What are the basic types of fingerprint sensor technology in use today?
Fingerprint sensors can use optical or capacitive technology. Each of these options suits different implementations. Like a digital camera, optical technology employs visible light to capture a digital image. It uses a light source to illuminate the finger’s surface while a charge-coupled device (CCD) captures the image, which is then converted to a digital signal. There are two types of capacitive technologies: passive and active. Passive capacitive technology uses a silicon-based structure containing an array of plates that are used to apply a small electrical current and measure the voltage discharge through the finger when it is placed over the sensor. The plates act as a parallel array of capacitors when the finger is placed on the sensor, and the capacitance at each point (pixel) is measured. Fingerprint ridges have a substantially greater discharge potential than valleys containing air, which may have little or no discharge (Fig. 2a). In either case, the result is an 8-bit high-
