How Is the Finger Imaged?

How Is the Finger Imaged ?

We have seen what constitutes a fingerprint for biometric use. How the finger is imaged needs to be explained. Since fingerprint biometrics are active biometrics, the user must submit to the scan and needs to place his/her finger on the scanner for imaging. The scanning can be done with different technologies, but in general, they all need to capture an image of the finger. A fingerprint scanner captures the image and then transmits either the image or, if it is a trusted device, a secure response to the fingerprint scan. The technology used to scan fingers falls into two basic categories:

1. Optical scanners

2. Silicon scanners

Optical scanners are composed of complementary metal-oxide semiconductor (CMOS) and charge- coupled device (CCD) technology. Silicon scanners include capacitive, thermal, and radio frequency (RF). Each scanner type is discussed in more detail in the following sections.

Optical Scanners

Optical scanners use optics to gather finger images. The optics are part of a camera system that captures reflected light from a light source, normally through a prism. Regardless of whether the camera technology being used is CCD or CMOS, the basic principals for optical imaging are employed.

To get an optical fingerprint image, the device will have:

• Platen ” Used for presenting the finger.

• Prism ” Used for reflecting the lighted image to the camera.

• Light source ” Used to illuminate the fingerprint. This is normally a grid of light-emitting diodes (LEDs).

• Camera ” Used to capture the finger image.

The difference between CCD and CMOS technologies comes down to their implementation and use.

CCD is the older of the two technologies. CCD came to the market in the early 1970s. In the following decades, it has been refined and tuned to produce better and cleaner images. CCD is used for almost any digital imaging requirements. The drawbacks of CCD technology are its need for high operating voltages and the cost of additional electronics to help manage complex clocking requirements. CCD also offers only an image captured as a sequential data stream. Thus, if an imaging application wanted only one particular area of interest in an image, it would need to stream in the whole image.

CMOS is a new silicon-based imaging technology. Since the imager is based on silicon, it can be readily manufactured on any production line that is currently making silicon chips. Its ability to utilize generalized production makes CMOS a lower-cost device. Since CMOS is a solid-state component, it can also be used as the basis for systems on a chip. In this type of chip design, all the electronics for a particular task can be consolidated into a single electronic package. That is to say, a single chip could handle the tasks of imaging, processing the signal, running a USB hub, and doing encryption. This way, the total cost of the system can be lowered .

CMOS technology comes in two flavors. The first is a passive arrangement whereby the output of a row or column of pixels is detected by a single amplifier for the row, column, or entire image. Thus, to use a single amplifier, a larger capacitance is required at both the input and output amplifiers . This can increase signal noise and reduce the sensitivity of the device. This can lead to CMOS's technology having a lower resolution when compared to CCD. This is being addressed through the development of active pixel technologies. In active pixel technologies, each pixel implements its own first-stage amplifier . In doing so, the required capacitance is diminished and the signal noise is decreased. In turn , the resolution of the image is increased.

The choice of a finger biometric using either CMOS or CCD technology really comes down to questions of cost and function. If the biometric application does not have a restriction on power consumption, then CCD will be fine. On the other hand, the relatively cheap production costs of CMOS imagers will lower the price of the biometric device more rapidly . It is important to know then if the CMOS imager to be used is being implemented with passive or active pixel technologies.

Silicon Scanners

In general, silicon-based scanners require the presentation of a fingerprint directly onto a piece of silicon. As we know, silicon is a sensitive material and very susceptible to electrostatic discharge . This material provides great promise in reduced cost even with its drawbacks. This reduced cost comes from two areas. First, like a CMOS device, it can be fabricated on almost any silicon-based assembly line. Second, with the work going on to reconstruct finger images, either through swiping the finger or multiple image reconstruction of the print, smaller pieces of silicon can be used. This second point drives the cost down since the smaller the piece of silicon required to image, the higher the silicon wafer yield.

Like optical technologies, regardless of the type of imaging being used in a silicon-based scanner, the basic operations are the same. The finger is sensed in some way, and that produces an output that can be measured by the appropriate type of silicon. Thus, to get an image, the following components are common:

• Platen ” Depending on the sensing technology, this may or may not be the silicon wafer itself.

• Signal generator ” This generates the sensing signal that will be picked up by the silicon.

• Contact plate ” The contact plate may be for electrostatic discharge or used as part of the signal generator.

• Silicon sensor ” This is the silicon part of the sensor that will receive the signal reflected back from the fingerprint.

Each specific type of silicon-based sensor is described in the following sections.

Capacitive

The capacitive was the first silicon-based scanner to make it to market. Early scanners suffered from frequent failure due to electrostatic discharge. Recent improvements in the packaging and design of the devices have reduced the frequency with which this happens, but it is still an issue to be considered . Capacitive-based sensors work on a capacitance principle. That is, as the ridges of a fingerprint contact the silicon layer, a greater capacitance is created than where the valleys do not touch the silicon. The details are as follows : The silicon contains a series of capacitors packed tightly on the chip. The drive circuit for each capacitor is an inverting operational amplifier. An inverting operational amplifier works by altering its output current based on changes in the input current. Thus, to gather an image from the capacitor , the two capacitive plates are discharged and then a fixed charge is applied to each plate. As the ridges of a finger get closer and finally make contact with the chip's surface, this drives up the capacitance of that circuit. The increase in capacitance indicates the presence of a ridge. The valley of a fingerprint does not increase capacitance since there is no contact with the skin. Thus, all the drive circuits are queried and a finger image is created for comparison.

Thermal

A thermal fingerprint sensor does not rely on generating a signal externally. Thermal sensors use the body's own heat as the signal to measure. As a finger contacts the surface of the sensor, the ridges and valleys provide a temperature difference. The temperature difference is measured between the nominal temperature of the sensor before the finger is presented and after. The difference generates a signal through a pyro-electric effect. The time required to measure the delta in temperature between the nominal temperature and the measured temperature is in microseconds. This allows for quick fingerprint imaging. As such, this makes this type of sensor ideal for a swipe form factor. In a swipe form factor, the finger is swiped across a thin piece of silicon and the finger image is reconstructed from distinct captured frames . Since the swipe form factor is very small, the cost of the silicon for the sensor is lower because more sensors can be taken from one wafer of silicon.

Radio frequency (RF)

An RF fingerprint sensor works by imaging the live layer below the skin. To accomplish this, the sensor uses two parallel plates that generate an electric field between them. As a finger comes into contact with an exposed drive plate, the finger causes the signal to attenuate, and this attenuation is captured by the sensors in the silicon below. The ridges on the finger cause the signal to attenuate more than the valleys. This type of sensor works very well for dry and calloused fingers. It also works relatively well with non-conductive coatings on the fingers.