Q1: What can a user do to avoid false rejections in a fingerprint authentication system?
The finger should be clean (free of sticky residue and grease),and depending on the sensor, should not be too damp or too dry (breathe on it!). The finger should always be applied on the sensor in the same manner (same position, same direction) and with uniform pressure (e.g., avoid pressing while twisting).

Q2: Which features of a fingerprint can be used in identification?
Three types of features are available for biometric identification:
* Coarse features (loops, arch, whorls etc.)
* Fine features (minutia)
* Pore structure
Coarse features have strong genotypic contributions and are suited for presorting during an identification with a very large database. The minutia is predominantly random type in nature and cause most of the uniqueness in a fingerprint. Therefore, either directly or indirectly (in picture correlation procedures), almost all fingerprint systems examine minutia. Pore structure is seldom used, due to large fluctuations in the quality of the scanning procedure.

Q3: What are minutiae?
Minutiae are the endings and the branching of the finger lines. Because these follow a strong random pattern, they are the carriers of uniqueness;.

Q4: Does everyone have fingerprints?
In principle, yes. Indeed, individual fingers can be damaged permanently (e.g. with rare skin diseases) or temporarily (e.g., dirty or worn down from abrasion), which can hinder or render impossible the recording and analysis of a fingerprint. With good sensors and analysis software, the failure to enroll rate is around 5% for everyone. If office workers are exclusively considered, the failure to enroll rate falls to under 1%.

Q5: Is there proof for the uniqueness of a fingerprint?
The uniqueness of a fingerprint is a working hypothesis which in the mathematical sense is difficult (if not impossible) to prove. The opposite is more provable, namely finding two identical fingers. Until now, no two fingerprints from different fingers have been found which are identical.
In a scientific sense, the term uniqueness has to be replaced by the probability to find two identical fingerprints from different fingers. This probability may be determined empirically by comparing all fingerprints of a forensic database against each other. For example, if such a collection contains 100 million fingerprints, a probability of less than 10-14 would be provable. However, such a large trial has not yet been undertaken until today. Furthermore, the probability for misnaming fingerprints (fingerprints from the same person/finger are filed under different names) is supposed to be much higher. This experience is well known from experiments with much smaller collections. As a result, the outcome of such a trial may become quite questionable.
A scientific investigation of the individuality of fingerprints has been published by [Pankant et al. 2001].

Q6: Fingerprint authentication is suitable for which applications?
* PC access
* PC network access (internet, intranet etc.)
* Access to rooms (key replacement)
* Safety on weapons: no access for children and other unauthorized users
* Mobile phones: network access, theft protection, mobile financial transactions and so on
* ID: company pass, personal identification, club ID and so on
* Credit cards, bank cards, EC cards
* Automobile: Seats, mirrors, temperature, and other personal settings
* Automation of hotels (e.g., check-in and room access)
* Company vending machines (soft drinks)
* Participation in sporting events
* Memberships (discotheques, tanning salons, slot machines, video stores etc.)
* Personal access to patient records

Q7: Which finger is most suitable for reaching high performance recognition?
In principle, every finger is suitable to give prints for authentication purposes. However, there are differences between the 10 fingers, which are expressed in different performance for FAR, FRR and FTE.
These differences are based on:
* Different finger qualities (use, moisture etc.)
* Different sizes
* Different ergonomics (e.g., systems ergonomically optimized for
the thumb are only usable by other fingers with contortion) Whereby the type of sensor also reacts in specific ways to these differences. In most cases one can assume that the index finger obtains the best performance regarding FAR and FRR.

Q8: How do fingerprint sensors work?
All fingerprint sensors try to generate a digital picture of the finger surface. This picture normally has a pixel resolution of 500 dpi. The picture generation can be different for every type of sensor.

* Static Capacitive Sensor Type 1
Here, one electrode is responsible for each pixel and measures the capacity compared to the neighbor electrode/ pixel (inter pixel measurement). The capacity, in turn, is dependent on the dielectric. If a pixel is on a groove (i.e. air), the capacity is substantially smaller than on a finger line (ridge). In this case, the dielectric is water, which is distinguished by a very high dielectric constant. The measurement of capacity is static in the sense that charging happens with fixed charge units and then voltage is measured. Practical systems are always a mix of type 1 and type 2.

* Static Capacitive Sensor Type 2
Also here one electrode per pixel is used, but the capacity is measured between pixel and ground, whereby the conductivity of the fingers does not play an insignificant role. The capacity measurement is in principle the same as in type 1. Practical systems are always
a mix between type 1 and type 2.

* Dynamic Capacitive Sensor
Here the capacity is measured by AC voltage. Inter pixel and pixel to ground measures can also be used here.

* Luminescent Capacitive Sensor
An electro luminescent foil with a transparent back electrode uses the finger at its front side as counter electrode. At the points where the finger ridges touch the foil surface, the field strength is largest, and, as a result, the light emission brightest. That way a glowing image of the ridge structure develops at the back side of the foil. This image may be acquired by a image sensor chip.

* Optical Reflexive Sensor
The finger lies on a prism surface for example. Where the finger ridges touch the glass, a total reflection of light inside of the glass is disturbed. This will supply a picture of the finger lines to a camera chip.

* Optical transmissible Sensors with fiber optical plate
Here a suitable light source illuminates through the finger. The finger lies directly on a fiber optical plate, which, in turn is directly connected to a camera chip. The fiber optical plate ensures that the finger does not touch the camera chip, nevertheless the light arrives at the camera chip without losing focus.

* Acoustic (Ultrasound) Sensors
Here a picture of the finger surface on the glass is recorded by very high frequency ltrasound (e.g., 50 MHz).

* Pressure Sensitive Sensors
With pressure sensors, the pressure per pixel of the finger is measured.

* Thermal Line Sensors
With these sensors, the finger is moved linearly over a narrow array of thermal sensors, similar to sensors for opening automatic doors on a larger scale. The thermal sensors register temperature differences over time, which vary between the finger lines and grooves.

* Capacitive and Optical Line Sensors
These sensor arrays work similar to thermal line sensors. Instead of temperature differences of time, the single sensors cells measure the capacity or the light, respectively, to build the image.

Q9: Which type of sensor is the best?
This question unfortunately offers no definitive answer, as every application has different requirements and each type of sensor has its specific advantages and disadvantages. The following criteria can assist in reaching an answer:
* Costs
* Degree of maturity
* Image quality in sub optimal conditions
* Indoor/ outdoor
* Personal/ public use
* Normal/ abnormal fingers
* Dry/ moist fingers
* Size
* Sensitivity against vandalism
* Temperature resistance
* Sensitivity against forgery
* ESD (electrostatic discharge) sensitivity

Requirement type of sensor currently best
Low costs capacitive silicon line sensor
High level of development optical reflexive sensor
High image quality optical reflexive sensor
Small size thermal/ capacitive line sensor
High vandalism protection optical transmissible sensor
High temperature span capacitive silicon sensor
High forgery protection optical transmissible sensor
High ESD strength optical reflexive sensor

Q10: Why is a good finger guide important?
Modern cost effective fingerprint sensors are generally smaller than a complete fingerprint, and therefore process only part of the fingerprint. Suitable mechanical finger guides nevertheless may lead to a good recognition performance. A good finger guide has the following characteristics:
* It will always record nearly the same part of the fingerprint
* It is suitable for both large and small fingers
* It also works with long fingernails
* It is comfortable
* It ensures that the finger covers the entire sensor surface
* Users can intuitively and correctly use it
* It allows use with all fingers from both the right and left hand
* It makes the application of fingerprint fakes more difficult