The image of a policeman, or a public official, taking fingerprints with the classic ink tampon is going down in history. In short, the stained fingers will remain as a memory of old black and white films. The technology has replaced the ink with scanners, and the process of digitizing old cards with fingerprints has allowed their storage in large databases. Today, in UMANICK, we approach the different types of scanners for fingerprinting.

For centuries, fingerprinting has been done by soaking the fingertips in ink and then pressing the finger against a paper card. When the fingerprint databases began to be digitized, these cards were scanned for digital storage. In the last decades, due to the great proliferation of electronic capture systems, fingerprints are no longer taken with ink.

Reconocimiento de huella dactilar

Fingerprint with ink. Photo of Exercise Tradewinds with CC License:

We will analyze the types of fingerprint scanners most used today. To start, we classify them into four blocks: optical, capacitive, thermal, and ultrasonic.

Optical scanners

They are of several types. The first one we analyze is the oldest and most used technology: the FTIR (Frustrated Total Internal Reflection). The finger is placed on a glass prism and illuminated with diffuse light. The different reflection between the crests and the valleys of the fingerprint is recorded on a CCD or CMOS sensor (like digital cameras). They provide quality images, but they can not be miniaturized, since the size of the prism determines the degree of distortion in the obtained image.

FTIR with laminar prism. Same as the first but with the smaller FTIR sensor. To achieve this, the internal surface of the scanner is carved in the form of microprisms, which simulate a larger prism. The quality of the images is, in general, worse than in the previous case.

Fiber Optic. This scanner creates a matrix of vertical optical fibers with which the finger comes into contact. The residual light emitted by the finger is picked up by a CCD or CMOS sensor that records the image. The image will have as much resolution as the number of fibers there are.

Electro-optics. This type of scanner works with a polymer that, if polarized, emits light depending on the voltage found on one of its faces. Thus, the voltage difference created by the ridges and valleys is recorded by a photodiode array or a CMOS sensor.

Direct Reading. This technique is the only one that does not require contact with the scanner. A very high quality photograph of the fingerprint is taken and processed to correct distortions. It is a little extended method, because it is complicated to obtain images with the necessary resolution and contrast.

Capacitive scanners

This type of scanners are very common. They are composed of a surface formed by a two-dimensional matrix of small capacitor plates. The skin acts as the second plate of the capacitors and, depending on the distance, the capacitance will vary and the crests of the valleys can be easily distinguished. The main problem with these scanners is their maintenance and durability, since the scanner coating must be thin enough to be precise, but not so thin that it can be damaged. Additionally, electrostatic discharges can cause irreversible damage to the scanner.

Reconocimiento de huella dactilar

Fingerprinting with an electronic device. Photo of West Midlands Police with CC License.

Thermal scanners

The thermal scanners are made of pyroelectric material. The temperature difference between the crests of the fingerprint, in contact with its surface, and the valleys can be easily captured. An important factor of error is that, when the thermal balance is reached by the pressure of the finger, the image disappears. To avoid this, the fingerprint is acquired by a sweep, that is, by sliding the finger across the surface of the scanner. These scanners allow to reduce the acquisition area, being able to integrate into portable systems and are more resistant to external damages than capacitive ones.

Ultrasound scanners

It sends acoustic signals to the surface of the finger and captures the received echo. This signal allows to reconstruct the shape of the fingerprint and the structure of ridges and valleys. Its main advantage is that it is invulnerable to dirt or even materials that come between the finger and the sensor, such as thin gloves. The great disadvantage is its complexity, which makes it difficult to integrate it into small devices, and its price which, compared to other types of fingerprint scanners, is relatively expensive. The acquisition of the images is also, in comparison with other systems, slower.

The small size of the scanners, their easy integration -even in telephones-, their usability and their low costs, make the fingerprint a very useful technology for its implantation as a biometric system for identifying people in offices, hospitals, homes and corporate buildings, for example.