Emerging Fingerprint Technologies for Smartphones

By Dr Alan Hodgson, Consultant in Technologies for

In the November 2022 edition of this publication, an article examined two use cases of fingerprint readers –smartphones and payment cards. The purpose of this article is to take the smartphone use case one step further, to examine emerging technologies for fingerprint readers on this platform. The aim is then to identify the opportunities and threats to existing business that this can bring to personal identity. The November article considered the security / convenience / trust balance from a consumer perspective in existing smartphone implementations. This work looks in a different direction, at technologies where the fingerprint reader is embedded beneath the display screen, and we will examine the implications of this. Finally, we will look at where ‘beneath screen’ technologies could take us into the future.

The issue of sensor size

It was noted in the November article that fingerprint sensors take a sample of the user’s fingerprint and, as a larger sensor gives a larger sample, it therefore provides higher security from two perspectives.

First, when the fingerprint is first taken on the device there is more data to inform authentication without multiple capture to populate the dataset. Second, during authentication the larger sensor captures more data to inform the process.

This is an area where fingerprint enabled payment cards currently have an advantage over smartphones. On a smartphone, ‘real estate’, particularly on the front surface, is at a premium as there are strong marketing drivers to increase the percentage area devoted to the display screen. In contrast to this there is a larger area potentially available to integrate the fingerprint sensor on a payment card.

The response from smartphone technology suppliers

Fingerprint readers are incorporated into smartphones to validate the identity of a smartphone user, mostly using capacitive sensor technology as part of a trusted system within the smartphone to guard against software attack. This type of system has become well embedded in smartphone design and is now implemented across a range of smartphone price points. However, there is still room for innovation in this area.

Secure Documents

One of the problems often cited with fingerprint readers in smartphones is that, although it is trusted technology, it requires a significant area on the smartphone front surface, a point of frustration to the design gurus. However, there are alternatives to this which include moving the sensor to the side or the back of the smartphone. Both of these can create further issues, not least of which is that they constrain the types of case that can then be used. But they have nevertheless found their way into a number of existing smartphone models. Moving the sensor to the side does have some interesting implementation implications. Using current technology, it can be integrated into the surface of the control actuators on the side of the smartphone. This then makes these buttons multifunctional, making better use of these areas and the potential to create a further marketing story, an important consideration in consumer electronic devices. For the needs of personal authentication, a better long-term solution may lie in the use of sophisticated electronics to embed the fingerprint sensor in the display area. Initial implementations already exist and this may well point to the trajectory the technology will take.

Under-display fingerprint readers

It is important to note that we are only at the start of this journey because the technology is demanding. Particularly in high end smartphone models, display image quality has become a marketing proposition. As a result of this, there is an imperative that any device hidden under the display must be invisible in normal use of the smartphone.

Initial implementations use a CMOS image sensor mounted under the display to detect and read a fingerprint. These only facilitate a fairly small sensor area (around 8mm x 8mm) so the display needs to indicate the touch area. And as a result, these are still limited to only one fingertip. A more secure solution may lie in systems that will allow the full display screen to become a fingerprint sensor, facilitating multi-finger authentication on the screen. Technology providers such as Isorg in France are developing full screen organic photodiode sensors that can go under smartphone OLED display screens (see IDN June 2023). This may add another layer of fingerprint authentication to smartphone systems while preserving the screen real estate.

While this sounds enticing as a solution to increase consumer identity security, it is important to add a significant caveat. Additional technologies add additional cost, so innovations such as this will percolate down, starting with the premium smartphone models. So rather than consider this as a consumer identity solution, we may be best placed to think of it as an enrolment or inspector technology. This is because this type of innovation has the potential to extend the use of smartphone fingerprint readers further into law enforcement and border control. At the moment, separate devices such as the GridLet iMatch series have to be purchased (and carried) to carry out finger enrolment. This may yet be another area where smartphones will become a disruptive technology.

There are a number of use cases where there is a potential need for large numbers of devices for multiple fingerprint enrolment and verification in the modern world of mass migration. Political, conflict and climate change issues have contributed to the mass movement of people and a resultant need for refugee and asylum seeker biometric systems. An example of this is the European asylum system EURODAC – could this technology find a home in systems such as this?

Other sensorunder-display technology

There are other sensor systems vying for location under the smartphone display that could have implications for identity management, for the same reasons as fingerprint sensors. The front face of some smartphones is also home to facial recognition camera systems and there are moves afoot to relocate these below the display screen too, utilising short wave infra-red (SWIR) technology.

The benefits of SWIR are a topic for another day, but it serves as an indication that innovation in the area of smartphone technology is far from over. The 2023 Consumer Electronics Show featured further optical technologies being implemented for the smartphone platform. Look out for a discussion of some of these at the 2023 Optical & Digital Document Security™ Conference, set for 17-19 April 2023 in Prague, Czech Republic (see page 8). opticaldigitalsecurity.com