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.
The authors began this series by reviewing the key concepts of optical features in identity documents. Here, they delve into the science that lies behind such optical features.
The Science Behind Optical Features
All current optical features found on ID documents are based on one or more of the following underlying physical and chemical principles:
- Physics (optics and light manipulation):
- Masking and unmasking of light
- Reflection/refraction
- Diffraction/Interference
- Lensing
- Chemistry:
- Energy absorption and emission (colours)
- Self-assembly
All of these topics are extensively researched from a scientific perspective and this knowledge is widely accessible. In what follows each principle and how it applies to optical features will be (very) briefly described.
Masking and unmasking of light
This consists of using elements (printed or otherwise) on one portion of a document to hide elements adjacent or on a different portion of the same document at certain angles of view and not others. FIG. 1 below shows two examples of how this can be exploited to make optical features.
FIG. 1: Examples of optical features by masking and unmasking of light. (a) Flip image by print on both sides of a transparent substrate. (b) Transient image by single-sided “deep” printing (e.g. intaglio) on an opaque substrate.
Light-masking based features are well-known2, in the wider print industry as a decorative element and, for example, in architecture to decorate facades and sound barriers. The key differentiation when used as a security feature is the resolution, corresponding to the period in FIG. 1. Current light-masking optical features for document authentication can have periods as low as 10 micrometers (equivalent to 2’540 dpi). Features exploiting light-masking include transient images and Moiré-based animations.
Reflection/refraction of light
Most surfaces will reflect light to a certain degree and absorb or – if the material is transparent – transmit the rest. Furthermore, when light travelling in air meets a transparent solid material, the light will be slowed down by the material giving rise to refraction at the interface between air and substrate and again as the light leaves the substrate. The latter gives rise to the phenomenon of total internal reflection (TIR), much exploited in the security feature industry3.
Reflection can be exploited to make optical features by arranging and/or shaping relief elements on a substrate in such a way that different images are displayed at different angles of view. These elements act as (for want of a better term) “micro-mirrors” where each element has one or more surfaces which deflect light at a different angle than that reflected from the substrate areas bearing no relief. This is shown in FIG 2.
FIG. 2 Light reflected off a document substrate is deflected (red path) by the presence of a relief element compared to areas bearing no relief (blue path).
In practice, the presence of multiple transparent layers on a typical ID document means that reflective optically variable image devices (ROVIDs) will always display a combination of reflection and refraction. Both components can be advantageously used to create a wide range of optical features including surface OVDs and so-called “pump” effects.
Interference/diffraction
The physical phenomenon of interference is widely used to make optical features. Simply put, interference is the interaction between the component wavelengths of incident light when reflected by or transmitted through a surface structure in such a way as to cause a phase shift between resulting output light rays from adjacent parts of the structure.
Because each wavelength, λ, composing the incoming light is shifted differently, the result is that interference-causing structures will separate the incoming light into its component wavelengths yielding the typically iridescent (rainbow) aspect of interference-based features. The different component colours are perforce visible at different angles of view.
While many different structures can yield interference, those composed of finely ruled parallel grooves – called gratings – give rise to diffraction, a form of interference. For normal (i.e. 90 degrees to the surface) incident light, a grating of period d will have intensity maxima at angles of view θm according to the relation:
The first order of diffraction (where m = 1) represents the smallest and most easily viewable angle at which colour-separation will occur. Two conclusions of note can be drawn from the above equation; firstly, that for reasonable angles of tilt (i.e.< 45 degrees), the period of the diffracting structures must be similar to the wavelengths which are to be diffracted. For visible light, this means periods between 400 and 750 nm. The second point is that even for much greater structure periods, diffraction still occurs, albeit at acute angles of view.
Beyond colour-shifting, gratings can be used to construct optically variable imagery by painting the different indicia to be displayed with grating elements differentiated by period or spatial orientation4. Light diffraction is extensively used for optical features and forms the basis for 2-D holograms, certain animated “holograms” and liquid-crystal based inks.
Finally, it is important to note that interference can be caused by other structures, for example superposed layers of transparent materials having different indices of refraction5.
Up Next: More about the science behind optical features
In the next article in this series, the authors will continue to explore the science behind optical features. Topics will include lensing, fluorescence, phosphorescence, selective absorption, and self-assembly.
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:
2.Van Renesse R, « Optical Document Security », ISBN 0-89006-619-1, Artech House (1994)
3.Commander LG, Eastell CJ, Isherwood R, Patent application WO2006/095161 (2006)
4.Gregor A, US patent 5,032,003 (1991)
5.Goodling AE et al, Nature, 566, 523-527 (2019)
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
Faten Ben Jemaa, Veridos
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.
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.
