity Documents Online: https://www.consilium.europa.eu/prado/en/prado-glossary/prado-glossary.pdf#nameddest=115

8.     Wikipedia – Kinebar: https://en.wikipedia.org/wiki/Kinebar

9.     Internal Hologram Manufacturers Association (IMHA) – Diffractive Features on Banknotes Special Report: https://ihma.org/diffractive-features-on-banknotes-special-report/

[1] Wikipedia - Optically Variable Device: https://en.wikipedia.org/wiki/Optical_variable_device

[2] DSA: www.documentsecurityalliance.org

[3] IEEE - Invariant characterization of DOVID security features using a photometric descriptor: https://ieeexplore.ieee.org/document/7351439

[4] Council of the European Union – PRADO – Public Register of Authentic Identity and Travel Documents: https://www.consilium.europa.eu/prado/en/CAN-HO-02002/index.html

[5] R. L. van Renesse, Optical Document Security, Third Edition, Artech House, Boston, London, 2005

[6] SPIE - Diffractive optical variable image devices generated by maskless interferometric lithography for optical security: https://www.spiedigitallibrary.org/conference-proceedings-of-spie/8001/800121/Diffractive-optical-variable-image-devices-generated-by-maskless-interferometric-lithography/10.1117/12.890750.short?SSO=1

[7] Council of the European Union – PRADO – Public Register of Authentic Identity and Travel Documents: https://www.consilium.europa.eu/prado/en/prado-glossary/prado-glossary.pdf#nameddest=115

[8] Wikipedia – Kinebar: https://en.wikipedia.org/wiki/Kinebar

[9] Internal Hologram Manufacturers Association (IMHA) – Diffractive Features on Banknotes Special Report: https://ihma.org/diffractive-features-on-banknotes-special-report/