What happens during a fingerprints scan?
What is a Fingerprint?
The skin surface of the fingers,
palms and soles of the feet is different to the rest of the body surface. If
you look at the inner surface of your hands and soles of the feet you will see
a series of lines made up of elevations which we call 'ridges' and depressions
which we call 'furrows'.
These ridges and furrows can be
recorded in many ways. For example, the ridges can be inked and placed on to a
piece of paper. This would leave a fingerprint like below. The black lines
represent the ridges and the white lines represent the furrows.
Within these patterns the ridges
can split or end creating ridge characteristics. There are 6 types of ridge
characteristics.
Everyone has a unique and
different distribution of these characteristics that develop in the womb and
are persistent throughout life.
It is the coincidence sequence of
these characteristics that allow me to make identifications. The coincidence
sequence is whereby I will find the same characteristics, in the same
order with the same relationship to each other in both the crime scene
fingerprint and the fingerprint on the form I am using.
Fingerprints unique:
It's pretty obvious why we
have fingerprints—the tiny friction ridges on the ends of our fingers and thumbs make it easier to grip things. By
making our fingers rougher, these ridges increase the force of friction between
our hands and the objects we hold, making it harder to drop things. You have
fingerprints even before you're born. In fact, fingerprints are completely
formed by the time you're seven months old in the womb. Unless you have
accidents with your hands, your fingerprints remain the same throughout your
life.
Enrollment and
verification
Suppose you're in charge of security for a large bank and
you want to put a fingerprint scanning system on the main entry turnstile where
your employees come in each morning. How exactly would it work?
There are two separate stages involved in using a system
like this. First you have to go through a process called enrollment,
where the system learns about all the people it will have to recognize each
day. During enrollment, each person's fingerprints are scanned, analyzed, and
then stored in a coded form on a secure database. Typically it takes less than
a half second to store a person's prints and the system works for over 99% of typical users (the failure rate is higher for manual workers than
for office workers).
Once enrollment is complete, the system is ready to
use—and this is the second stage, known as verification. Anyone who wants to gain access
has to put their finger on a scanner. The scanner takes their fingerprint,
checks it against all the prints in the database stored during enrollment, and
decides whether the person is entitled to gain access or not. Sophisticated
fingerprint systems can verify and match up to 40,000 prints per second!
How fingerprint scanners
work
a computer has to scan the
surface of your finger very quickly and then turn the scanned representation
into a code it can check against its database. How does this happen?
There are two main ways of scanning fingers. An optical
scanner works by shining a bright light over your fingerprint and
taking what is effectively a digital photograph. If you've ever photocopied your
hand, you'll know exactly how this works. Instead of producing a dirty black
photocopy, the image feeds into a computer scanner. The scanner uses a
light-sensitive microchip (either a CCD, charge-coupled device, or a CMOS
image sensor) to produce a digital image. The computer analyzes the image
automatically, selecting just the fingerprint, and then uses sophisticated
pattern-matching software to turn it into a code.
Another type of scanner, known as a capacitive
scanner, measures your finger electrically. When your finger rests on a
surface, the ridges in your fingerprints touch the surface while the hollows
between the ridges stand slightly clear of it. In other words, there are
varying distances between each part of your finger and the surface below. A
capacitive scanner builds up a picture of your fingerprint by measuring these
distances. Scanners like this are a bit like the touchscreens on
things like iPhones and iPads.
Unlike ordinary digital photos, scans have to capture
exactly the right amount of detail—brightness and contrast—so that the
individual ridges and other details in the fingerprint can be accurately
matched to scans taken previously. Remember that fingerprints might be used as
evidence in criminal trials, where a conviction could result in a long jail
sentence or even the death penalty. That's why "quality control" is
such an important part of the fingerprint scanning process.
Here's how the process
works with a simple optical scanner:
1.
A row of LEDs scans
bright light onto the glass (or plastic) surface on which your finger is
pressing (sometimes called the platen).
2.
The quality of the
image will vary according to how you're pressing, how clean or greasy your
fingers are, how clean the scanning surface is, the light level in the room,
and so on.
3.
Reflected light
bounces back from your finger, through the glass, onto a CCD or CMOS image
sensor.
4.
The longer this
image-capture process takes, the brighter the image formed on the image sensor.
5.
If the image is too
bright, areas of the fingerprint (including important details) may be washed
out completely—like an indoor digital photo where the flash is too close or too
bright. If it's too dark, the whole image will look black and details will be
invisible for the opposite reason.
6.
An algorithm tests
whether the image is too light or too dark; if so, an audible beep or LED
indicator alerts the operator and we go back to step 1 to try again.
7.
If the image is
roughly acceptable, another algorithm tests the level of detail, typically by
counting the number of ridges and making sure there are alternate light and
dark areas (as you'd expect to find in a decent fingerprint image). If the
image fails this test, we go back to step 1 and try again.
8.
Providing the image
passes these two tests, the scanner signals that the image is OK to the
operator (again, either by beeping or with a different LED indicator). The
image is stored as an acceptable scan in flash memory, ready to be
transmitted (by USB cable, wireless, Bluetooth, or some similar
method) to a "host" computer where it can be processed further.
Typically, images captured this way are 512×512 pixels (the dimensions used by
the FBI), and the standard image is 2.5cm (1 inch) square, 500 dots per inch,
and 256 shades of gray.
9.
The host computer can
either store the image on a database (temporarily or indefinitely) or
automatically compare it against one or many other fingerprints to find a
match.
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The matching algorithm finds out whether there is
a match by comparing two templates extracted by the characteristic point
extraction algorithm, specifically by comparing the positions of each
characteristic point and the structure.
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