Security System

3M security systems are very much in demand in the world today. One area which 3M looks into is the problem of illegal immigration - the use of fake passports and certifications. 3M works closely with the law enforcement agencies to understand their needs in the security set-up.

One other popular advertisement which 3M had was for their Security Glass. 3M was so sure that their Security Glass was unbreakable that they put up stacks of money in a panel at the bus stop and asked every passer-by to attempt to break open.

external image money_security.jpg

3M Congent Blue Check®

The fingerprint identification system (which was an enormous machine many years ago) has been downscaled to a portable equipment today. Its portability has brought about much ease to identify persons, especially in law enforcement where police can quickly identify suspects.


BlueCheck by 3M enables users to perform two types of searches. Users can capture fingerprints and search against fingerprint templates stored on the device or securely transfer the data to a host (such as a PDA, laptop or smart phone) via Bluetooth or USB connection. The host device can submit ANSI-NIST format files via SMTP or FTP (network protocols for file transfer) to a remote server or to an Automated Fingerprint Identification System (AFIS) for fingerprint identification.

Here is a product brochure for BlueCheck:

Concepts involved - Fingerprint Identifaction

A fingerprint scanner system has two basic jobs -- to get an image and to determine whether the pattern in this image matches the pattern of ridges and valleys in pre-scanned images. The most common methods today are optical scanning (used in BlueCheck® II) and capacitance scanning (which uses silicon scanning; used in BlueCheck® II U).

  • Optical Scanner

At the heart of an optical scanner, it is a charge coupled device (CCD) - a light sensor system used in digital cameras and camcorders.

A CCD is an array of light-sensitive diodes called photosites, which generate an electrical signal in response to light photons. Each photosite 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 (in this case, a finger). An analog-to-digital converter in the scanner system processes the analog electrical signal to generate a digital representation of this image.

The scanning process starts when you place your finger on a glass plate. A CCD camera takes a picture. The scanner has its own light source (some light-emitting diodes) to illuminate the ridges of the finger. The CCD system 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).

The scanner system will run checks to correct the brightness and definition of the image. After the processor finds that the image crisp and properly exposed, it carries on to compare the captured fingerprint with fingerprints on file.

external image 3M_MD6000_Reader_Device_Detectors_gadgets.jpg

  • Capacitance Scanner

Capacitive fingerprint scanners generate an image of the ridges and valleys that make up a fingerprint. Instead of using light, the capacitors use electrical current.

The diagram above shows a simple capacitive sensor connected to an integrator.
The diagram above shows a simple capacitive sensor connected to an integrator.

The integrator is an electrical circuit built around an inverting operational amplifier. The inverting amplifier is a complex device made up of a number of transistors, resistors and capacitors - of which the details of its operation would take a few thousand more words to explain. But, here we can get a general sense of what happens in a capacitance scanner.

Like any amplifier, an inverting amplifier alters one current based on fluctuations in another current. In this case, the inverting amplifier alters a supply voltage. The alteration is based on the relative voltage of two inputs - the inverting terminal and the non-inverting terminal. The non-inverting terminal is connected to ground and the inverting terminal is connected to a reference voltage supply and a feedback loop. The feedback loop, which is connected to the amplifier output, includes the two conductor plates (blue layer in the diagram).

As you may have recognized, the two conductor plates form a basic capacitor (capacitors are electrical components that can store up charges - read more here!) The surface of the finger acts as a third capacitor plate, separated by a pocket of air due to the fingerprint valleys (see beige colour in diagram). Moving the finger closer or farther away from the conducting plates changes the total capacitance (ability to store charge) of the capacitor. Thus, the capacitor in a cell under a ridge will have a greater capacitance than the capacitor in a cell under a valley.

To scan the finger,

  1. The processor closes the reset switch for each cell - shorting each amplifier's input and output to "balance" the integrator circuit.
  2. When the switch is opened and the processor applies a fixed charge to the integrator circuit, the capacitors charge up
  3. Capacitance of the feedback loop's capacitor affects the voltage at the amplifier's input, which affects the amplifier's output.
  4. Since the distance to the finger alters capacitance, a finger ridge will result in a different voltage output than a finger valley

The scanner processor reads this voltage output and determines whether it is characteristic of a ridge or a valley. By reading every cell in the array, the processor put together an overall picture of the fingerprint.

The main advantage of a capacitive scanner is that it requires a real fingerprint-type shape, rather than the pattern of light and dark that makes up the visual impression of a fingerprint. This makes the system harder to trick.

Related resources: