This device is formed by a matrix of pressure sensitve sensors. Each time that a mechanical force is applied on them, an electrical signal is produced. The magninute of the generated current depends on the pressure applied by the finger when touching the sensor surface. Since the fingerprint ridges and valleys are at different distances from the sensor surface, the amount of current differs. The main drawback of this method is that is traditionally captures low qualitiy images because of its low sensitivity properties.
In ultrasound devices, a set of acoustic signals are send toward the fingertip and the echo signal, that computes the range image of the fingerprint, is captured. The echo signal represents therefore the ridge structure itself. This type of device comprises two main components: the sender and the receiver. The sender is responsible for generating the acoustic signals, whereas the receiver captures the echo responses when the signals touch the fingerprint surface. The main drawbacks of this device consist in the image noise that may be produced due to the oily or dirty regions on the fingerprints as well as the cost for producing it.
This type of device is composed by one or more semiconductor chips. Each chip comprises a matrix of small cells. At the same time, each cell has a capacitance plate coated by an isolant layer. Typically, the cells are smaller than the size of the fingerprint ridges.
When the finger is placed on the chips, small electrical charges are produced and since the distance between regions of the fingerprints (valleys and ridges) differ, different capacitance patterns are generated. The sensor then process the output of each of the cells to determine whether the pattern belongs to a ridge or a valley.
This is maybe one of the oldest and most popular techniques for live-scan fingerprint acquisition. The fingerprint is initially placed on a glass prism, whilst a laser light is directed toward the prism. Then a CCD (Carge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor) is used to capture the reflected light. While the light rays absorbed in the fingerprint ridges produce brighter regions in the image, reflected rays on the fingerprint valleys result in darker image regions. One disadvantage of this kind of devices is related to the image distortions that may be generated when the reflected light is not focused properly. Moreover, because there should exist a proper distance between the prism and the sensor, these devices cannot be miniaturized.
In this kind of devices, a set of cells of pyro-electric material are used to generate current based on temperature differentials. The fingerprint ridges that are directly in contact with the sensor surface produce a different temperature than the fingerprint valleys that are away from that surface. The difference of temperature represents then the basis of these devices and is used to generate the fingerprint image. Typically, the sensors are maintained at high temperatures by warming them up.