|Overview of Fingerprint Recognition
Fingerprint recognition equipment is relatively low-priced compared to other biometric system and R&D investments are very robust in this field.
Advantages and Disadvantages of Fingerprint Recognition
|Since fingerprints are the composition of protruding sweat glands, everyone has unique fingerprints.
||Vulnerable to noise and distortion brought on by dirt and twists.
|They do not change naturally.
||Some people may feel offended about placing their fingers on the same place where many other people have continuously touched.
|Its reliability and stability is higher compared to the iris, voice, and face recognition method.
||Some people have damaged or eliminated fingerprints.
|Fingerprint recognition equipment is relatively low-priced compared to other biometric system and R&D investments are very robust in this field.
Optical Fingerprint Sensor
There are two main types of sensors for inputting fingerprints. One is an optical sensor using a prism or hologram, and the other type is a non-optical sensor. Recently, products employing both optical and non-optical methods have been introduced. In the past, a semiconductor sensor was the only non-optical choice, but now equipment with ultrasonic sensors, another type of non-optical sensors, are on the market.
Figure 1 explains the basic principle of absorption in an optical fingerprint sensor.
* Figure 1: Principle of Absorption in Fingerprint Sensor
An absorption optical fingerprint sensor is composed of a right-angled triangle prism (4), light source (20), a diffusion plate (3), a lens group and an image sensor (6).
When a fingerprint is placed on the contact surface, its ridges are closely pressed onto the surface while its valleys are detached from it.
The light radiated from light source becomes uniform after undergoing the diffusion plate. The light reaches the fingerprint contact surface after passing through the prism. If the light touches the valley, total internal reflection happens so that it reaches the image sensor composed of CCD (Carge Coupled Device) element or CMOS (Complementary Metal Oxide Semiconductor) element after going through the lens group. On the other hand, if the light reaches the ridges closely pushed onto the surface, some light goes to the image sensor after the total internal reflection and some light is absorbed in the ridges.
There are changes in luminous intensity between light reflected from valleys and light from ridges and the image sensor obtains the fingerprint image by calculating the changes in the reflected light intensity between the two. The absorption optical fingerprint sensor needs several LEDs (15-20) since the light should be two-dimensionally uniform after going through the diffusion plate. To capture a fingerprint image without distortion brought on by different optical paths, enough distance is required between the prism and the image sensor.
The scattering optical fingerprint sensor is mainly comprised of a rectangular-triangle prism (13), light source (20), a lens group (15), and an image sensor (16). When a fingerprint is placed on the contact surface, its ridges are closely pressed onto the surface while its valleys are detached from it.
The light radiated from the source passes through the prism and reaches the surface. The light perpendicularly goes through the surface unlike the absorption sensor. If the light reaches the valleys, it goes through the surface, radiating to the outside. If it touches upon the ridges, scattering happens at the ridges. The scattered light gets to the image sensor composed of CCD or CMOS element through the lens group. The light radiated to outside near the valleys seldom reaches the image sensor. Only the scattered light near the ridges gets to the sensor. As a result, a fingerprint image can be captured since the valley area is dark and the ridge area is bright.
The scattering optical fingerprint sensor doesn't need a diffusion plate and its contrast is great. However, it needs the rectangular prism, more expensive than the triangle prism.
* Figure 2: Principle of scattering fingerprint sensor
Semiconductor Fingerprint Sensor
A semiconductor fingerprint sensor is a prime example of non-optical sensors. Figure 5.3 shown below describes the basic principle of the semiconductor fingerprint sensor.
* Figure 3: Basic principle of the semiconductor fingerprint sensor
The semiconductor fingerprint sensor measures the electrostatic capacity between sensor surface and skin, and translates it into an image. If a user places his or her fingerprint on the surface, its ridges are closely pressed on the surface and its valleys have some space from the surface. In the case of the ridges, the distance (d) between ridges and surface is short so that the electrostatic capacity is high. On the other hand, the valleys are distant from the surface compared to the ridges, so the electrostatic capacity is low.
The fingerprint image can be captured by composing signals obtained from an array of sensors on the semiconductor surface. The semiconductor fingerprint sensor can be lighter and smaller. But it is vulnerable to external shocks and chemical substances such as sodium chloride from people's skin due to the physical traits of a silicon wafer, which is fundamental to the sensor. To address these disadvantages, the contact surface is being coated. Developing a physically strong coating is one of the major tasks facing semiconductor fingerprint sensors.
||Pressure, heat, contact, static electricity, ultrasound
||Very safe. High perception rates. Strong against external shocks and scratch.
||Impossible to duplicate. Able to minimize the size. Low production and maintenance costs.
||Relatively big module. High production and maintenance costs.
||Sensitive to environmental changes such as static electricity and temperatures.
* Table 1: Comparison between optical and non-optical methods