Physiologically, a fingerprint is a so-called papillary pattern — a configuration of protrusions (crests) containing individual pores separated by depressions. Under the skin of the finger is a network of blood vessels. In addition, a fingerprint is associated with certain electrical and thermal characteristics of the skin. This means that light, heat, or capacitance (or a combination of them) can be used to obtain a fingerprint image.
A fingerprint is formed during the development of the fetus and does not change throughout a person’s life. Moreover, if damaged, it will restore its original structure in time. Even identical twins do not have identical fingerprints. In terms of reliability, fingerprint scanning is second only to DNA analysis, as well as scans of the iris or retina.
Examples of the use of fingerprint scanners:
1. Instead of a signature
People have been interested in the specifics of fingertip patterns since ancient times, and they were even found to be used hundreds of years ago. In 6th century China, a fingerprint was used instead of a signature or stamp. However, one cannot say with certainty whether the uniqueness of the trace left on the document to confirm its authenticity actually mattered. This type of “signature” had a primarily mystical character — it was important that the person, having touched the paper, left a trace of his personality on it.
2. Instead of a key
Thanks to Hollywood movies, the practice of using prints as a “key” to unlock locks or safes has become quite well known. To do this, complex equipment called AFIS, or Automated Fingerprint Identification Systems, are used. The scanners used in them can be optical, semiconductor-based, or use ultrasound. The last two types are the most reliable and effective, but, of course, the optical application looks more effective on a screen when a light strip passes along the print, so these scanners have become more widespread. They are used in much more prosaic areas than the protection of bank vaults and secret laboratories. For example, biometric terminals can be installed to account for the time an employee spends at work.
3. Instead of a password
The patterns drawn on the fingertips have gained great popularity among the developers of security systems. In particular, a finger scan can be used instead of a password or PIN code for computers and gadgets.
Among these new developments, a noteworthy one is ITBiometrics’ hardware “cold” fingerprint wallet, which takes into account the special needs of digital currencies holders for biometric authentication. The user’s fingerprint is captured using very sophisticated encryption and uses this algorithm to enter a password. An interesting feature of this wallet is not only its ability to identify the owner’s fingerprint, but also to determine whether it’s really that person trying to gain access to his wallet by reading the pulse and blood flow in the finger.
Since the 80s, small biometric devices have been introduced into personal computers, and today they can often be found in laptops or flash drives. As a rule, the finger must be put up against a small chip that reads the surface of the skin using a light source. Actually, an entire thumbprint is not required for identification. A unique fragment, such as an intersection of lines, or a special bend, etc., is sufficient.
4. Instead of a bank card
In 2007, an interesting experiment was conducted in southern Germany: some banks’ clients were offered the choice to link their accounts not to a plastic card, but to the unique patterns on their fingers. In a number of supermarkets and other retail outlets, special scanners were installed, to which one could apply his or her finger when making a purchase. Some schoolchildren even started paying for school lunches this way. Since the probability of mistaken prints is 1 in 10 million, the owners of such “means of payment” can be less concerned about the safety of their money than the holders of ordinary cards.
What are the types of fingerprint scanners?
All existing fingerprint scanners can be divided into three groups: optical, semiconductor-based and ultrasonic. In addition, each method offers several types of implementation.
1. Optical scanners
Optical scanners — operation is based on the use of optical imaging methods. There are several fundamental ways to implement the optical method:
- The Optical Reflection Method:
This method uses the effect of broken total reflection (Frustrated Total Internal Reflection). This effect occurs when light falls on the interface between two mediums, and the light energy is divided into two parts — one is reflected from the border, the other penetrates through the border into the second medium. The proportion of reflected energy depends on the angle of incidence of the light flux. Starting from a certain value of a given angle, all the light energy is reflected from the interface.
This phenomenon is called total internal reflection. In the case of contact of a more dense optical medium (the surface of a finger) with a less dense one at the point of total internal reflection, a beam of light passes through this boundary. Thus, only beams of light that hit certain points of total internal reflection, to which a finger’s papillary pattern was not attached, will be reflected from the border. To capture the resulting light image of the surface of a finger, a special image sensor is used (CMOS or CCD, depending on the scanner).
Disadvantages of the method:
• Ineffective protection against fakes
• Sensitivity to contamination
The leading manufacturers of these scanners are BioLink, Digital Persona, and Identix.
