How fingerprint sensors work?

Abstract

We all use at least one biometric security method, be it fingerprint, face id, voice recognition, etc. but have you ever thought about how the finger sensors behind these authentication methods actually work? If you are looking for answers now, you are in the right place as the fingerprint is among one of the most used security method nowadays.

Time to read: 5 min

Difficulty: ***/10

Introduction

First of all, for getting us into theme, we need to know what biometric sensors are. A biometric sensor is a type of device that can be used to identify or authenticate a person. It uses various physical attributes such as fingerprints, facial images, and voice recognition to verify or identify a person.

Human fingerprint — To this day, no two fingerprints have been found to be identical.

What types of fingerprint sensors exists and how do they work?

1. Optical fingerprint sensors
   These sensors use a small camera(CMOS sensor) to take a picture of the fingerprint(which consists in different valleys and ridges), which is then analyzed by software. These sensors are often used in smartphones and laptops.

Working principle of Optical fingerprint sensor source: www.teachmemicro.com

2. Capacitive fingerprint sensors:
This consist in a small array of capacitors to detect the ridges and valleys of a fingerprint. The capacitors measure the tiny electrical charges that are generated by the friction ridges of the fingerprint. These sensors are often used in smartphones and other electronic devices. Once an edge is detected it means that is quite close to the capacitor plate and has a lower capacitance as a result. When there is a valley, the distance through the capacitor plate increases, which causes the capacitance to increase. Every capacitor’s capacitance is transmitted to the op-amp and simply recorded with an ADC. This creates a digital fingerprint scan based on capacitive touch sensing.

working principle of capactive sensor soruce https://www.circuitstoday.com/

3. Ultrasonic fingerprint sensors
They use ultrasonic waves to create a 3D image of the fingerprint, which is then analyzed by software. These sensors are considered to be more accurate than other types of sensors. Compared to the first 2 sensors these type of sensors are more robust and hard to crack with 3d molds.

soruce: Yipeng Lu

4. Thermal fingerprint sensors
Here, heat signature of a fingerprint is used to create an image. This sensor measures the heat transfer from the sensor to the fingerprint as it scans the surface of the finger.

soruce: Yipeng Lu

5. RF (Radio Frequency) fingerprint sensors: These use radio frequency to create a image of the fingerprint, which is then analyzed by software. RF sensors are constructed using the same concepts as capacitive sensors

source: DOI:10.1109/INMIC.2008.4777740

6. SWIR (Short-Wave Infrared) fingerprint sensors:
It is using the infrared light to create a image of the fingerprint.

Advantages of finger print sensors

1. Convenience: Since users don’t need to memorize and input a password or PIN, fingerprint sensors make it simple and quick to identify and authenticate people.
2. Security: Fingerprint sensors offer a high level of security because each person’s fingerprint is distinctive and challenging to imitate.
3. Reliability: Fingerprint sensors are accurate and dependable, which lowers the possibility of mistakes or erroneous rejections.
4. Fingerprints cannot be exchanged or transferred, removing the possibility of lost or stolen credentials.
5. Fingerprint sensors are widely utilized in a variety of fields, such as banking, healthcare, and government, which makes it simple for consumers to use the same technology on several systems.

Disadvantages of finger print sensors

1. Limited functionality: Some people, such as those whose fingerprints are dry or damaged, may cause fingerprint sensors to not work properly, which could result in false rejections.
2. Privacy worries: There are worries that the fingerprints gathered might be utilized for other purposes without the person’s permission.
3. False acceptance: A high-quality copy of a fingerprint can trick fingerprint scanners, which could give access to an illegal person.
4. Fingerprint sensors only employ one factor — the user’s fingerprint — whereas other multi-factor authentication techniques may also use the user’s knowledge or identity.

In conclusion, fingerprints sensors are a good step forward towards security but there still is work to do to make it even more secure. In case you lose a key from your door you can change it easily(including the mechanism) but, if someone has your finger print there is no way to replace it.

Written in collaboration with: Eva Montejano

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