The main part of any digital imager is its image sensor.
Most biometric sensors also contain some kind of image
sensor, especially fingerprint, finger-, hand-vein scanners
and iris scanners. There are two main types of image
sensors, CCD and CMOS (APS). An image sensor is an
analog device which basically consists of an array of photosensitive
cells, called pixels. Like every human being and
every electronic device also an image sensor ages. Image
sensors develop more and more defective pixels as they become
older even if they are not in use. These defective
pixels show different characteristics than at manufacturing
time. They appear as spiky noise in the output image. Pixel
defects are permanent, their number increases linearly with
time and they are randomly distributed