Optical Scanner
A fingerprint scanner system has two basic jobs -- it needs to get an image of your finger, and it needs to determine whether the pattern of ridges and valleys in this image matches the pattern of ridges and valleys in pre-scanned images.
There are a number of different ways to get an image of somebody's finger. The most common methods today are optical scanning and capacitance scanning. Both types come up with the same sort of image, but they go about it in completely different ways.
The heart of an optical scanner is a charge coupled device (CCD), the same light sensor system used in digital cameras and camcorders. A CCD is simply an array of light-sensitive diodes called photo sites, which generate an electrical signal in response to light photons. Each photo site records a pixel, a tiny dot representing the light that hit that spot. Collectively, the light and dark pixels form an image of the scanned scene (a finger, for example). Typically, an analogue to digital converter in the scanner system processes the analogue electrical signal to generate a digital representation of this image. See How Digital Cameras Work for details on CCDs and digital conversion.
The scanning process starts when you place your finger on a glass plate, and a CCD camera takes a picture. The scanner has its own light source, typically an array of light-emitting diodes, to illuminate the ridges of the finger. The CCD system actually generates an inverted image of the finger, with darker areas representing more reflected light (the ridges of the finger) and lighter areas representing less reflected light (the valleys between the ridges).
Before comparing the print to stored data, the scanner processor makes sure the CCD has captured a clear image. It checks the average pixel darkness, or the overall values in a small sample, and rejects the scan if the overall image is too dark or too light. If the image is rejected, the scanner adjusts the exposure time to let in more or less light, and then tries the scan again.
If the darkness level is adequate, the scanner system goes on to check the image definition (how sharp the fingerprint scan is). The processor looks at several straight lines moving horizontally and vertically across the image. If the fingerprint image has good definition, a line running perpendicular to the ridges will be made up of alternating sections of very dark pixels and very light pixels.
If the processor finds that the image is crisp and properly exposed, it proceeds to comparing the captured fingerprint with fingerprints on file.
Analysis
In movies and TV shows, automated fingerprint analyzers typically overlay
various fingerprint images to find a match. In actuality, this isn't a particularly
practical way to compare fingerprints. Smudging can make two images of the
same print look pretty different, so you're rarely going to get a perfect image
overlay. Additionally, using the entire fingerprint image in comparative analysis
uses a lot of processing power, and it also makes it easier for somebody to steal
th
e print data.
Instead, most fingerprint scanner systems compare specific features of the fingerprint, generally known as minutiae. Typically, human and computer investigators concentrate on points where ridge lines end or where one ridge splits into two (bifurcations). Collectively, these and other distinctive features are sometimes called typica.
The scanner system software uses highly complex algorithms to recognize and analyze these minutiae. The basic idea is to measure the relative positions of minutiae, in the same sort of way you might recognize a part of the sky by the relative positions of stars. A simple way to think of it is to consider the shapes that various minutia form when you draw straight lines between them. If two prints have three ridge endings and two bifurcations, forming the same shape with the same dimensions, there's a high likelihood they're from the same print.
To get a match, the scanner system doesn't have to find the entire pattern of minutiae both in the sample and in the print on record; it simply has to find a sufficient number of minutiae patterns that the two prints have in common. The exact number varies according to the scanner programming.
Does the Digital Persona reader comply with applicable FCC regulations?
This device has been tested and found to comply with the limits for a Class B digital device under Part 15 of the Federal Communication Commission (FCC) rules, and it is subject to the following conditions: a) It may not cause harmful interference, and b) It must accept any interference received, including interference that may cause undesired operation. Any changes or modifications not expressly approved by DigitalPersona could void your authority to operate this equipment.
This device conforms to emission product standards EN55022 (B) and EN50082-1 of the European Economic Community; AS/NZS 3548 Class B of Australia and New Zealand; VCC1 Class B of Japan.
This digital apparatus does not exceed the Class B limit for radio noise emission from digital apparatus specified in the interference-causing equipment standard entitled "Digital Apparatus," ICES-003 of the Department of Communication of Canada.
Additional reading...
How to implement biometrics
A guide to how you can successfully implement Biometrics. Read the full article.
Explanation letter for Cashless Catering installation...
A sample letter to be sent to parents to inform them of the proposed Cashless Catering installation. The sample can be seen on this page.