• The Optical Illumination Method:
Scanners of this type utilize a fiber-optic matrix in which all the waveguides at the output are connected to photo sensors.
The sensitivity of each sensor allows you to capture the residual light passing through the finger at the point of contact of the finger with the surface of the matrix. The image of the entire fingerprint is formed according to the data read by each photo sensor.
This method has many more advantages:
• High reading reliability
• Resistance to fakery
However, this method also has a significant drawback — its complexity of implementation:
This type of scanner is manufactured by Security First Corp.
• Optical touchless scanners:
Optical non-contact scanners (touchless scanners), unbelievably do not require direct contact of the finger with the surface of the scanning device. The finger is applied to a hole in the scanner, and several light sources illuminate it from below from different sides; in the center of the scanner is a lens through which the collected information is projected onto a CMOS camera that converts the data into a fingerprint image.
The leading manufacturer of scanners of this type is Touchless Sensor Technology.
2. Semiconductor Scanners
The basis of semiconductor scanners is the use of the properties of semiconductors to obtain the image of the finger’s surface of a finger by changing at the points of contact of the crests of the papillary pattern with the surface of the scanner.
• Capacitive Scanners:
Capacitive scanners are the most common semiconductor devices for fingerprint imaging in use today. Their operation is based on the effect of a change in the capacitance of the PN junction of a semiconductor when the comb of a papillary pattern touches the element of the semiconductor matrix.
It has advantages due to its widespread use:
• Low cost
• Ineffective protection against fakery
The leading manufacturers of scanners of this type are Infineon, STMicroelectronics, and Veridicom.
• Radio frequency scanners:
With radio frequency scanners (RF-Field Scanners), a matrix of elements is used, each of which operates as a miniature antenna. The radio frequency module generates a low-intensity signal and sends it to the scanned surface of the finger. Each of the sensitive elements of the matrix receives a signal reflected from the papillary pattern. The magnitude of the emf induced in each miniature antenna depends on the presence or absence of a crest of a papillary pattern near it. The resulting stress matrix is converted into a digital fingerprint image.
• Since the physiological properties of the skin are analyzed, the probability of cheating this scanner tends towards zero.
• Unstable operation if there is poor finger contact
A well-known manufacturer of radio frequency scanners is Authentec.
• Scanners which use the pressure method:
Pressure-sensitive scanners (Pressure Scanners) use a matrix of piezoelectric elements in their design that are sensitive to pressure. When a finger is applied to the scanning surface, the papillary ridge projections exert pressure on a subset of the elements of the matrix. The depressions of the skin pattern do not exert any pressure. Thus, the totality of the voltages obtained from the piezoelectric elements is converted into an image of a fingerprint.
This method has several disadvantages:
• Low sensitivity
• Ineffective protection against fakery
• Susceptibility to damage due to excessive force used while operating
Pressure sensitive scanners are available from BMF.
In thermal scanner devices, sensors are used which consist of pyroelectric elements. This allows for the fixing of the temperature difference and its conversion into voltage.
When the finger is applied to the scanner, a temperature map of the finger surface is constructed based on the temperature of the protrusions of the papillary pattern touching the pyroelectric elements and the temperature of the air in the cavities, which is subsequently converted into a digital image.
The temperature method has many advantages:
• High resistance to electrostatic discharge
• Steady operation in a wide range of temperatures
• Effective protection from fakes.
The disadvantages of this method include the fact that the image disappears quickly. When you put your finger on, in the first moment, the temperature difference is significant and the signal level, respectively, is high. After a short time (less than one tenth of a second), the image disappears as the finger and the sensor come to temperature equilibrium.
3. Ultrasonic method
In this group, so far, there is only one method, and that is what it is called: Ultrasonic. Ultrasonic scanners scan the surface of a finger with ultrasonic waves. The distances between the source of the waves and the ridge protrusions and depressions of the papillary pattern are measured by the echo reflected from them.
The quality of the image is ten times better than that of any other method available on the biometric market. In addition, this method is almost completely impervious to fakes, because, in addition to the fingerprint papillary pattern, it can obtain information about other characteristics as well, such as pulse.
• High cost
The leading manufacturer of this type of scanner is Ultra-Scan Corporation.