Capacitive sensors use an array capacitor plates to image the fingerprint. Skin is conductive enough to provide a capacitive coupling with an individual capacitive element on the array. Ridges, being closer to the detector, have a higher capacitance and valleys have a lower capacitance. Some capacitive sensors apply a small voltage to the finger to enhance the signal and create better image contrast.
Capacitive sensors can be sensitive to electrostatic discharge (shock) but they are insensitive to ambient lighting and are more resist contamination issues than some optical designs.
Optical sensors use arrays of photodiode or phototransistor detectors to convert the energy in light incident on the detector into electrical charge. The sensor package usually includes a light-emitting-diode (LED) to illuminate the finger.
There are two detector types used by optical sensors, charge-coupled-devices (CCD) and CMOS based optical imagers. CCD detectors are sensitive to low light levels and are capable of making excellent grayscale pictures. However, CCD fabrication is relatively expensive and neither low-light sensitivity or grayscale imaging are required for fingerprint recognition. CMOS optical imagers are manufactured in quantity and can be made with some of the image processing steps built into the chip resulting in a lower cost.
Optical sensors for fingerprints may be affected by a number of real world factors such as stray light and surface contamination, possibly even a fingerprint impression left by a prior user. Common contaminates that deteriorate image quality include oil and dirt, scratches on the sensor surface, and condensation or ice. Some suppliers have tried to sidestep the contamination problem by directly taking a 3D image from the surface of a finger. 3D imaging technology is more hygienic but introduces a whole new set of problems and was not included in this study.
Impostor prints are more of a problem for optical sensors than it is for other detectors because it is relatively easy to present the scanner with a convincing picture of a fingerprint. Suppliers have come up with several techniques to validate a live finger. For example optical sensors can be enhanced and made more resistant to deception with Electro-Optical imaging. This works by placing a voltage across a light-emitting polymer film. When a finger is presented, the ridges provide a ground to the polymer surface creating a small current that generating light. The fingerprint valleys remain dark so a high contrast image is produced. The polymer is directly coupled to an optical detector
Thermal sensors use the same pyro-electric material that is used in infrared cameras. When a finger is presented to the sensor, the fingerprint ridges make contact with the sensor surface and the contact temperature is measured, the valleys do not make contact and are not measured. A fingerprint image is created by the skin-temperature ridges and the ambient temperature measure for valleys.
The biggest drawback of this technique is that the temperature change is dynamic and it only takes about a tenth of a second for the sensor surface touching ridges and valleys to come to the same temperature, erasing the fingerprint image. Additionally, this technology has many of the same contamination and wear issues as other sensors. While it can operation over a wide range of temperatures, if the ambient temperature is close to the finger surface temperature the sensor requires heating to create a temperature difference of at least 1 degree Centigrade.
Pressure sensing scanners can be made very thin and are often used in electronic devices. Early pressure sensing scanners had to make a tradeoff between durability and quality because any protective layer on the detector surface would diminish the contrast of the impression. There are two types of pressure sensing detectors available, conductive film detectors and micro electro-mechanical devices (MEMS). Conductive film sensors use a double-layer electrode on flexible films. MEMS is a newer technology that uses extremely tiny silicon switches on a silicon chip. When a fingerprint ridge touches a switch, it closes and is detected electronically
A low radio frequency (RF) signal is applied to the userís finger and then read by the detector array, with each pixel operating like a tiny antenna. The advantage of this detector is that it reads the fingerprint from the dermal layer underneath the surface making it less susceptible to damaged or dry fingertips.
Ultrasonic scanners have an advantage of being able to see beneath the skin. This provides not only verification of a live finger, it provides more information as a biometric measure. But this technology is slow, expensive, bulky, and too data intensive for most access control applications