If you’re overwhelmed by the vast number and types of optical features available to add security to identity documents, you’re not alone. In this series, a group of experts from the Secure Identity Alliance (SIA) will clarify the options—and when and how to apply them. This series is based on a paper presented by the SIA at the Optical & Digital Document Security (ODDS) 2024 conference in Lisbon, Portugal.
Sight is our most important sense. The human eye is a complex sensor, backed by the vast processing power of the human brain, accounting for up to 60% of its activity1. Indeed, even for the smartphone-toting primate of the 21st century, seeing is still believing. It is therefore no surprise that optical features play a primordial role in authenticating all manner of things including banknotes, certificates and of course identity documents.
Advances in fabrication technology and, especially, its’ increasingly widespread availability have driven the development of optical features in the past twenty years. Where once holograms and a few specialty inks dominated the optical feature market, now stand a bustle of technologies based on plasmon resonance, liquid crystals, micro-lenses and more; adding to a growing halo of acronyms and confusing terminology.
Starting from the basic principle of Optical Variability, this article aims to cut through the haze and provide an impartial review of the state of the art for informed but non-technical persons involved in ID document specification and design. After a brief overview of the fundamental physical and chemical phenomena which underpin all contemporary optical features, we categorize and describe the different types of optical security elements which can currently be applied to ID documents, whether by print, by overlay, by personalization or during the lamination process. This article focuses exclusively on features that can be authenticated by the naked eye with the occasional assistance of an independent light source (smartphone “white,” UV or IR). In technical terms, our scope is limited to Level 1 and some Level 2 – call them level one-and-a-half – features.
We also address briefly address the subject of “perception,” how “good enough is close enough” as far as the counterfeiter is concerned and how a judicious selection of optical features best protects a document.
Key Concepts
Optical Variability: Since the advent of cheap photographic reproduction (photocopying, digital scanning/printing), the key mechanism for all optical features is a clear and visible change when the viewing angle between document and observer is changed. This mechanism remains pertinent today because the viewing angle of photographic reproduction equipment is fixed and generally limited to visible light.
Furthermore, there are only two types of shift possible:
-
- Colour-shifting: The viewed wavelength of light changes over the feature.
- Image shifting: The graphics displayed by the feature change.
Some features may combine both a colour and an image shift.
Material vs. Relief: Optical variability can be achieved through either:
-
- Material properties which cause the perceived light reflected or transmitted by the material to be different from that of incoming light.
- Forming physical relief elements on a material such that light is reflected or diffracted in a desired way.
Generally, features relying on material properties will display some kind of colour-shifting effect regardless of the angle of incoming light. Relief-based features on the other hand can modify both perceived colour and display shifting images but will always be dependent on the direction of incoming light. To allow authentication under as many conditions as possible, it is judicious to have at least one of each type of feature on a document.
Some features may combine both material properties and physical relief to achieve the desired optical effect.
Perception: Counterfeiters do not need to copy an optical feature exactly; they only need to trick your brain into seeing it. In addition, optical features (if they are checked at all) are only studied for a fraction of a second during inspection. The counterfeiter’s principle copying strategy for an optical feature will therefore be “close enough is good enough”. Good security design accounts for how easy it is to counterfeit-by-substitution, bearing in mind that successful counterfeiters may have access to high quality printing equipment and specialist knowledge – at least with respect to the underlying technical principles behind most optical features.
Perception difficulties in identifying optical features are typically addressed by attempting to reduce the possibility of subjective interpretation through a stronger static contrast or more vivid dynamic movements for image-shifting features and higher brilliance for colour-switches. In this context, when designing a new ID document, the key questions to ask from a perception perspective are:
- Can a sufficiently similar image-switch / animation be achieved with a lower resolution alternative?
- Can a sufficiently similar colour-shift be achieved with a commercially available off-the-shelf alternative?
Finally, no matter how visible and high-tech, a single optical feature will always pose a higher counterfeiting risk than a combination of two or more optical features distributed along the cross section (i.e., surfaces and body) of the document.
Specificity of ID Documents: An ID document differs from other papers of value (e.g., banknotes) in the following ways:
- Lifetime: An ID document will undergo regular use and must last 10 years or more.
- Function: IDs are personal, supporting their principal function which is to prove right of access.
- Form: Passport datapages and IDs are between 5 and 10 times thicker than banknotes.
Beyond the implications for feature selection and placement, ID documents will typically be vested with a higher value in a user’s mind than most other items they habitually carry on their person.
Security Level: Last but not least and for the sake of coherence across the industry, security features for documents are typically categorized as follows:
- Level 1: Authentication by the un-assisted eye only
- Level 2: Authentication by eye with assistance of a simple tool easily available to the general public such as a magnifying glass or a light source (including UV/IR).
- Level 3: Authentication possible only with specialized equipment, not easily available to the general public.
The present review concentrates on Level 1 features only.
Up Next: The science behind optical features
The next article in this series will delve into the science behind optical features—based on underlying physical and chemical principles.
The Secure Identity Alliance (SIA) is an expert and globally recognised not-for-profit organisation. We bring together public, private and non-government organisations to foster international collaboration, help shape policy, provide technical guidance and share best practice in the implementation of identity programmes. Underpinning our work is the belief that unlocking the full power of identity is critical to enable people, economy and society to thrive.
Sources/References:
1.Sheth BR et al. Science vol 274 / 5295 / pp. 2110-15 (1996)
Yit-shun Leung Ki holds a PhD in Microengineering from the Swiss Federal Institute of Technology (EPFL). He has been active in the Security Print Industry for over twenty years and is currently with 4Plate GmbH. As an inventor of optical features, he likes to play with light.
Christophe Halopé has served as Executive Director of Crime Science Technology since 2019. Before, he served as Director of R&D at Arjowiggins Security, overseeing various business units, including Banknote, ID, Brand Protection, and managing the company’s IP. He also served as R&D Process Director at ASK (now Paragon ID). Christophe has a degree in chemical engineering and a master’s degree in industrial processes.
F. Daniel Françoise is an industry-recognized expert in Security Features for documents of value. After more than twenty years in senior positions at Hologram Industries/Surys /IN Group, she is currently the founder and director of PIKit an IP consultancy in the Paris region.
Renaud Laffont-Leenhardt is Senior Product Line Manager in the Identity & Biometric Solution Business Line of Thales, overseeing passport products, including secure embedded software, visual security features and issuance solutions/services. He joined Thales 28 years ago and has held various marketing and business positions in the government programs division for the past 21 years, notably handling business development in Africa and Latin America for secure electronic documents projects.
Serge Wsevolojskoy, Idemia
Robert Dvorak has been working on the development of optical security features since 2008. He has been participating in the implementation of DOVIDs into polymer banknotes and has developed a unique technology for the integration of optical security features into polycarbonate documents. His team is currently working on the development of materials that will enable a major increase in the complexity of optical protection of identity documents as well as banknotes.
Faten Ben Jemaa, Veridos
Maickel van Oijen has worked for 19 years in government positions at the Dutch Immigration Office and the Forgery Department at Schiphol Airport Amsterdam. He joined Keesing Technologies 8 years ago, where he now works as Manager Operations and Senior Document Expert, consulting on fraud cases and helping Keesing’s solutions evolve.
