Gone are the days when fingerprint scanners were limited to only the top-of-the-line smartphones. These days, phones priced as low as $100, come equipped with a fingerprint scanner to authenticate users on the device. Moreover, it’s 2019, and we have not just one, but instead, a few different types of fingerprint scanners around. From the old, optical-based fingerprint scanners prevalent during the inception of biometric authentication – to the current relevant capacitive scanners – to the newest of the lot, the ultrasonic scanners. Although currently, the most commonly used type of fingerprint scanner has to be the capacitive scanner, we are starting to see some smartphone manufacturers adopting the all-new ultrasonic fingerprint scanner on their smartphones.
To help you understand the aforementioned types of fingerprint scanners and the differences between each of them here’s everything that you need to know about these fingerprint scanners.
Fingerprints are essentially ridges and lines present at the end of the fingertips. They provide users with a grip between the fingers and the object by creating friction, to help them grip objects better and prevent it from dropping off of their hand. Every individual has a unique fingerprint, and it is highly unlikely for two people to possess the same fingerprint pattern. This uniqueness is what makes fingerprints the preferred choice for biometric security on smartphones.
To be able to use fingerprints to authenticate yourself on the device, the entire process involves two steps:
Enrolment – It is the initial step in the process and involves users registering their fingerprint of the preferred finger, which would be used in the future to authenticate them on the device. The process essentially includes scanning, analyzing, and storing the fingerprints in coded form on a secure database, for future reference.
Verification – Once a fingerprint is registered and saved, the same will be used in the future to verify and authenticate users on the device every time they try to gain access. For this, a scanner first scans the fingerprint, saves all its detailed related data, and then verifies it with the ones present on its database. Next, depending on the outcome, it either allows or denies the device’s access to the user.
Types of Fingerprint Scanners
1. Optical Fingerprint Scanners
As the name suggests, an optical scanner involves the use of optics (light) to capture and scan fingerprints on a device. Essentially, the scanner works by capturing a digital photograph of the fingerprint and then using algorithms to find unique patterns of lines and ridges, spread across the different lighter and darker areas of the image. This digital photograph is a 2D depiction of the different patterns of ridges and lines present on the finger, and since it comprises of details in the darker sections of the image as well, the same is lit-up using a light source, typically an LED, to capture a detailed image. The quality of the image sensor plays a crucial role in getting a high-definition and detailed image of the fingerprint, which would make it easier to extract more data from the image, increasing security.
Optical Scanners have been prevalent in the early days of fingerprint authentication technology, and are nowadays, rarely used on most smartphones, except for a few budget-friendly ones. The possible reasons for a fall in the adoption of optical scanners seem to be – the bulky circuitry design that makes it difficult for manufacturers to fit it into a smaller form factor design, or the low-level of security offered with the 2D picture of the fingerprint, which can be easily tricked using prosthetics or high-resolution pictures.
2. Capacitive Fingerprint Scanners
Going by the name, one can get an idea of the involvement of capacitors in Capacitive Scanners. For those unaware, a capacitor is an electronic component that stores electrical energy in an electric field. In case you’re wondering about its role in Capacitive Scanners, it’s important to first understand that unlike optical scanners, which capture a 2D image of the fingerprint, capacitive scanners capture different details of the fingerprint using just the electrical signals. For this, it uses a series of tiny capacitors circuits, arranged in an array, to store data of the captured fingerprints. During the process of enrolment, the change in fingerprint patterns (ridges and lines) causes a change in the registration process, as the charge would be different for a finger placed over the capacitive plate and different for the air gap between the ridges and lines. This change, in the charge of the capacitor, is further determined using an op-amp (Operational Amplifier), and then recorded with the help of an ADC (Analogue-to-Digital Converter).
Once a fingerprint is captured, all its related data is further analyzed for unique fingerprint information and then saved for comparison at the time of user authentication, in the future. Since no 2D image capturing is involved in this process, the fingerprint data is far more secure than the data from an Optical Scanner. And therefore, a Capacitive Scanner cannot be easily tricked with the help of prosthetics or photographs of the fingerprint, which is why these scanners are more popular and widely used across a wide range of smartphones currently in the market.
3. Ultrasonic Fingerprint Scanners
It is the newest fingerprint scanning technology, that has started to appear in smartphones, recently. Unlike the other two types of fingerprint scanners, which involve the use of light or capacitor, an ultrasonic scanner, on the other hand, utilizes a very high-frequency ultrasonic sound. Additionally, it also requires the use of a combination of an ultrasonic transmitter and an ultrasonic receiver. The process involves the use of an ultrasonic pulse, which is sent through the ultrasonic transmitter towards the finger resting on the scanner. As soon as this pulse strikes the finger, some part of it is transmitted, while some part is reflected back. This reflected pulse is then picked up by an ultrasonic receiver, which depending upon the intensity of the pulse, captures a 3D depiction of the fingerprint. This change in the intensity of pulse is caused due to the texture of the finger, which constitutes ridges and lines.
To be able to pick the change in intensity of the reflected ultrasonic pulse, the ultrasonic receiver takes into consideration the mechanical stress of the fingerprint on the scanner. The longer a finger stays on the scanner, the more details it can capture, and produce a detailed 3D depiction of the fingerprint. However, a drawback of this type of fingerprint scanning method is that it is not as fast as the other methods of fingerprint scanning. On the other hand, the scanner does a great job of allowing manufacturers to get rid of or minimize the bezels around the screen, which is only possible because the scanner can be easily implemented under the display.
In spite of the different types of fingerprint scanning technology used by the optical, capacitive, or ultrasonic scanners, the basic idea behind using a fingerprint scanner is the same – to ensure a high level of security on the device, without requiring users to remember each and every username or password combination. To make things more secure, the basic hardware and software present on all the three different types of fingerprint scanners is also accompanied by a few other hardware components, along with software. For instance, a dedicated IC is used along with the scanners to store the data about the fingerprints and transmitting it further to other processing components. The information is kept securely on the device to prevent any kind of interference from other processes and also preventing other components of the device from accessing such important data.
In case of an Android smartphone, this task is taken care of by an ARM processor, which securely keeps the fingerprint data in a secure chip, that it calls, Trusted Execution Environment (TEE), whereas, on an Apple device, the same is securely saved on to a similar arrangement, that Apple calls, Secure Enclave. Despite the change of operating systems and their internal services, the basic idea is the same, securing the fingerprint data and keeping it in a form that is not easily accessible by other components on the device, to enhance the security.