Startek Engineering Inc. – Leader of fingerprint module, reader, & authentication systems.

Biometrics is an automated system of recognizing a person based upon physical or behavior characteristics. It is a system that recognizes a person based on "who you are" instead of "what you have" or "what you know". There are four stages for security recognition:

What you know: Password, Pin Code
What you have: Token, Key, ID, License, Passport, Card
What you are: Biometrics
When and where you act: Combination of Time and Location

Things that you know or you have like password or card could be lost, stolen, or duplicated. They do not necessarily authenticate the person using the card or knowing the password, while biometrics recognize a person by his/her natural born human characteristics or his/her costumed behavior. Fingerprint is the most frequently applied physical characteristics in biometrics.

Other physical characteristics applied to biometrics are iris, palm (shape and print), voice and face. The behaviors applied to biometrics are voice, signature and keystroke. Recently, vein is also applied on biometrics. Among those, fingerprint verification is still the most recognized technology for good performance and price. Combination of either two or three of the above mentioned four stages will improve security even better; ex. card + fingerprint + pin no. or fingerprints + priority of different fingerprints. However, no matter which is included in the combination, at least one kind of biometrics is the only insurance of connecting authentication to the real and correct person.

Capacitive sensors use an array capacitor plates to image the fingerprint. Skin is conductive enough to provide a capacitive coupling with an individual capacitive element on the array. Ridges, being closer to the detector, have a higher capacitance and valleys have a lower capacitance. Some capacitive sensors apply a small voltage to the finger to enhance the signal and create better image contrast.
Capacitive sensors can be sensitive to electrostatic discharge (shock) but they are insensitive to ambient lighting and are more resist contamination issues than some optical designs.

Optical sensors use arrays of photodiode or phototransistor detectors to convert the energy in light incident on the detector into electrical charge. The sensor package usually includes a light-emitting-diode (LED) to illuminate the finger.

There are two detector types used by optical sensors, charge-coupled-devices (CCD) and CMOS based optical imagers. CCD detectors are sensitive to low light levels and are capable of making excellent grayscale pictures. However, CCD fabrication is relatively expensive and neither low-light sensitivity or grayscale imaging are required for fingerprint recognition. CMOS optical imagers are manufactured in quantity and can be made with some of the image processing steps built into the chip resulting in a lower cost.

Optical sensors for fingerprints may be affected by a number of real world factors such as stray light and surface contamination, possibly even a fingerprint impression left by a prior user. Common contaminates that deteriorate image quality include oil and dirt, scratches on the sensor surface, and condensation or ice. Some suppliers have tried to sidestep the contamination problem by directly taking a 3D image from the surface of a finger. 3D imaging technology is more hygienic but introduces a whole new set of problems and was not included in this study.

Impostor prints are more of a problem for optical sensors than it is for other detectors because it is relatively easy to present the scanner with a convincing picture of a fingerprint. Suppliers have come up with several techniques to validate a live finger. For example optical sensors can be enhanced and made more resistant to deception with Electro-Optical imaging. This works by placing a voltage across a light-emitting polymer film. When a finger is presented, the ridges provide a ground to the polymer surface creating a small current that generating light. The fingerprint valleys remain dark so a high contrast image is produced. The polymer is directly coupled to an optical detector

A Swipe Fingerprint Reader/Sensor requires the user drag the upper part of the finger across the sensor. The complete image is pieced together by accumulating the partial image as the finger moves across the scan area.

Pros of a Swipe Fingerprint Reader/Sensor:

Smaller imaging array
Cheaper - It requires a smaller IC chip, which significantly reduces the overall sensor cost
Swiping motion reduces contamination buildup on the sensor, which is particularly a problem for optical sensor.

Cons of a Swipe Fingerprint Reader/Sensor:

Less accuracte - Dynamic swiping may decrease the accuracy due to increased distortion of the finger surface as it is pulled across the sensor.
Swiping creates additional wear on the sensor surface
More difficult to use.

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