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. They then began an in-depth discussion of the science behind optical features. Here, they continue to explore the science, specifically lensing, fluorescence, phosphorescence, selective absorption, and self-assembly.
The Science Behind Optical Features
Lensing: While lens-based features are a sub-category of reflection/refraction devices, they merit a category of their own by virtue of being inherently suitable to the multi-layer construction of a Card. A typical lens-based optical feature functions as shown in FIG. 3 below.
FIG. 3: Schematic of MLI lens-based feature function. Images 1 and 2 are preferentially visible at different angles of view due to the lenticular surface structure.
For an array of cylindrical lenses as shown in FIG. 3, the number of different images which can be displayed – while theoretically limited only by the resolution at which the image data can be printed – is in practice not more than three different images. By nature of their shape, an array of spherical lenses can display more images. This opens the possibility of displaying quite striking animations upon manipulation of the card substrate, animations which furthermore appear to “float” on a different plane compared to the rest of the print on the document. Straightforward display of variable (personal) data within an optically variable device is the principal authentication advantage of lens-based optical features.
The most common lens-based feature on ID documents is the Multiple or Changeable Laser Image (MLI/CLI) where laser-engraving is used at two or more different angles to produce two or more different images visible at the same number of different angles of view. Lens arrays can be used as a fine “key” within a Moiré-type image system to yield strikingly fast and high-resolution animations [6].
Fluorescence and phosphorescence: While classed as Level 2 features, intended for verification with a simple tool – in this case a UV light – UV fluorescent print is a requirement for convergence with internationally recognized standards such as ICAO 9303. As such, all modern ID documents include UV fluorescent print.
Fluorescence is a short-lived emission of light which ceases immediately after the excitation is stopped. Fluorescent security inks emit different colors when excited by energy of a different (usually lower) wavelength. The vast majority of fluorescent inks used in the wider Security Print industry are excited by ultraviolet radiation (wavelength 254 – 365 nm). Visible fluorescent inks show one color under normal light and another color under UV light, whereas Invisible fluorescent inks are visible only under UV lighting.
Some invisible fluorescent inks can emit different colors under different excitation wavelengths. Typically, Bi-fluorescent inks will display one colour under 365 nm UV and another when exposed to 254 nm UV.
Full color fluorescent images can be realized by combining inks that are fluorescent in the three additive primary colors.
Finally, phosphorescent inks are not only visible under UV light but also after UV light is interrupted, the ink remains visible in the dark for a certain period depending on formulation and ink deposit. Unlike fluorescent inks which no longer emit once excitation is interrupted, phosphorescent inks will continue to glow for some time.
A word of caution however, UV fluorescent and phosphorescent inks are easily purchased on the open market (see FIG. 4 below).
FIG. 4: Counterfeiter’s paradise, page 1 of 50 when searching for “UV fluorescent ink” on Alibaba.com
Fortunately, there are also other types of fluorescence, which are more difficult to source. Anti stokes inks emit fluorescent radiation of shorter wavelength than of the exciting source. With an exciting source in the infrared (980 nm), you may have an anti-Stokes radiation in red, green, or blue visible wavelengths.
Selective absorption: Selective absorption chemical inks are based on selective absorption depending on the background through a transparent area. The observer sees the intrinsic colour of the ink on a clear background and the secondary colour of the ink on a dark background (see FIG. 5).
FIG 5: View of a selective absorption based ink applied to a clear window against a split black (left) and white(right) background, displaying secondary and intrinsic colour respectively.
In contrast to UV-fluorescence, Selective absorption optical effects can be made discernable entirely within the visible spectrum and are good candidates for Level 1 optical features.
Self-assembly: Self-assembly is a phenomenon by which molecules re-arrange themselves when exposed to an external influence (typically temperature). The arrangements are periodic both in and out of plane and can thus be engineered to exhibit strong structural colours when illuminated. Liquid crystals are an example of self-assembly and form the basis of certain colour-shifting security inks. The principal advantage of self-assembling molecular systems is precisely the possibility of using them to make inks, considerably easing industrial application.
Up Next: Types of optical features
Many types of optical features are used in identity documents, as the authors will explain in the next article. Get ready to learn the technical details of more than a dozen of these sophisticated security features.
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:
6.Steenblik RA et al., US patent 7,333,268 (2008)
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.
